-----------------------------------------------------------------------------
This file contains a concatenation of the PCRE2 man pages, converted to plain
text format for ease of searching with a text editor, or for use on systems
that do not have a man page processor. The small individual files that give
synopses of each function in the library have not been included. Neither has
the pcre2demo program. There are separate text files for the pcre2grep and
pcre2test commands.
-----------------------------------------------------------------------------


PCRE2(3)                   Library Functions Manual                   PCRE2(3)



NAME
       PCRE2 - Perl-compatible regular expressions (revised API)

INTRODUCTION

       PCRE2 is the name used for a revised API for the PCRE library, which is
       a set of functions, written in C,  that  implement  regular  expression
       pattern matching using the same syntax and semantics as Perl, with just
       a few differences. Some features that appeared in Python and the origi-
       nal  PCRE  before  they  appeared  in Perl are also available using the
       Python syntax. There is also some support for one or two .NET and Onig-
       uruma  syntax  items,  and  there are options for requesting some minor
       changes that give better ECMAScript (aka JavaScript) compatibility.

       The source code for PCRE2 can be compiled to support 8-bit, 16-bit,  or
       32-bit  code units, which means that up to three separate libraries may
       be installed.  The original work to extend PCRE to  16-bit  and  32-bit
       code  units  was  done  by Zoltan Herczeg and Christian Persch, respec-
       tively. In all three cases, strings can be interpreted  either  as  one
       character  per  code  unit, or as UTF-encoded Unicode, with support for
       Unicode general category properties. Unicode  support  is  optional  at
       build  time  (but  is  the default). However, processing strings as UTF
       code units must be enabled explicitly at run time. The version of  Uni-
       code in use can be discovered by running

         pcre2test -C

       The  three  libraries  contain  identical sets of functions, with names
       ending in _8,  _16,  or  _32,  respectively  (for  example,  pcre2_com-
       pile_8()).  However,  by defining PCRE2_CODE_UNIT_WIDTH to be 8, 16, or
       32, a program that uses just one code unit width can be  written  using
       generic names such as pcre2_compile(), and the documentation is written
       assuming that this is the case.

       In addition to the Perl-compatible matching function, PCRE2 contains an
       alternative  function that matches the same compiled patterns in a dif-
       ferent way. In certain circumstances, the alternative function has some
       advantages.   For  a discussion of the two matching algorithms, see the
       pcre2matching page.

       Details of exactly which Perl regular expression features are  and  are
       not  supported  by  PCRE2  are  given  in  separate  documents. See the
       pcre2pattern and pcre2compat pages. There is a syntax  summary  in  the
       pcre2syntax page.

       Some  features  of PCRE2 can be included, excluded, or changed when the
       library is built. The pcre2_config() function makes it possible  for  a
       client  to  discover  which  features are available. The features them-
       selves are described in the pcre2build page. Documentation about build-
       ing  PCRE2 for various operating systems can be found in the README and
       NON-AUTOTOOLS_BUILD files in the source distribution.

       The libraries contains a number of undocumented internal functions  and
       data  tables  that  are  used by more than one of the exported external
       functions, but which are not intended  for  use  by  external  callers.
       Their  names  all begin with "_pcre2", which hopefully will not provoke
       any name clashes. In some environments, it is possible to control which
       external  symbols  are  exported when a shared library is built, and in
       these cases the undocumented symbols are not exported.


SECURITY CONSIDERATIONS

       If you are using PCRE2 in a non-UTF application that permits  users  to
       supply  arbitrary  patterns  for  compilation, you should be aware of a
       feature that allows users to turn on UTF support from within a pattern.
       For  example, an 8-bit pattern that begins with "(*UTF)" turns on UTF-8
       mode, which interprets patterns and subjects as strings of  UTF-8  code
       units instead of individual 8-bit characters. This causes both the pat-
       tern and any data against which it is matched to be checked  for  UTF-8
       validity.  If the data string is very long, such a check might use suf-
       ficiently many resources as to cause your application to  lose  perfor-
       mance.

       One  way  of guarding against this possibility is to use the pcre2_pat-
       tern_info() function  to  check  the  compiled  pattern's  options  for
       PCRE2_UTF.  Alternatively,  you can set the PCRE2_NEVER_UTF option when
       calling pcre2_compile(). This causes an compile time error if a pattern
       contains a UTF-setting sequence.

       The  use  of Unicode properties for character types such as \d can also
       be enabled from within the pattern, by specifying "(*UCP)".  This  fea-
       ture can be disallowed by setting the PCRE2_NEVER_UCP option.

       If  your  application  is one that supports UTF, be aware that validity
       checking can take time. If the same data string is to be  matched  many
       times,  you  can  use  the PCRE2_NO_UTF_CHECK option for the second and
       subsequent matches to avoid running redundant checks.

       The use of the \C escape sequence in a UTF-8 or UTF-16 pattern can lead
       to  problems,  because  it  may leave the current matching point in the
       middle of  a  multi-code-unit  character.  The  PCRE2_NEVER_BACKSLASH_C
       option can be used by an application to lock out the use of \C, causing
       a compile-time error if it is encountered. It is also possible to build
       PCRE2 with the use of \C permanently disabled.

       Another  way  that  performance can be hit is by running a pattern that
       has a very large search tree against a string that  will  never  match.
       Nested  unlimited repeats in a pattern are a common example. PCRE2 pro-
       vides some protection against  this:  see  the  pcre2_set_match_limit()
       function in the pcre2api page.


USER DOCUMENTATION

       The  user  documentation for PCRE2 comprises a number of different sec-
       tions. In the "man" format, each of these is a separate "man page".  In
       the  HTML  format, each is a separate page, linked from the index page.
       In the plain  text  format,  the  descriptions  of  the  pcre2grep  and
       pcre2test programs are in files called pcre2grep.txt and pcre2test.txt,
       respectively. The remaining sections, except for the pcre2demo  section
       (which  is a program listing), and the short pages for individual func-
       tions, are concatenated in pcre2.txt, for ease of searching.  The  sec-
       tions are as follows:

         pcre2              this document
         pcre2-config       show PCRE2 installation configuration information
         pcre2api           details of PCRE2's native C API
         pcre2build         building PCRE2
         pcre2callout       details of the callout feature
         pcre2compat        discussion of Perl compatibility
         pcre2demo          a demonstration C program that uses PCRE2
         pcre2grep          description of the pcre2grep command (8-bit only)
         pcre2jit           discussion of just-in-time optimization support
         pcre2limits        details of size and other limits
         pcre2matching      discussion of the two matching algorithms
         pcre2partial       details of the partial matching facility
         pcre2pattern       syntax and semantics of supported regular
                              expression patterns
         pcre2perform       discussion of performance issues
         pcre2posix         the POSIX-compatible C API for the 8-bit library
         pcre2sample        discussion of the pcre2demo program
         pcre2stack         discussion of stack usage
         pcre2syntax        quick syntax reference
         pcre2test          description of the pcre2test command
         pcre2unicode       discussion of Unicode and UTF support

       In  the  "man"  and HTML formats, there is also a short page for each C
       library function, listing its arguments and results.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge, England.

       Putting an actual email address here is a spam magnet. If you  want  to
       email  me,  use  my two initials, followed by the two digits 10, at the
       domain cam.ac.uk.


REVISION

       Last updated: 16 October 2015
       Copyright (c) 1997-2015 University of Cambridge.
------------------------------------------------------------------------------


PCRE2API(3)                Library Functions Manual                PCRE2API(3)



NAME
       PCRE2 - Perl-compatible regular expressions (revised API)

       #include <pcre2.h>

       PCRE2  is  a  new API for PCRE. This document contains a description of
       all its functions. See the pcre2 document for an overview  of  all  the
       PCRE2 documentation.


PCRE2 NATIVE API BASIC FUNCTIONS

       pcre2_code *pcre2_compile(PCRE2_SPTR pattern, PCRE2_SIZE length,
         uint32_t options, int *errorcode, PCRE2_SIZE *erroroffset,
         pcre2_compile_context *ccontext);

       void pcre2_code_free(pcre2_code *code);

       pcre2_match_data *pcre2_match_data_create(uint32_t ovecsize,
         pcre2_general_context *gcontext);

       pcre2_match_data *pcre2_match_data_create_from_pattern(
         const pcre2_code *code, pcre2_general_context *gcontext);

       int pcre2_match(const pcre2_code *code, PCRE2_SPTR subject,
         PCRE2_SIZE length, PCRE2_SIZE startoffset,
         uint32_t options, pcre2_match_data *match_data,
         pcre2_match_context *mcontext);

       int pcre2_dfa_match(const pcre2_code *code, PCRE2_SPTR subject,
         PCRE2_SIZE length, PCRE2_SIZE startoffset,
         uint32_t options, pcre2_match_data *match_data,
         pcre2_match_context *mcontext,
         int *workspace, PCRE2_SIZE wscount);

       void pcre2_match_data_free(pcre2_match_data *match_data);


PCRE2 NATIVE API AUXILIARY MATCH FUNCTIONS

       PCRE2_SPTR pcre2_get_mark(pcre2_match_data *match_data);

       uint32_t pcre2_get_ovector_count(pcre2_match_data *match_data);

       PCRE2_SIZE *pcre2_get_ovector_pointer(pcre2_match_data *match_data);

       PCRE2_SIZE pcre2_get_startchar(pcre2_match_data *match_data);


PCRE2 NATIVE API GENERAL CONTEXT FUNCTIONS

       pcre2_general_context *pcre2_general_context_create(
         void *(*private_malloc)(PCRE2_SIZE, void *),
         void (*private_free)(void *, void *), void *memory_data);

       pcre2_general_context *pcre2_general_context_copy(
         pcre2_general_context *gcontext);

       void pcre2_general_context_free(pcre2_general_context *gcontext);


PCRE2 NATIVE API COMPILE CONTEXT FUNCTIONS

       pcre2_compile_context *pcre2_compile_context_create(
         pcre2_general_context *gcontext);

       pcre2_compile_context *pcre2_compile_context_copy(
         pcre2_compile_context *ccontext);

       void pcre2_compile_context_free(pcre2_compile_context *ccontext);

       int pcre2_set_bsr(pcre2_compile_context *ccontext,
         uint32_t value);

       int pcre2_set_character_tables(pcre2_compile_context *ccontext,
         const unsigned char *tables);

       int pcre2_set_max_pattern_length(pcre2_compile_context *ccontext,
         PCRE2_SIZE value);

       int pcre2_set_newline(pcre2_compile_context *ccontext,
         uint32_t value);

       int pcre2_set_parens_nest_limit(pcre2_compile_context *ccontext,
         uint32_t value);

       int pcre2_set_compile_recursion_guard(pcre2_compile_context *ccontext,
         int (*guard_function)(uint32_t, void *), void *user_data);


PCRE2 NATIVE API MATCH CONTEXT FUNCTIONS

       pcre2_match_context *pcre2_match_context_create(
         pcre2_general_context *gcontext);

       pcre2_match_context *pcre2_match_context_copy(
         pcre2_match_context *mcontext);

       void pcre2_match_context_free(pcre2_match_context *mcontext);

       int pcre2_set_callout(pcre2_match_context *mcontext,
         int (*callout_function)(pcre2_callout_block *, void *),
         void *callout_data);

       int pcre2_set_match_limit(pcre2_match_context *mcontext,
         uint32_t value);

       int pcre2_set_offset_limit(pcre2_match_context *mcontext,
         PCRE2_SIZE value);

       int pcre2_set_recursion_limit(pcre2_match_context *mcontext,
         uint32_t value);

       int pcre2_set_recursion_memory_management(
         pcre2_match_context *mcontext,
         void *(*private_malloc)(PCRE2_SIZE, void *),
         void (*private_free)(void *, void *), void *memory_data);


PCRE2 NATIVE API STRING EXTRACTION FUNCTIONS

       int pcre2_substring_copy_byname(pcre2_match_data *match_data,
         PCRE2_SPTR name, PCRE2_UCHAR *buffer, PCRE2_SIZE *bufflen);

       int pcre2_substring_copy_bynumber(pcre2_match_data *match_data,
         uint32_t number, PCRE2_UCHAR *buffer,
         PCRE2_SIZE *bufflen);

       void pcre2_substring_free(PCRE2_UCHAR *buffer);

       int pcre2_substring_get_byname(pcre2_match_data *match_data,
         PCRE2_SPTR name, PCRE2_UCHAR **bufferptr, PCRE2_SIZE *bufflen);

       int pcre2_substring_get_bynumber(pcre2_match_data *match_data,
         uint32_t number, PCRE2_UCHAR **bufferptr,
         PCRE2_SIZE *bufflen);

       int pcre2_substring_length_byname(pcre2_match_data *match_data,
         PCRE2_SPTR name, PCRE2_SIZE *length);

       int pcre2_substring_length_bynumber(pcre2_match_data *match_data,
         uint32_t number, PCRE2_SIZE *length);

       int pcre2_substring_nametable_scan(const pcre2_code *code,
         PCRE2_SPTR name, PCRE2_SPTR *first, PCRE2_SPTR *last);

       int pcre2_substring_number_from_name(const pcre2_code *code,
         PCRE2_SPTR name);

       void pcre2_substring_list_free(PCRE2_SPTR *list);

       int pcre2_substring_list_get(pcre2_match_data *match_data,
         PCRE2_UCHAR ***listptr, PCRE2_SIZE **lengthsptr);


PCRE2 NATIVE API STRING SUBSTITUTION FUNCTION

       int pcre2_substitute(const pcre2_code *code, PCRE2_SPTR subject,
         PCRE2_SIZE length, PCRE2_SIZE startoffset,
         uint32_t options, pcre2_match_data *match_data,
         pcre2_match_context *mcontext, PCRE2_SPTR replacementzfP,
         PCRE2_SIZE rlength, PCRE2_UCHAR *outputbuffer,
         PCRE2_SIZE *outlengthptr);


PCRE2 NATIVE API JIT FUNCTIONS

       int pcre2_jit_compile(pcre2_code *code, uint32_t options);

       int pcre2_jit_match(const pcre2_code *code, PCRE2_SPTR subject,
         PCRE2_SIZE length, PCRE2_SIZE startoffset,
         uint32_t options, pcre2_match_data *match_data,
         pcre2_match_context *mcontext);

       void pcre2_jit_free_unused_memory(pcre2_general_context *gcontext);

       pcre2_jit_stack *pcre2_jit_stack_create(PCRE2_SIZE startsize,
         PCRE2_SIZE maxsize, pcre2_general_context *gcontext);

       void pcre2_jit_stack_assign(pcre2_match_context *mcontext,
         pcre2_jit_callback callback_function, void *callback_data);

       void pcre2_jit_stack_free(pcre2_jit_stack *jit_stack);


PCRE2 NATIVE API SERIALIZATION FUNCTIONS

       int32_t pcre2_serialize_decode(pcre2_code **codes,
         int32_t number_of_codes, const uint8_t *bytes,
         pcre2_general_context *gcontext);

       int32_t pcre2_serialize_encode(const pcre2_code **codes,
         int32_t number_of_codes, uint8_t **serialized_bytes,
         PCRE2_SIZE *serialized_size, pcre2_general_context *gcontext);

       void pcre2_serialize_free(uint8_t *bytes);

       int32_t pcre2_serialize_get_number_of_codes(const uint8_t *bytes);


PCRE2 NATIVE API AUXILIARY FUNCTIONS

       pcre2_code *pcre2_code_copy(const pcre2_code *code);

       int pcre2_get_error_message(int errorcode, PCRE2_UCHAR *buffer,
         PCRE2_SIZE bufflen);

       const unsigned char *pcre2_maketables(pcre2_general_context *gcontext);

       int pcre2_pattern_info(const pcre2 *code, uint32_t what, void *where);

       int pcre2_callout_enumerate(const pcre2_code *code,
         int (*callback)(pcre2_callout_enumerate_block *, void *),
         void *user_data);

       int pcre2_config(uint32_t what, void *where);


PCRE2 8-BIT, 16-BIT, AND 32-BIT LIBRARIES

       There  are  three PCRE2 libraries, supporting 8-bit, 16-bit, and 32-bit
       code units, respectively. However,  there  is  just  one  header  file,
       pcre2.h.   This  contains the function prototypes and other definitions
       for all three libraries. One, two, or all three can be installed simul-
       taneously.  On  Unix-like  systems the libraries are called libpcre2-8,
       libpcre2-16, and libpcre2-32, and they can also co-exist with the orig-
       inal PCRE libraries.

       Character  strings are passed to and from a PCRE2 library as a sequence
       of unsigned integers in code units  of  the  appropriate  width.  Every
       PCRE2  function  comes  in three different forms, one for each library,
       for example:

         pcre2_compile_8()
         pcre2_compile_16()
         pcre2_compile_32()

       There are also three different sets of data types:

         PCRE2_UCHAR8, PCRE2_UCHAR16, PCRE2_UCHAR32
         PCRE2_SPTR8,  PCRE2_SPTR16,  PCRE2_SPTR32

       The UCHAR types define unsigned code units of the  appropriate  widths.
       For  example,  PCRE2_UCHAR16 is usually defined as `uint16_t'. The SPTR
       types are constant pointers to the equivalent  UCHAR  types,  that  is,
       they are pointers to vectors of unsigned code units.

       Many  applications use only one code unit width. For their convenience,
       macros are defined whose names are the generic forms such as pcre2_com-
       pile()  and  PCRE2_SPTR.  These  macros  use  the  value  of  the macro
       PCRE2_CODE_UNIT_WIDTH to generate the appropriate width-specific  func-
       tion and macro names.  PCRE2_CODE_UNIT_WIDTH is not defined by default.
       An application must define it to be  8,  16,  or  32  before  including
       pcre2.h in order to make use of the generic names.

       Applications  that use more than one code unit width can be linked with
       more than one PCRE2 library, but must define  PCRE2_CODE_UNIT_WIDTH  to
       be  0  before  including pcre2.h, and then use the real function names.
       Any code that is to be included in an environment where  the  value  of
       PCRE2_CODE_UNIT_WIDTH  is  unknown  should  also  use the real function
       names. (Unfortunately, it is not possible in C code to save and restore
       the value of a macro.)

       If  PCRE2_CODE_UNIT_WIDTH  is  not  defined before including pcre2.h, a
       compiler error occurs.

       When using multiple libraries in an application,  you  must  take  care
       when  processing  any  particular  pattern to use only functions from a
       single library.  For example, if you want to run a match using  a  pat-
       tern  that  was  compiled  with pcre2_compile_16(), you must do so with
       pcre2_match_16(), not pcre2_match_8().

       In the function summaries above, and in the rest of this  document  and
       other  PCRE2  documents,  functions  and data types are described using
       their generic names, without the 8, 16, or 32 suffix.


PCRE2 API OVERVIEW

       PCRE2 has its own native API, which  is  described  in  this  document.
       There are also some wrapper functions for the 8-bit library that corre-
       spond to the POSIX regular expression API, but they do not give  access
       to all the functionality. They are described in the pcre2posix documen-
       tation. Both these APIs define a set of C function calls.

       The native API C data types, function prototypes,  option  values,  and
       error codes are defined in the header file pcre2.h, which contains def-
       initions of PCRE2_MAJOR and PCRE2_MINOR, the major  and  minor  release
       numbers  for the library. Applications can use these to include support
       for different releases of PCRE2.

       In a Windows environment, if you want to statically link an application
       program  against  a non-dll PCRE2 library, you must define PCRE2_STATIC
       before including pcre2.h.

       The functions pcre2_compile(), and pcre2_match() are used for compiling
       and  matching regular expressions in a Perl-compatible manner. A sample
       program that demonstrates the simplest way of using them is provided in
       the file called pcre2demo.c in the PCRE2 source distribution. A listing
       of this program is  given  in  the  pcre2demo  documentation,  and  the
       pcre2sample documentation describes how to compile and run it.

       Just-in-time  compiler support is an optional feature of PCRE2 that can
       be built in appropriate hardware environments. It greatly speeds up the
       matching  performance of many patterns. Programs can request that it be
       used if available, by calling pcre2_jit_compile() after a  pattern  has
       been successfully compiled by pcre2_compile(). This does nothing if JIT
       support is not available.

       More complicated programs might need to  make  use  of  the  specialist
       functions    pcre2_jit_stack_create(),    pcre2_jit_stack_free(),   and
       pcre2_jit_stack_assign() in order to  control  the  JIT  code's  memory
       usage.

       JIT matching is automatically used by pcre2_match() if it is available,
       unless the PCRE2_NO_JIT option is set. There is also a direct interface
       for  JIT  matching,  which gives improved performance. The JIT-specific
       functions are discussed in the pcre2jit documentation.

       A second matching function, pcre2_dfa_match(), which is  not  Perl-com-
       patible,  is  also  provided.  This  uses a different algorithm for the
       matching. The alternative algorithm finds all possible  matches  (at  a
       given  point  in  the subject), and scans the subject just once (unless
       there are lookbehind assertions).  However,  this  algorithm  does  not
       return  captured  substrings.  A  description of the two matching algo-
       rithms  and  their  advantages  and  disadvantages  is  given  in   the
       pcre2matching    documentation.   There   is   no   JIT   support   for
       pcre2_dfa_match().

       In addition to the main compiling and  matching  functions,  there  are
       convenience functions for extracting captured substrings from a subject
       string that has been matched by pcre2_match(). They are:

         pcre2_substring_copy_byname()
         pcre2_substring_copy_bynumber()
         pcre2_substring_get_byname()
         pcre2_substring_get_bynumber()
         pcre2_substring_list_get()
         pcre2_substring_length_byname()
         pcre2_substring_length_bynumber()
         pcre2_substring_nametable_scan()
         pcre2_substring_number_from_name()

       pcre2_substring_free() and pcre2_substring_list_free()  are  also  pro-
       vided, to free the memory used for extracted strings.

       The  function  pcre2_substitute()  can be called to match a pattern and
       return a copy of the subject string with substitutions for  parts  that
       were matched.

       Functions  whose  names begin with pcre2_serialize_ are used for saving
       compiled patterns on disc or elsewhere, and reloading them later.

       Finally, there are functions for finding out information about  a  com-
       piled  pattern  (pcre2_pattern_info()) and about the configuration with
       which PCRE2 was built (pcre2_config()).

       Functions with names ending with _free() are used  for  freeing  memory
       blocks  of  various  sorts.  In all cases, if one of these functions is
       called with a NULL argument, it does nothing.


STRING LENGTHS AND OFFSETS

       The PCRE2 API uses string lengths and  offsets  into  strings  of  code
       units  in  several  places. These values are always of type PCRE2_SIZE,
       which is an unsigned integer type, currently always defined as  size_t.
       The  largest  value  that  can  be  stored  in  such  a  type  (that is
       ~(PCRE2_SIZE)0) is reserved as a special indicator for  zero-terminated
       strings  and  unset offsets.  Therefore, the longest string that can be
       handled is one less than this maximum.


NEWLINES

       PCRE2 supports five different conventions for indicating line breaks in
       strings:  a  single  CR (carriage return) character, a single LF (line-
       feed) character, the two-character sequence CRLF, any of the three pre-
       ceding,  or any Unicode newline sequence. The Unicode newline sequences
       are the three just mentioned, plus the single characters  VT  (vertical
       tab, U+000B), FF (form feed, U+000C), NEL (next line, U+0085), LS (line
       separator, U+2028), and PS (paragraph separator, U+2029).

       Each of the first three conventions is used by at least  one  operating
       system as its standard newline sequence. When PCRE2 is built, a default
       can be specified.  The default default is LF, which is the  Unix  stan-
       dard.  However, the newline convention can be changed by an application
       when calling pcre2_compile(), or it can be specified by special text at
       the start of the pattern itself; this overrides any other settings. See
       the pcre2pattern page for details of the special character sequences.

       In the PCRE2 documentation the word "newline"  is  used  to  mean  "the
       character or pair of characters that indicate a line break". The choice
       of newline convention affects the handling of the dot, circumflex,  and
       dollar metacharacters, the handling of #-comments in /x mode, and, when
       CRLF is a recognized line ending sequence, the match position  advance-
       ment for a non-anchored pattern. There is more detail about this in the
       section on pcre2_match() options below.

       The choice of newline convention does not affect the interpretation  of
       the \n or \r escape sequences, nor does it affect what \R matches; this
       has its own separate convention.


MULTITHREADING

       In a multithreaded application it is important to keep  thread-specific
       data  separate  from data that can be shared between threads. The PCRE2
       library code itself is thread-safe: it contains  no  static  or  global
       variables.  The  API  is  designed to be fairly simple for non-threaded
       applications while at the same time ensuring that multithreaded  appli-
       cations can use it.

       There are several different blocks of data that are used to pass infor-
       mation between the application and the PCRE2 libraries.

   The compiled pattern

       A pointer to the compiled form of a pattern is  returned  to  the  user
       when pcre2_compile() is successful. The data in the compiled pattern is
       fixed, and does not change when the pattern is matched.  Therefore,  it
       is  thread-safe, that is, the same compiled pattern can be used by more
       than one thread simultaneously. For example, an application can compile
       all its patterns at the start, before forking off multiple threads that
       use them. However, if the just-in-time optimization  feature  is  being
       used,  it  needs  separate  memory stack areas for each thread. See the
       pcre2jit documentation for more details.

       In a more complicated situation, where patterns are compiled only  when
       they  are  first needed, but are still shared between threads, pointers
       to compiled patterns must be protected  from  simultaneous  writing  by
       multiple threads, at least until a pattern has been compiled. The logic
       can be something like this:

         Get a read-only (shared) lock (mutex) for pointer
         if (pointer == NULL)
           {
           Get a write (unique) lock for pointer
           pointer = pcre2_compile(...
           }
         Release the lock
         Use pointer in pcre2_match()

       Of course, testing for compilation errors should also  be  included  in
       the code.

       If JIT is being used, but the JIT compilation is not being done immedi-
       ately, (perhaps waiting to see if the pattern  is  used  often  enough)
       similar logic is required. JIT compilation updates a pointer within the
       compiled code block, so a thread must gain unique write access  to  the
       pointer     before    calling    pcre2_jit_compile().    Alternatively,
       pcre2_code_copy() can be used to obtain a private copy of the  compiled
       code.

   Context blocks

       The  next main section below introduces the idea of "contexts" in which
       PCRE2 functions are called. A context is nothing more than a collection
       of parameters that control the way PCRE2 operates. Grouping a number of
       parameters together in a context is a convenient way of passing them to
       a  PCRE2  function without using lots of arguments. The parameters that
       are stored in contexts are in some sense  "advanced  features"  of  the
       API. Many straightforward applications will not need to use contexts.

       In a multithreaded application, if the parameters in a context are val-
       ues that are never changed, the same context can be  used  by  all  the
       threads. However, if any thread needs to change any value in a context,
       it must make its own thread-specific copy.

   Match blocks

       The matching functions need a block of memory for working space and for
       storing  the  results  of  a  match.  This includes details of what was
       matched, as well as additional  information  such  as  the  name  of  a
       (*MARK) setting. Each thread must provide its own copy of this memory.


PCRE2 CONTEXTS

       Some  PCRE2  functions have a lot of parameters, many of which are used
       only by specialist applications, for example,  those  that  use  custom
       memory  management  or  non-standard character tables. To keep function
       argument lists at a reasonable size, and at the same time to  keep  the
       API  extensible,  "uncommon" parameters are passed to certain functions
       in a context instead of directly. A context is just a block  of  memory
       that  holds  the  parameter  values.   Applications that do not need to
       adjust any of the context parameters  can  pass  NULL  when  a  context
       pointer is required.

       There  are  three different types of context: a general context that is
       relevant for several PCRE2 operations, a compile-time  context,  and  a
       match-time context.

   The general context

       At  present,  this  context  just  contains  pointers to (and data for)
       external memory management  functions  that  are  called  from  several
       places in the PCRE2 library. The context is named `general' rather than
       specifically `memory' because in future other fields may be  added.  If
       you  do not want to supply your own custom memory management functions,
       you do not need to bother with a general context. A general context  is
       created by:

       pcre2_general_context *pcre2_general_context_create(
         void *(*private_malloc)(PCRE2_SIZE, void *),
         void (*private_free)(void *, void *), void *memory_data);

       The  two  function pointers specify custom memory management functions,
       whose prototypes are:

         void *private_malloc(PCRE2_SIZE, void *);
         void  private_free(void *, void *);

       Whenever code in PCRE2 calls these functions, the final argument is the
       value of memory_data. Either of the first two arguments of the creation
       function may be NULL, in which case the system memory management  func-
       tions  malloc()  and free() are used. (This is not currently useful, as
       there are no other fields in a general context,  but  in  future  there
       might  be.)   The  private_malloc()  function  is used (if supplied) to
       obtain memory for storing the context, and all three values  are  saved
       as part of the context.

       Whenever  PCRE2  creates a data block of any kind, the block contains a
       pointer to the free() function that matches the malloc() function  that
       was  used.  When  the  time  comes  to free the block, this function is
       called.

       A general context can be copied by calling:

       pcre2_general_context *pcre2_general_context_copy(
         pcre2_general_context *gcontext);

       The memory used for a general context should be freed by calling:

       void pcre2_general_context_free(pcre2_general_context *gcontext);


   The compile context

       A compile context is required if you want to change the default  values
       of any of the following compile-time parameters:

         What \R matches (Unicode newlines or CR, LF, CRLF only)
         PCRE2's character tables
         The newline character sequence
         The compile time nested parentheses limit
         The maximum length of the pattern string
         An external function for stack checking

       A  compile context is also required if you are using custom memory man-
       agement.  If none of these apply, just pass NULL as the  context  argu-
       ment of pcre2_compile().

       A  compile context is created, copied, and freed by the following func-
       tions:

       pcre2_compile_context *pcre2_compile_context_create(
         pcre2_general_context *gcontext);

       pcre2_compile_context *pcre2_compile_context_copy(
         pcre2_compile_context *ccontext);

       void pcre2_compile_context_free(pcre2_compile_context *ccontext);

       A compile context is created with default values  for  its  parameters.
       These can be changed by calling the following functions, which return 0
       on success, or PCRE2_ERROR_BADDATA if invalid data is detected.

       int pcre2_set_bsr(pcre2_compile_context *ccontext,
         uint32_t value);

       The value must be PCRE2_BSR_ANYCRLF, to specify that  \R  matches  only
       CR,  LF,  or CRLF, or PCRE2_BSR_UNICODE, to specify that \R matches any
       Unicode line ending sequence. The value is used by the JIT compiler and
       by   the   two   interpreted   matching  functions,  pcre2_match()  and
       pcre2_dfa_match().

       int pcre2_set_character_tables(pcre2_compile_context *ccontext,
         const unsigned char *tables);

       The value must be the result of a  call  to  pcre2_maketables(),  whose
       only argument is a general context. This function builds a set of char-
       acter tables in the current locale.

       int pcre2_set_max_pattern_length(pcre2_compile_context *ccontext,
         PCRE2_SIZE value);

       This sets a maximum length, in code units, for the pattern string  that
       is  to  be  compiled.  If the pattern is longer, an error is generated.
       This facility is provided so that  applications  that  accept  patterns
       from  external sources can limit their size. The default is the largest
       number that a PCRE2_SIZE variable can hold, which is effectively unlim-
       ited.

       int pcre2_set_newline(pcre2_compile_context *ccontext,
         uint32_t value);

       This specifies which characters or character sequences are to be recog-
       nized as newlines. The value must be one of PCRE2_NEWLINE_CR  (carriage
       return only), PCRE2_NEWLINE_LF (linefeed only), PCRE2_NEWLINE_CRLF (the
       two-character sequence CR followed by LF),  PCRE2_NEWLINE_ANYCRLF  (any
       of the above), or PCRE2_NEWLINE_ANY (any Unicode newline sequence).

       When a pattern is compiled with the PCRE2_EXTENDED option, the value of
       this parameter affects the recognition of white space and  the  end  of
       internal comments starting with #. The value is saved with the compiled
       pattern for subsequent use by the JIT compiler and by  the  two  inter-
       preted matching functions, pcre2_match() and pcre2_dfa_match().

       int pcre2_set_parens_nest_limit(pcre2_compile_context *ccontext,
         uint32_t value);

       This parameter ajusts the limit, set when PCRE2 is built (default 250),
       on the depth of parenthesis nesting in  a  pattern.  This  limit  stops
       rogue patterns using up too much system stack when being compiled.

       int pcre2_set_compile_recursion_guard(pcre2_compile_context *ccontext,
         int (*guard_function)(uint32_t, void *), void *user_data);

       There  is at least one application that runs PCRE2 in threads with very
       limited system stack, where running out of stack is to  be  avoided  at
       all  costs. The parenthesis limit above cannot take account of how much
       stack is actually available. For a finer  control,  you  can  supply  a
       function  that  is  called whenever pcre2_compile() starts to compile a
       parenthesized part of a pattern. This function  can  check  the  actual
       stack size (or anything else that it wants to, of course).

       The  first  argument to the callout function gives the current depth of
       nesting, and the second is user data that is set up by the  last  argu-
       ment   of  pcre2_set_compile_recursion_guard().  The  callout  function
       should return zero if all is well, or non-zero to force an error.

   The match context

       A match context is required if you want to change the default values of
       any of the following match-time parameters:

         A callout function
         The offset limit for matching an unanchored pattern
         The limit for calling match() (see below)
         The limit for calling match() recursively

       A match context is also required if you are using custom memory manage-
       ment.  If none of these apply, just pass NULL as the  context  argument
       of pcre2_match(), pcre2_dfa_match(), or pcre2_jit_match().

       A  match  context  is created, copied, and freed by the following func-
       tions:

       pcre2_match_context *pcre2_match_context_create(
         pcre2_general_context *gcontext);

       pcre2_match_context *pcre2_match_context_copy(
         pcre2_match_context *mcontext);

       void pcre2_match_context_free(pcre2_match_context *mcontext);

       A match context is created with  default  values  for  its  parameters.
       These can be changed by calling the following functions, which return 0
       on success, or PCRE2_ERROR_BADDATA if invalid data is detected.

       int pcre2_set_callout(pcre2_match_context *mcontext,
         int (*callout_function)(pcre2_callout_block *, void *),
         void *callout_data);

       This sets up a "callout" function, which PCRE2 will call  at  specified
       points during a matching operation. Details are given in the pcre2call-
       out documentation.

       int pcre2_set_offset_limit(pcre2_match_context *mcontext,
         PCRE2_SIZE value);

       The offset_limit parameter limits how  far  an  unanchored  search  can
       advance  in  the  subject string. The default value is PCRE2_UNSET. The
       pcre2_match()     and      pcre2_dfa_match()      functions      return
       PCRE2_ERROR_NOMATCH  if  a match with a starting point before or at the
       given offset is not found. For example, if the pattern /abc/ is matched
       against  "123abc"  with  an  offset  limit  less  than 3, the result is
       PCRE2_ERROR_NO_MATCH.  A match can never be found  if  the  startoffset
       argument of pcre2_match() or pcre2_dfa_match() is greater than the off-
       set limit.

       When using this facility,  you  must  set  PCRE2_USE_OFFSET_LIMIT  when
       calling  pcre2_compile() so that when JIT is in use, different code can
       be compiled. If a match is started with a non-default match limit  when
       PCRE2_USE_OFFSET_LIMIT is not set, an error is generated.

       The  offset limit facility can be used to track progress when searching
       large subject strings.  See  also  the  PCRE2_FIRSTLINE  option,  which
       requires a match to start within the first line of the subject. If this
       is set with an offset limit, a match must occur in the first  line  and
       also  within  the  offset limit.  In other words, whichever limit comes
       first is used.

       int pcre2_set_match_limit(pcre2_match_context *mcontext,
         uint32_t value);

       The match_limit parameter provides a means  of  preventing  PCRE2  from
       using up too many resources when processing patterns that are not going
       to match, but which have a very large number of possibilities in  their
       search  trees. The classic example is a pattern that uses nested unlim-
       ited repeats.

       Internally, pcre2_match() uses a  function  called  match(),  which  it
       calls  repeatedly (sometimes recursively). The limit set by match_limit
       is imposed on the number of times this  function  is  called  during  a
       match, which has the effect of limiting the amount of backtracking that
       can take place. For patterns that are not anchored, the count  restarts
       from  zero  for  each position in the subject string. This limit is not
       relevant to pcre2_dfa_match(), which ignores it.

       When pcre2_match() is called with a pattern that was successfully  pro-
       cessed by pcre2_jit_compile(), the way in which matching is executed is
       entirely different. However, there is still the possibility of  runaway
       matching  that  goes  on  for  a very long time, and so the match_limit
       value is also used in this case (but in a different way) to  limit  how
       long the matching can continue.

       The  default  value  for  the limit can be set when PCRE2 is built; the
       default default is 10 million, which handles all but the  most  extreme
       cases.    If    the    limit   is   exceeded,   pcre2_match()   returns
       PCRE2_ERROR_MATCHLIMIT. A value for the match limit may  also  be  sup-
       plied by an item at the start of a pattern of the form

         (*LIMIT_MATCH=ddd)

       where  ddd  is  a  decimal  number.  However, such a setting is ignored
       unless ddd is less than the limit set by the  caller  of  pcre2_match()
       or, if no such limit is set, less than the default.

       int pcre2_set_recursion_limit(pcre2_match_context *mcontext,
         uint32_t value);

       The recursion_limit parameter is similar to match_limit, but instead of
       limiting the total number of times that match() is  called,  it  limits
       the  depth  of  recursion. The recursion depth is a smaller number than
       the total number of calls, because not all calls to match() are  recur-
       sive.  This limit is of use only if it is set smaller than match_limit.

       Limiting the recursion depth limits the amount of system stack that can
       be used, or, when PCRE2 has been compiled to use  memory  on  the  heap
       instead  of the stack, the amount of heap memory that can be used. This
       limit is not relevant, and is ignored, when matching is done using  JIT
       compiled code or by the pcre2_dfa_match() function.

       The  default  value for recursion_limit can be set when PCRE2 is built;
       the default default is the same value as the default  for  match_limit.
       If  the limit is exceeded, pcre2_match() returns PCRE2_ERROR_RECURSION-
       LIMIT. A value for the recursion limit may also be supplied by an  item
       at the start of a pattern of the form

         (*LIMIT_RECURSION=ddd)

       where  ddd  is  a  decimal  number.  However, such a setting is ignored
       unless ddd is less than the limit set by the  caller  of  pcre2_match()
       or, if no such limit is set, less than the default.

       int pcre2_set_recursion_memory_management(
         pcre2_match_context *mcontext,
         void *(*private_malloc)(PCRE2_SIZE, void *),
         void (*private_free)(void *, void *), void *memory_data);

       This function sets up two additional custom memory management functions
       for use by pcre2_match() when PCRE2 is compiled to  use  the  heap  for
       remembering backtracking data, instead of recursive function calls that
       use the system stack. There is a discussion about PCRE2's  stack  usage
       in  the  pcre2stack documentation. See the pcre2build documentation for
       details of how to build PCRE2.

       Using the heap for recursion is a non-standard way of  building  PCRE2,
       for  use  in  environments  that  have  limited  stacks. Because of the
       greater use of memory management, pcre2_match() runs more slowly. Func-
       tions  that  are  different  to the general custom memory functions are
       provided so that special-purpose external code can  be  used  for  this
       case,  because  the memory blocks are all the same size. The blocks are
       retained by pcre2_match() until it is about to exit so that they can be
       re-used  when  possible during the match. In the absence of these func-
       tions, the normal custom memory management functions are used, if  sup-
       plied, otherwise the system functions.


CHECKING BUILD-TIME OPTIONS

       int pcre2_config(uint32_t what, void *where);

       The  function  pcre2_config()  makes  it possible for a PCRE2 client to
       discover which optional features have  been  compiled  into  the  PCRE2
       library.  The  pcre2build  documentation  has  more details about these
       optional features.

       The first argument for pcre2_config() specifies  which  information  is
       required.  The  second  argument  is a pointer to memory into which the
       information is placed. If NULL is  passed,  the  function  returns  the
       amount  of  memory  that  is  needed for the requested information. For
       calls that return  numerical  values,  the  value  is  in  bytes;  when
       requesting  these  values,  where should point to appropriately aligned
       memory. For calls that return strings, the required length is given  in
       code units, not counting the terminating zero.

       When  requesting information, the returned value from pcre2_config() is
       non-negative on success, or the negative error code  PCRE2_ERROR_BADOP-
       TION  if the value in the first argument is not recognized. The follow-
       ing information is available:

         PCRE2_CONFIG_BSR

       The output is a uint32_t integer whose value indicates  what  character
       sequences  the  \R  escape  sequence  matches  by  default.  A value of
       PCRE2_BSR_UNICODE  means  that  \R  matches  any  Unicode  line  ending
       sequence;  a  value of PCRE2_BSR_ANYCRLF means that \R matches only CR,
       LF, or CRLF. The default can be overridden when a pattern is compiled.

         PCRE2_CONFIG_JIT

       The output is a uint32_t integer that is set  to  one  if  support  for
       just-in-time compiling is available; otherwise it is set to zero.

         PCRE2_CONFIG_JITTARGET

       The  where  argument  should point to a buffer that is at least 48 code
       units long.  (The  exact  length  required  can  be  found  by  calling
       pcre2_config()  with  where  set  to NULL.) The buffer is filled with a
       string that contains the name of the architecture  for  which  the  JIT
       compiler  is  configured,  for  example  "x86  32bit  (little  endian +
       unaligned)". If JIT support is not available, PCRE2_ERROR_BADOPTION  is
       returned,  otherwise the number of code units used is returned. This is
       the length of the string, plus one unit for the terminating zero.

         PCRE2_CONFIG_LINKSIZE

       The output is a uint32_t integer that contains the number of bytes used
       for  internal  linkage  in  compiled regular expressions. When PCRE2 is
       configured, the value can be set to 2, 3, or 4, with the default  being
       2.  This is the value that is returned by pcre2_config(). However, when
       the 16-bit library is compiled, a value of 3 is rounded up  to  4,  and
       when  the  32-bit  library  is compiled, internal linkages always use 4
       bytes, so the configured value is not relevant.

       The default value of 2 for the 8-bit and 16-bit libraries is sufficient
       for  all but the most massive patterns, since it allows the size of the
       compiled pattern to be up to 64K code units. Larger values allow larger
       regular  expressions  to be compiled by those two libraries, but at the
       expense of slower matching.

         PCRE2_CONFIG_MATCHLIMIT

       The output is a uint32_t integer that gives the default limit  for  the
       number  of  internal  matching function calls in a pcre2_match() execu-
       tion. Further details are given with pcre2_match() below.

         PCRE2_CONFIG_NEWLINE

       The output is a uint32_t integer  whose  value  specifies  the  default
       character  sequence that is recognized as meaning "newline". The values
       are:

         PCRE2_NEWLINE_CR       Carriage return (CR)
         PCRE2_NEWLINE_LF       Linefeed (LF)
         PCRE2_NEWLINE_CRLF     Carriage return, linefeed (CRLF)
         PCRE2_NEWLINE_ANY      Any Unicode line ending
         PCRE2_NEWLINE_ANYCRLF  Any of CR, LF, or CRLF

       The default should normally correspond to  the  standard  sequence  for
       your operating system.

         PCRE2_CONFIG_PARENSLIMIT

       The  output is a uint32_t integer that gives the maximum depth of nest-
       ing of parentheses (of any kind) in a pattern. This limit is imposed to
       cap  the  amount of system stack used when a pattern is compiled. It is
       specified when PCRE2 is built; the default is 250. This limit does  not
       take  into  account  the  stack that may already be used by the calling
       application. For  finer  control  over  compilation  stack  usage,  see
       pcre2_set_compile_recursion_guard().

         PCRE2_CONFIG_RECURSIONLIMIT

       The  output  is a uint32_t integer that gives the default limit for the
       depth of recursion when calling the internal  matching  function  in  a
       pcre2_match()  execution.  Further details are given with pcre2_match()
       below.

         PCRE2_CONFIG_STACKRECURSE

       The output is a uint32_t integer that is set to one if internal  recur-
       sion  when  running  pcre2_match() is implemented by recursive function
       calls that use the system stack to remember their state.  This  is  the
       usual  way that PCRE2 is compiled. The output is zero if PCRE2 was com-
       piled to use blocks of data on the heap instead of  recursive  function
       calls.

         PCRE2_CONFIG_UNICODE_VERSION

       The  where  argument  should point to a buffer that is at least 24 code
       units long.  (The  exact  length  required  can  be  found  by  calling
       pcre2_config()  with  where  set  to  NULL.) If PCRE2 has been compiled
       without Unicode support, the buffer is filled with  the  text  "Unicode
       not  supported".  Otherwise,  the  Unicode version string (for example,
       "8.0.0") is inserted. The number of code units used is  returned.  This
       is the length of the string plus one unit for the terminating zero.

         PCRE2_CONFIG_UNICODE

       The  output is a uint32_t integer that is set to one if Unicode support
       is available; otherwise it is set to zero. Unicode support implies  UTF
       support.

         PCRE2_CONFIG_VERSION

       The  where  argument  should point to a buffer that is at least 12 code
       units long.  (The  exact  length  required  can  be  found  by  calling
       pcre2_config()  with  where set to NULL.) The buffer is filled with the
       PCRE2 version string, zero-terminated. The number of code units used is
       returned. This is the length of the string plus one unit for the termi-
       nating zero.


COMPILING A PATTERN

       pcre2_code *pcre2_compile(PCRE2_SPTR pattern, PCRE2_SIZE length,
         uint32_t options, int *errorcode, PCRE2_SIZE *erroroffset,
         pcre2_compile_context *ccontext);

       void pcre2_code_free(pcre2_code *code);

       pcre2_code *pcre2_code_copy(const pcre2_code *code);

       The pcre2_compile() function compiles a pattern into an internal  form.
       The  pattern  is  defined  by a pointer to a string of code units and a
       length. If the pattern is zero-terminated, the length can be  specified
       as  PCRE2_ZERO_TERMINATED. The function returns a pointer to a block of
       memory that contains the compiled pattern and related data, or NULL  if
       an error occurred.

       If  the  compile context argument ccontext is NULL, memory for the com-
       piled pattern  is  obtained  by  calling  malloc().  Otherwise,  it  is
       obtained  from  the  same memory function that was used for the compile
       context. The caller must free the memory by  calling  pcre2_code_free()
       when it is no longer needed.

       The function pcre2_code_copy() makes a copy of the compiled code in new
       memory, using the same memory allocator as was used for  the  original.
       However,  if  the  code  has  been  processed  by the JIT compiler (see
       below), the JIT information cannot be copied (because it  is  position-
       dependent).  The new copy can initially be used only for non-JIT match-
       ing, though it can be passed to pcre2_jit_compile()  if  required.  The
       pcre2_code_copy()  function  provides a way for individual threads in a
       multithreaded application to acquire a private copy of shared  compiled
       code.

       NOTE:  When  one  of  the matching functions is called, pointers to the
       compiled pattern and the subject string are set in the match data block
       so  that  they can be referenced by the substring extraction functions.
       After running a match, you must not free a compiled pattern (or a  sub-
       ject  string)  until  after all operations on the match data block have
       taken place.

       The options argument for pcre2_compile() contains various bit  settings
       that  affect  the  compilation.  It  should  be  zero if no options are
       required. The available options are described below. Some of  them  (in
       particular,  those  that  are  compatible with Perl, but some others as
       well) can also be set and  unset  from  within  the  pattern  (see  the
       detailed description in the pcre2pattern documentation).

       For  those options that can be different in different parts of the pat-
       tern, the contents of the options argument specifies their settings  at
       the  start  of  compilation.  The PCRE2_ANCHORED and PCRE2_NO_UTF_CHECK
       options can be set at the time of matching as well as at compile time.

       Other, less frequently required compile-time parameters  (for  example,
       the newline setting) can be provided in a compile context (as described
       above).

       If errorcode or erroroffset is NULL, pcre2_compile() returns NULL imme-
       diately.  Otherwise,  the  variables to which these point are set to an
       error code and an offset (number of code  units)  within  the  pattern,
       respectively,  when  pcre2_compile() returns NULL because a compilation
       error has occurred. The values are not defined when compilation is suc-
       cessful and pcre2_compile() returns a non-NULL value.

       The  pcre2_get_error_message() function (see "Obtaining a textual error
       message" below) provides a textual message for each error code.  Compi-
       lation errors have positive error codes; UTF formatting error codes are
       negative. For an invalid UTF-8 or UTF-16 string, the offset is that  of
       the first code unit of the failing character.

       Some  errors are not detected until the whole pattern has been scanned;
       in these cases, the offset passed back is the length  of  the  pattern.
       Note  that  the  offset is in code units, not characters, even in a UTF
       mode. It may sometimes point into the middle of a UTF-8 or UTF-16 char-
       acter.

       This  code  fragment shows a typical straightforward call to pcre2_com-
       pile():

         pcre2_code *re;
         PCRE2_SIZE erroffset;
         int errorcode;
         re = pcre2_compile(
           "^A.*Z",                /* the pattern */
           PCRE2_ZERO_TERMINATED,  /* the pattern is zero-terminated */
           0,                      /* default options */
           &errorcode,             /* for error code */
           &erroffset,             /* for error offset */
           NULL);                  /* no compile context */

       The following names for option bits are defined in the  pcre2.h  header
       file:

         PCRE2_ANCHORED

       If this bit is set, the pattern is forced to be "anchored", that is, it
       is constrained to match only at the first matching point in the  string
       that  is being searched (the "subject string"). This effect can also be
       achieved by appropriate constructs in the pattern itself, which is  the
       only way to do it in Perl.

         PCRE2_ALLOW_EMPTY_CLASS

       By  default, for compatibility with Perl, a closing square bracket that
       immediately follows an opening one is treated as a data  character  for
       the  class.  When  PCRE2_ALLOW_EMPTY_CLASS  is  set,  it terminates the
       class, which therefore contains no characters and so can never match.

         PCRE2_ALT_BSUX

       This option request alternative handling  of  three  escape  sequences,
       which  makes  PCRE2's  behaviour more like ECMAscript (aka JavaScript).
       When it is set:

       (1) \U matches an upper case "U" character; by default \U causes a com-
       pile time error (Perl uses \U to upper case subsequent characters).

       (2) \u matches a lower case "u" character unless it is followed by four
       hexadecimal digits, in which case the hexadecimal  number  defines  the
       code  point  to match. By default, \u causes a compile time error (Perl
       uses it to upper case the following character).

       (3) \x matches a lower case "x" character unless it is followed by  two
       hexadecimal  digits,  in  which case the hexadecimal number defines the
       code point to match. By default, as in Perl, a  hexadecimal  number  is
       always expected after \x, but it may have zero, one, or two digits (so,
       for example, \xz matches a binary zero character followed by z).

         PCRE2_ALT_CIRCUMFLEX

       In  multiline  mode  (when  PCRE2_MULTILINE  is  set),  the  circumflex
       metacharacter  matches at the start of the subject (unless PCRE2_NOTBOL
       is set), and also after any internal  newline.  However,  it  does  not
       match after a newline at the end of the subject, for compatibility with
       Perl. If you want a multiline circumflex also to match after  a  termi-
       nating newline, you must set PCRE2_ALT_CIRCUMFLEX.

         PCRE2_ALT_VERBNAMES

       By  default, for compatibility with Perl, the name in any verb sequence
       such as (*MARK:NAME) is  any  sequence  of  characters  that  does  not
       include  a  closing  parenthesis. The name is not processed in any way,
       and it is not possible to include a closing parenthesis  in  the  name.
       However,  if  the  PCRE2_ALT_VERBNAMES  option is set, normal backslash
       processing is applied to verb  names  and  only  an  unescaped  closing
       parenthesis  terminates the name. A closing parenthesis can be included
       in a name either as \) or between \Q  and  \E.  If  the  PCRE2_EXTENDED
       option is set, unescaped whitespace in verb names is skipped and #-com-
       ments are recognized, exactly as in the rest of the pattern.

         PCRE2_AUTO_CALLOUT

       If this bit  is  set,  pcre2_compile()  automatically  inserts  callout
       items, all with number 255, before each pattern item. For discussion of
       the callout facility, see the pcre2callout documentation.

         PCRE2_CASELESS

       If this bit is set, letters in the pattern match both upper  and  lower
       case  letters in the subject. It is equivalent to Perl's /i option, and
       it can be changed within a pattern by a (?i) option setting.

         PCRE2_DOLLAR_ENDONLY

       If this bit is set, a dollar metacharacter in the pattern matches  only
       at  the  end  of the subject string. Without this option, a dollar also
       matches immediately before a newline at the end of the string (but  not
       before  any other newlines). The PCRE2_DOLLAR_ENDONLY option is ignored
       if PCRE2_MULTILINE is set. There is no equivalent  to  this  option  in
       Perl, and no way to set it within a pattern.

         PCRE2_DOTALL

       If  this  bit  is  set,  a dot metacharacter in the pattern matches any
       character, including one that indicates a  newline.  However,  it  only
       ever matches one character, even if newlines are coded as CRLF. Without
       this option, a dot does not match when the current position in the sub-
       ject  is  at  a newline. This option is equivalent to Perl's /s option,
       and it can be changed within a pattern by a (?s) option setting. A neg-
       ative class such as [^a] always matches newline characters, independent
       of the setting of this option.

         PCRE2_DUPNAMES

       If this bit is set, names used to identify capturing  subpatterns  need
       not be unique. This can be helpful for certain types of pattern when it
       is known that only one instance of the named  subpattern  can  ever  be
       matched.  There  are  more details of named subpatterns below; see also
       the pcre2pattern documentation.

         PCRE2_EXTENDED

       If this bit is set, most white space  characters  in  the  pattern  are
       totally  ignored  except when escaped or inside a character class. How-
       ever, white space is not allowed within  sequences  such  as  (?>  that
       introduce various parenthesized subpatterns, nor within numerical quan-
       tifiers such as {1,3}.  Ignorable white space is permitted  between  an
       item  and a following quantifier and between a quantifier and a follow-
       ing + that indicates possessiveness.

       PCRE2_EXTENDED also causes characters between an unescaped # outside  a
       character  class  and the next newline, inclusive, to be ignored, which
       makes it possible to include comments inside complicated patterns. Note
       that  the  end of this type of comment is a literal newline sequence in
       the pattern; escape sequences that happen to represent a newline do not
       count.  PCRE2_EXTENDED is equivalent to Perl's /x option, and it can be
       changed within a pattern by a (?x) option setting.

       Which characters are interpreted as newlines can be specified by a set-
       ting  in  the compile context that is passed to pcre2_compile() or by a
       special sequence at the start of the pattern, as described in the  sec-
       tion  entitled "Newline conventions" in the pcre2pattern documentation.
       A default is defined when PCRE2 is built.

         PCRE2_FIRSTLINE

       If this option is set, an  unanchored  pattern  is  required  to  match
       before  or  at  the  first  newline  in  the subject string, though the
       matched text may continue over the  newline.  See  also  PCRE2_USE_OFF-
       SET_LIMIT,   which  provides  a  more  general  limiting  facility.  If
       PCRE2_FIRSTLINE is set with an offset limit, a match must occur in  the
       first  line and also within the offset limit. In other words, whichever
       limit comes first is used.

         PCRE2_MATCH_UNSET_BACKREF

       If this option is set, a back reference to an  unset  subpattern  group
       matches  an  empty  string (by default this causes the current matching
       alternative to fail).  A pattern such as  (\1)(a)  succeeds  when  this
       option  is set (assuming it can find an "a" in the subject), whereas it
       fails by default, for Perl compatibility.  Setting  this  option  makes
       PCRE2 behave more like ECMAscript (aka JavaScript).

         PCRE2_MULTILINE

       By  default,  for  the purposes of matching "start of line" and "end of
       line", PCRE2 treats the subject string as consisting of a  single  line
       of  characters,  even  if  it actually contains newlines. The "start of
       line" metacharacter (^) matches only at the start of  the  string,  and
       the  "end  of  line"  metacharacter  ($) matches only at the end of the
       string,  or  before  a  terminating  newline  (except  when  PCRE2_DOL-
       LAR_ENDONLY  is  set).  Note, however, that unless PCRE2_DOTALL is set,
       the "any character" metacharacter (.) does not match at a newline. This
       behaviour (for ^, $, and dot) is the same as Perl.

       When  PCRE2_MULTILINE  it is set, the "start of line" and "end of line"
       constructs match immediately following or immediately  before  internal
       newlines  in  the  subject string, respectively, as well as at the very
       start and end. This is equivalent to Perl's /m option, and  it  can  be
       changed within a pattern by a (?m) option setting. Note that the "start
       of line" metacharacter does not match after a newline at the end of the
       subject,  for compatibility with Perl.  However, you can change this by
       setting the PCRE2_ALT_CIRCUMFLEX option. If there are no newlines in  a
       subject  string,  or  no  occurrences  of  ^ or $ in a pattern, setting
       PCRE2_MULTILINE has no effect.

         PCRE2_NEVER_BACKSLASH_C

       This option locks out the use of \C in the pattern that is  being  com-
       piled.   This  escape  can  cause  unpredictable  behaviour in UTF-8 or
       UTF-16 modes, because it may leave the current matching  point  in  the
       middle  of  a  multi-code-unit  character. This option may be useful in
       applications that process patterns from  external  sources.  Note  that
       there is also a build-time option that permanently locks out the use of
       \C.

         PCRE2_NEVER_UCP

       This option locks out the use of Unicode properties  for  handling  \B,
       \b, \D, \d, \S, \s, \W, \w, and some of the POSIX character classes, as
       described for the PCRE2_UCP option below. In  particular,  it  prevents
       the  creator of the pattern from enabling this facility by starting the
       pattern with (*UCP). This option may be  useful  in  applications  that
       process patterns from external sources. The option combination PCRE_UCP
       and PCRE_NEVER_UCP causes an error.

         PCRE2_NEVER_UTF

       This option locks out interpretation of the pattern as  UTF-8,  UTF-16,
       or UTF-32, depending on which library is in use. In particular, it pre-
       vents the creator of the pattern from switching to  UTF  interpretation
       by  starting  the  pattern  with  (*UTF).  This option may be useful in
       applications that process patterns from external sources. The  combina-
       tion of PCRE2_UTF and PCRE2_NEVER_UTF causes an error.

         PCRE2_NO_AUTO_CAPTURE

       If this option is set, it disables the use of numbered capturing paren-
       theses in the pattern. Any opening parenthesis that is not followed  by
       ?  behaves as if it were followed by ?: but named parentheses can still
       be used for capturing (and they acquire  numbers  in  the  usual  way).
       There  is  no  equivalent  of  this  option in Perl. Note that, if this
       option is set, references  to  capturing  groups  (back  references  or
       recursion/subroutine  calls) may only refer to named groups, though the
       reference can be by name or by number.

         PCRE2_NO_AUTO_POSSESS

       If this option is set, it disables "auto-possessification", which is an
       optimization  that,  for example, turns a+b into a++b in order to avoid
       backtracks into a+ that can never be successful. However,  if  callouts
       are  in  use,  auto-possessification means that some callouts are never
       taken. You can set this option if you want the matching functions to do
       a  full  unoptimized  search and run all the callouts, but it is mainly
       provided for testing purposes.

         PCRE2_NO_DOTSTAR_ANCHOR

       If this option is set, it disables an optimization that is applied when
       .*  is  the  first significant item in a top-level branch of a pattern,
       and all the other branches also start with .* or with \A or  \G  or  ^.
       The  optimization  is  automatically disabled for .* if it is inside an
       atomic group or a capturing group that is the subject of a back  refer-
       ence,  or  if  the pattern contains (*PRUNE) or (*SKIP). When the opti-
       mization is not disabled, such a pattern is automatically  anchored  if
       PCRE2_DOTALL is set for all the .* items and PCRE2_MULTILINE is not set
       for any ^ items. Otherwise, the fact that any match must  start  either
       at  the start of the subject or following a newline is remembered. Like
       other optimizations, this can cause callouts to be skipped.

         PCRE2_NO_START_OPTIMIZE

       This is an option whose main effect is at matching time.  It  does  not
       change what pcre2_compile() generates, but it does affect the output of
       the JIT compiler.

       There are a number of optimizations that may occur at the  start  of  a
       match,  in  order  to speed up the process. For example, if it is known
       that an unanchored match must start  with  a  specific  character,  the
       matching  code searches the subject for that character, and fails imme-
       diately if it cannot find it, without actually running the main  match-
       ing  function.  This means that a special item such as (*COMMIT) at the
       start of a pattern is not considered until after  a  suitable  starting
       point  for  the  match  has  been found. Also, when callouts or (*MARK)
       items are in use, these "start-up" optimizations can cause them  to  be
       skipped  if  the pattern is never actually used. The start-up optimiza-
       tions are in effect a pre-scan of the subject that takes  place  before
       the pattern is run.

       The PCRE2_NO_START_OPTIMIZE option disables the start-up optimizations,
       possibly causing performance to suffer,  but  ensuring  that  in  cases
       where  the  result is "no match", the callouts do occur, and that items
       such as (*COMMIT) and (*MARK) are considered at every possible starting
       position in the subject string.

       Setting  PCRE2_NO_START_OPTIMIZE  may  change the outcome of a matching
       operation.  Consider the pattern

         (*COMMIT)ABC

       When this is compiled, PCRE2 records the fact that a match  must  start
       with  the  character  "A".  Suppose the subject string is "DEFABC". The
       start-up optimization scans along the subject, finds "A" and  runs  the
       first  match attempt from there. The (*COMMIT) item means that the pat-
       tern must match the current starting position, which in this  case,  it
       does.  However,  if  the same match is run with PCRE2_NO_START_OPTIMIZE
       set, the initial scan along the subject string  does  not  happen.  The
       first  match  attempt  is  run  starting  from "D" and when this fails,
       (*COMMIT) prevents any further matches  being  tried,  so  the  overall
       result is "no match". There are also other start-up optimizations.  For
       example, a minimum length for the subject may be recorded. Consider the
       pattern

         (*MARK:A)(X|Y)

       The  minimum  length  for  a  match is one character. If the subject is
       "ABC", there will be attempts to match "ABC", "BC", and "C". An attempt
       to match an empty string at the end of the subject does not take place,
       because PCRE2 knows that the subject is  now  too  short,  and  so  the
       (*MARK)  is  never encountered. In this case, the optimization does not
       affect the overall match result, which is still "no match", but it does
       affect the auxiliary information that is returned.

         PCRE2_NO_UTF_CHECK

       When  PCRE2_UTF  is set, the validity of the pattern as a UTF string is
       automatically checked. There are  discussions  about  the  validity  of
       UTF-8  strings,  UTF-16 strings, and UTF-32 strings in the pcre2unicode
       document.  If an invalid UTF sequence is found, pcre2_compile() returns
       a negative error code.

       If you know that your pattern is valid, and you want to skip this check
       for performance reasons, you can  set  the  PCRE2_NO_UTF_CHECK  option.
       When  it  is set, the effect of passing an invalid UTF string as a pat-
       tern is undefined. It may cause your program to  crash  or  loop.  Note
       that   this   option   can   also   be   passed  to  pcre2_match()  and
       pcre_dfa_match(), to suppress validity checking of the subject string.

         PCRE2_UCP

       This option changes the way PCRE2 processes \B, \b, \D, \d, \S, \s, \W,
       \w,  and  some  of  the POSIX character classes. By default, only ASCII
       characters are recognized, but if PCRE2_UCP is set, Unicode  properties
       are  used instead to classify characters. More details are given in the
       section on generic character types in the pcre2pattern page. If you set
       PCRE2_UCP,  matching one of the items it affects takes much longer. The
       option is available only if PCRE2 has been compiled with  Unicode  sup-
       port.

         PCRE2_UNGREEDY

       This  option  inverts  the "greediness" of the quantifiers so that they
       are not greedy by default, but become greedy if followed by "?". It  is
       not  compatible  with Perl. It can also be set by a (?U) option setting
       within the pattern.

         PCRE2_USE_OFFSET_LIMIT

       This option must be set for pcre2_compile() if pcre2_set_offset_limit()
       is  going  to be used to set a non-default offset limit in a match con-
       text for matches that use this pattern. An error  is  generated  if  an
       offset  limit  is  set  without  this option. For more details, see the
       description of pcre2_set_offset_limit() in the section  that  describes
       match contexts. See also the PCRE2_FIRSTLINE option above.

         PCRE2_UTF

       This  option  causes  PCRE2  to regard both the pattern and the subject
       strings that are subsequently processed as strings  of  UTF  characters
       instead  of  single-code-unit  strings.  It  is available when PCRE2 is
       built to include Unicode support (which is  the  default).  If  Unicode
       support  is  not  available,  the use of this option provokes an error.
       Details of how this option changes the behaviour of PCRE2 are given  in
       the pcre2unicode page.


COMPILATION ERROR CODES

       There  are over 80 positive error codes that pcre2_compile() may return
       (via errorcode) if it finds an error in the  pattern.  There  are  also
       some  negative error codes that are used for invalid UTF strings. These
       are the same as given by pcre2_match() and pcre2_dfa_match(),  and  are
       described in the pcre2unicode page. The pcre2_get_error_message() func-
       tion (see "Obtaining a textual error message" below) can be  called  to
       obtain a textual error message from any error code.


JUST-IN-TIME (JIT) COMPILATION

       int pcre2_jit_compile(pcre2_code *code, uint32_t options);

       int pcre2_jit_match(const pcre2_code *code, PCRE2_SPTR subject,
         PCRE2_SIZE length, PCRE2_SIZE startoffset,
         uint32_t options, pcre2_match_data *match_data,
         pcre2_match_context *mcontext);

       void pcre2_jit_free_unused_memory(pcre2_general_context *gcontext);

       pcre2_jit_stack *pcre2_jit_stack_create(PCRE2_SIZE startsize,
         PCRE2_SIZE maxsize, pcre2_general_context *gcontext);

       void pcre2_jit_stack_assign(pcre2_match_context *mcontext,
         pcre2_jit_callback callback_function, void *callback_data);

       void pcre2_jit_stack_free(pcre2_jit_stack *jit_stack);

       These  functions  provide  support  for  JIT compilation, which, if the
       just-in-time compiler is available, further processes a  compiled  pat-
       tern into machine code that executes much faster than the pcre2_match()
       interpretive matching function. Full details are given in the  pcre2jit
       documentation.

       JIT  compilation  is  a heavyweight optimization. It can take some time
       for patterns to be analyzed, and for one-off matches  and  simple  pat-
       terns  the benefit of faster execution might be offset by a much slower
       compilation time.  Most, but not all patterns can be optimized  by  the
       JIT compiler.


LOCALE SUPPORT

       PCRE2  handles caseless matching, and determines whether characters are
       letters, digits, or whatever, by reference to a set of tables,  indexed
       by  character  code  point.  This applies only to characters whose code
       points are less than 256. By default, higher-valued code  points  never
       match  escapes  such  as \w or \d.  However, if PCRE2 is built with UTF
       support, all characters can be tested with  \p  and  \P,  or,  alterna-
       tively,  the  PCRE2_UCP  option  can be set when a pattern is compiled;
       this causes \w and friends to use Unicode property support  instead  of
       the built-in tables.

       The  use  of  locales  with Unicode is discouraged. If you are handling
       characters with code points greater than 128,  you  should  either  use
       Unicode support, or use locales, but not try to mix the two.

       PCRE2  contains  an  internal  set of character tables that are used by
       default.  These are sufficient for  many  applications.  Normally,  the
       internal tables recognize only ASCII characters. However, when PCRE2 is
       built, it is possible to cause the internal tables to be rebuilt in the
       default "C" locale of the local system, which may cause them to be dif-
       ferent.

       The internal tables can be overridden by tables supplied by the  appli-
       cation  that  calls  PCRE2.  These may be created in a different locale
       from the default.  As more and more applications change to  using  Uni-
       code, the need for this locale support is expected to die away.

       External  tables  are built by calling the pcre2_maketables() function,
       in the relevant locale. The result can be passed to pcre2_compile()  as
       often   as  necessary,  by  creating  a  compile  context  and  calling
       pcre2_set_character_tables() to set the  tables  pointer  therein.  For
       example,  to  build  and use tables that are appropriate for the French
       locale (where accented characters with  values  greater  than  128  are
       treated as letters), the following code could be used:

         setlocale(LC_CTYPE, "fr_FR");
         tables = pcre2_maketables(NULL);
         ccontext = pcre2_compile_context_create(NULL);
         pcre2_set_character_tables(ccontext, tables);
         re = pcre2_compile(..., ccontext);

       The  locale  name "fr_FR" is used on Linux and other Unix-like systems;
       if you are using Windows, the name for the French locale  is  "french".
       It  is the caller's responsibility to ensure that the memory containing
       the tables remains available for as long as it is needed.

       The pointer that is passed (via the compile context) to pcre2_compile()
       is  saved  with  the  compiled pattern, and the same tables are used by
       pcre2_match() and pcre_dfa_match(). Thus, for any single pattern,  com-
       pilation,  and  matching  all  happen in the same locale, but different
       patterns can be processed in different locales.


INFORMATION ABOUT A COMPILED PATTERN

       int pcre2_pattern_info(const pcre2 *code, uint32_t what, void *where);

       The pcre2_pattern_info() function returns general information  about  a
       compiled pattern. For information about callouts, see the next section.
       The first argument for pcre2_pattern_info() is a pointer  to  the  com-
       piled pattern. The second argument specifies which piece of information
       is required, and the third argument is  a  pointer  to  a  variable  to
       receive  the data. If the third argument is NULL, the first argument is
       ignored, and the function returns the size in  bytes  of  the  variable
       that is required for the information requested. Otherwise, The yield of
       the function is zero for success, or one of the following negative num-
       bers:

         PCRE2_ERROR_NULL           the argument code was NULL
         PCRE2_ERROR_BADMAGIC       the "magic number" was not found
         PCRE2_ERROR_BADOPTION      the value of what was invalid
         PCRE2_ERROR_UNSET          the requested field is not set

       The  "magic  number" is placed at the start of each compiled pattern as
       an simple check against passing an arbitrary memory pointer. Here is  a
       typical  call of pcre2_pattern_info(), to obtain the length of the com-
       piled pattern:

         int rc;
         size_t length;
         rc = pcre2_pattern_info(
           re,               /* result of pcre2_compile() */
           PCRE2_INFO_SIZE,  /* what is required */
           &length);         /* where to put the data */

       The possible values for the second argument are defined in pcre2.h, and
       are as follows:

         PCRE2_INFO_ALLOPTIONS
         PCRE2_INFO_ARGOPTIONS

       Return a copy of the pattern's options. The third argument should point
       to a  uint32_t  variable.  PCRE2_INFO_ARGOPTIONS  returns  exactly  the
       options  that were passed to pcre2_compile(), whereas PCRE2_INFO_ALLOP-
       TIONS returns the compile options as modified by any  top-level  (*XXX)
       option settings such as (*UTF) at the start of the pattern itself.

       For   example,   if  the  pattern  /(*UTF)abc/  is  compiled  with  the
       PCRE2_EXTENDED  option,  the  result   for   PCRE2_INFO_ALLOPTIONS   is
       PCRE2_EXTENDED  and  PCRE2_UTF.   Option settings such as (?i) that can
       change within a pattern do not affect the result  of  PCRE2_INFO_ALLOP-
       TIONS, even if they appear right at the start of the pattern. (This was
       different in some earlier releases.)

       A pattern compiled without PCRE2_ANCHORED is automatically anchored  by
       PCRE2 if the first significant item in every top-level branch is one of
       the following:

         ^     unless PCRE2_MULTILINE is set
         \A    always
         \G    always
         .*    sometimes - see below

       When .* is the first significant item, anchoring is possible only  when
       all the following are true:

         .* is not in an atomic group
         .* is not in a capturing group that is the subject
              of a back reference
         PCRE2_DOTALL is in force for .*
         Neither (*PRUNE) nor (*SKIP) appears in the pattern.
         PCRE2_NO_DOTSTAR_ANCHOR is not set.

       For  patterns  that are auto-anchored, the PCRE2_ANCHORED bit is set in
       the options returned for PCRE2_INFO_ALLOPTIONS.

         PCRE2_INFO_BACKREFMAX

       Return the number of the highest back reference  in  the  pattern.  The
       third  argument should point to an uint32_t variable. Named subpatterns
       acquire numbers as well as names, and these count towards  the  highest
       back  reference.   Back  references such as \4 or \g{12} match the cap-
       tured characters of the given group, but in addition, the check that  a
       capturing group is set in a conditional subpattern such as (?(3)a|b) is
       also a back reference. Zero is returned if there  are  no  back  refer-
       ences.

         PCRE2_INFO_BSR

       The output is a uint32_t whose value indicates what character sequences
       the \R escape sequence matches. A value of PCRE2_BSR_UNICODE means that
       \R  matches any Unicode line ending sequence; a value of PCRE2_BSR_ANY-
       CRLF means that \R matches only CR, LF, or CRLF.

         PCRE2_INFO_CAPTURECOUNT

       Return the highest capturing subpattern number in the pattern. In  pat-
       terns where (?| is not used, this is also the total number of capturing
       subpatterns.  The third argument should point to an uint32_t variable.

         PCRE2_INFO_FIRSTBITMAP

       In the absence of a single first code unit for a non-anchored  pattern,
       pcre2_compile()  may construct a 256-bit table that defines a fixed set
       of values for the first code unit in any match. For example, a  pattern
       that  starts  with  [abc]  results in a table with three bits set. When
       code unit values greater than 255 are supported, the flag bit  for  255
       means  "any  code unit of value 255 or above". If such a table was con-
       structed, a pointer to it is returned. Otherwise NULL is returned.  The
       third argument should point to an const uint8_t * variable.

         PCRE2_INFO_FIRSTCODETYPE

       Return information about the first code unit of any matched string, for
       a non-anchored pattern. The third argument should point to an  uint32_t
       variable.  If there is a fixed first value, for example, the letter "c"
       from a pattern such as (cat|cow|coyote), 1 is returned, and the charac-
       ter  value can be retrieved using PCRE2_INFO_FIRSTCODEUNIT. If there is
       no fixed first value, but it is known that a match can  occur  only  at
       the  start  of  the subject or following a newline in the subject, 2 is
       returned. Otherwise, and for anchored patterns, 0 is returned.

         PCRE2_INFO_FIRSTCODEUNIT

       Return the value of the first code unit of any matched  string  in  the
       situation where PCRE2_INFO_FIRSTCODETYPE returns 1; otherwise return 0.
       The third argument should point to an uint32_t variable. In  the  8-bit
       library,  the  value is always less than 256. In the 16-bit library the
       value can be up to 0xffff. In the 32-bit library  in  UTF-32  mode  the
       value can be up to 0x10ffff, and up to 0xffffffff when not using UTF-32
       mode.

         PCRE2_INFO_HASBACKSLASHC

       Return 1 if the pattern contains any instances of \C, otherwise 0.  The
       third argument should point to an uint32_t variable.

         PCRE2_INFO_HASCRORLF

       Return  1  if  the  pattern  contains any explicit matches for CR or LF
       characters, otherwise 0. The third argument should point to an uint32_t
       variable.  An explicit match is either a literal CR or LF character, or
       \r or \n.

         PCRE2_INFO_JCHANGED

       Return 1 if the (?J) or (?-J) option setting is used  in  the  pattern,
       otherwise  0.  The third argument should point to an uint32_t variable.
       (?J) and (?-J) set and unset the local PCRE2_DUPNAMES  option,  respec-
       tively.

         PCRE2_INFO_JITSIZE

       If  the  compiled  pattern was successfully processed by pcre2_jit_com-
       pile(), return the size of the  JIT  compiled  code,  otherwise  return
       zero. The third argument should point to a size_t variable.

         PCRE2_INFO_LASTCODETYPE

       Returns  1 if there is a rightmost literal code unit that must exist in
       any matched string, other than at its start. The third argument  should
       point  to  an  uint32_t  variable.  If  there  is  no  such value, 0 is
       returned. When 1 is  returned,  the  code  unit  value  itself  can  be
       retrieved  using PCRE2_INFO_LASTCODEUNIT. For anchored patterns, a last
       literal value is recorded only if  it  follows  something  of  variable
       length.  For example, for the pattern /^a\d+z\d+/ the returned value is
       1 (with "z" returned from PCRE2_INFO_LASTCODEUNIT), but  for  /^a\dz\d/
       the returned value is 0.

         PCRE2_INFO_LASTCODEUNIT

       Return  the value of the rightmost literal data unit that must exist in
       any matched string, other than at its start, if such a value  has  been
       recorded.  The  third argument should point to an uint32_t variable. If
       there is no such value, 0 is returned.

         PCRE2_INFO_MATCHEMPTY

       Return 1 if the pattern might match an empty string, otherwise  0.  The
       third  argument  should  point  to an uint32_t variable. When a pattern
       contains recursive subroutine calls it is not always possible to deter-
       mine  whether  or  not it can match an empty string. PCRE2 takes a cau-
       tious approach and returns 1 in such cases.

         PCRE2_INFO_MATCHLIMIT

       If the pattern set a match limit by  including  an  item  of  the  form
       (*LIMIT_MATCH=nnnn)  at  the  start,  the  value is returned. The third
       argument should point to an unsigned 32-bit integer. If no  such  value
       has  been  set,  the  call  to  pcre2_pattern_info()  returns the error
       PCRE2_ERROR_UNSET.

         PCRE2_INFO_MAXLOOKBEHIND

       Return the number of characters (not code units) in the longest lookbe-
       hind  assertion  in  the pattern. The third argument should point to an
       unsigned 32-bit integer. This information is useful when  doing  multi-
       segment  matching  using the partial matching facilities. Note that the
       simple assertions \b and \B require a one-character lookbehind. \A also
       registers  a  one-character  lookbehind,  though  it  does not actually
       inspect the previous character. This is to ensure  that  at  least  one
       character  from  the old segment is retained when a new segment is pro-
       cessed. Otherwise, if there are no lookbehinds in the pattern, \A might
       match incorrectly at the start of a new segment.

         PCRE2_INFO_MINLENGTH

       If  a  minimum  length  for  matching subject strings was computed, its
       value is returned. Otherwise the returned value is 0. The  value  is  a
       number  of characters, which in UTF mode may be different from the num-
       ber of code units.  The third argument  should  point  to  an  uint32_t
       variable.  The  value  is  a  lower bound to the length of any matching
       string. There may not be any strings of that length  that  do  actually
       match, but every string that does match is at least that long.

         PCRE2_INFO_NAMECOUNT
         PCRE2_INFO_NAMEENTRYSIZE
         PCRE2_INFO_NAMETABLE

       PCRE2 supports the use of named as well as numbered capturing parenthe-
       ses. The names are just an additional way of identifying the  parenthe-
       ses, which still acquire numbers. Several convenience functions such as
       pcre2_substring_get_byname() are provided for extracting captured  sub-
       strings  by  name. It is also possible to extract the data directly, by
       first converting the name to a number in order to  access  the  correct
       pointers  in the output vector (described with pcre2_match() below). To
       do the conversion, you need to use the  name-to-number  map,  which  is
       described by these three values.

       The  map  consists  of a number of fixed-size entries. PCRE2_INFO_NAME-
       COUNT gives the number of entries, and  PCRE2_INFO_NAMEENTRYSIZE  gives
       the  size  of each entry in code units; both of these return a uint32_t
       value. The entry size depends on the length of the longest name.

       PCRE2_INFO_NAMETABLE returns a pointer to the first entry of the table.
       This  is  a  PCRE2_SPTR  pointer to a block of code units. In the 8-bit
       library, the first two bytes of each entry are the number of  the  cap-
       turing parenthesis, most significant byte first. In the 16-bit library,
       the pointer points to 16-bit code units, the first  of  which  contains
       the  parenthesis  number.  In the 32-bit library, the pointer points to
       32-bit code units, the first of which contains the parenthesis  number.
       The rest of the entry is the corresponding name, zero terminated.

       The  names are in alphabetical order. If (?| is used to create multiple
       groups with the same number, as described in the section  on  duplicate
       subpattern  numbers  in  the pcre2pattern page, the groups may be given
       the same name, but there is only one  entry  in  the  table.  Different
       names for groups of the same number are not permitted.

       Duplicate  names  for subpatterns with different numbers are permitted,
       but only if PCRE2_DUPNAMES is set. They appear  in  the  table  in  the
       order  in  which  they were found in the pattern. In the absence of (?|
       this is the order of increasing number; when (?| is used  this  is  not
       necessarily the case because later subpatterns may have lower numbers.

       As  a  simple  example of the name/number table, consider the following
       pattern after compilation by the 8-bit library  (assume  PCRE2_EXTENDED
       is set, so white space - including newlines - is ignored):

         (?<date> (?<year>(\d\d)?\d\d) -
         (?<month>\d\d) - (?<day>\d\d) )

       There  are  four  named subpatterns, so the table has four entries, and
       each entry in the table is eight bytes long. The table is  as  follows,
       with non-printing bytes shows in hexadecimal, and undefined bytes shown
       as ??:

         00 01 d  a  t  e  00 ??
         00 05 d  a  y  00 ?? ??
         00 04 m  o  n  t  h  00
         00 02 y  e  a  r  00 ??

       When writing code to extract data  from  named  subpatterns  using  the
       name-to-number  map,  remember that the length of the entries is likely
       to be different for each compiled pattern.

         PCRE2_INFO_NEWLINE

       The output is a uint32_t with one of the following values:

         PCRE2_NEWLINE_CR       Carriage return (CR)
         PCRE2_NEWLINE_LF       Linefeed (LF)
         PCRE2_NEWLINE_CRLF     Carriage return, linefeed (CRLF)
         PCRE2_NEWLINE_ANY      Any Unicode line ending
         PCRE2_NEWLINE_ANYCRLF  Any of CR, LF, or CRLF

       This specifies the default character sequence that will  be  recognized
       as meaning "newline" while matching.

         PCRE2_INFO_RECURSIONLIMIT

       If  the  pattern set a recursion limit by including an item of the form
       (*LIMIT_RECURSION=nnnn) at the start, the value is returned. The  third
       argument  should  point to an unsigned 32-bit integer. If no such value
       has been set,  the  call  to  pcre2_pattern_info()  returns  the  error
       PCRE2_ERROR_UNSET.

         PCRE2_INFO_SIZE

       Return  the  size  of  the  compiled  pattern  in  bytes (for all three
       libraries). The third argument should point to a size_t variable.  This
       value  includes  the  size  of the general data block that precedes the
       code units of the compiled pattern itself. The value that is used  when
       pcre2_compile()  is  getting memory in which to place the compiled pat-
       tern may be slightly larger than the value  returned  by  this  option,
       because  there are cases where the code that calculates the size has to
       over-estimate. Processing a pattern with  the  JIT  compiler  does  not
       alter the value returned by this option.


INFORMATION ABOUT A PATTERN'S CALLOUTS

       int pcre2_callout_enumerate(const pcre2_code *code,
         int (*callback)(pcre2_callout_enumerate_block *, void *),
         void *user_data);

       A script language that supports the use of string arguments in callouts
       might like to scan all the callouts in a  pattern  before  running  the
       match. This can be done by calling pcre2_callout_enumerate(). The first
       argument is a pointer to a compiled pattern, the  second  points  to  a
       callback  function,  and the third is arbitrary user data. The callback
       function is called for every callout in the pattern  in  the  order  in
       which they appear. Its first argument is a pointer to a callout enumer-
       ation block, and its second argument is the user_data  value  that  was
       passed  to  pcre2_callout_enumerate(). The contents of the callout enu-
       meration block are described in the pcre2callout  documentation,  which
       also gives further details about callouts.


SERIALIZATION AND PRECOMPILING

       It  is  possible  to  save  compiled patterns on disc or elsewhere, and
       reload them later, subject to a number of restrictions.  The  functions
       whose names begin with pcre2_serialize_ are used for this purpose. They
       are described in the pcre2serialize documentation.


THE MATCH DATA BLOCK

       pcre2_match_data *pcre2_match_data_create(uint32_t ovecsize,
         pcre2_general_context *gcontext);

       pcre2_match_data *pcre2_match_data_create_from_pattern(
         const pcre2_code *code, pcre2_general_context *gcontext);

       void pcre2_match_data_free(pcre2_match_data *match_data);

       Information about a successful or unsuccessful match  is  placed  in  a
       match  data  block,  which  is  an opaque structure that is accessed by
       function calls. In particular, the match data block contains  a  vector
       of  offsets into the subject string that define the matched part of the
       subject and any substrings that were captured.  This  is  know  as  the
       ovector.

       Before  calling  pcre2_match(), pcre2_dfa_match(), or pcre2_jit_match()
       you must create a match data block by calling one of the creation func-
       tions  above.  For pcre2_match_data_create(), the first argument is the
       number of pairs of offsets in the  ovector.  One  pair  of  offsets  is
       required  to  identify  the string that matched the whole pattern, with
       another pair for each captured substring. For example,  a  value  of  4
       creates  enough space to record the matched portion of the subject plus
       three captured substrings. A minimum of at least 1 pair is  imposed  by
       pcre2_match_data_create(), so it is always possible to return the over-
       all matched string.

       The second argument of pcre2_match_data_create() is a pointer to a gen-
       eral  context, which can specify custom memory management for obtaining
       the memory for the match data block. If you are not using custom memory
       management, pass NULL, which causes malloc() to be used.

       For  pcre2_match_data_create_from_pattern(),  the  first  argument is a
       pointer to a compiled pattern. The ovector is created to be exactly the
       right size to hold all the substrings a pattern might capture. The sec-
       ond argument is again a pointer to a general context, but in this  case
       if NULL is passed, the memory is obtained using the same allocator that
       was used for the compiled pattern (custom or default).

       A match data block can be used many times, with the same  or  different
       compiled  patterns. You can extract information from a match data block
       after  a  match  operation  has  finished,  using  functions  that  are
       described  in  the  sections  on  matched  strings and other match data
       below.

       When a call of pcre2_match() fails, valid  data  is  available  in  the
       match    block    only   when   the   error   is   PCRE2_ERROR_NOMATCH,
       PCRE2_ERROR_PARTIAL, or one of the  error  codes  for  an  invalid  UTF
       string. Exactly what is available depends on the error, and is detailed
       below.

       When one of the matching functions is called, pointers to the  compiled
       pattern  and the subject string are set in the match data block so that
       they can be referenced by the extraction  functions.  After  running  a
       match,  you  must not free a compiled pattern or a subject string until
       after all operations on the match data  block  (for  that  match)  have
       taken place.

       When  a match data block itself is no longer needed, it should be freed
       by calling pcre2_match_data_free().


MATCHING A PATTERN: THE TRADITIONAL FUNCTION

       int pcre2_match(const pcre2_code *code, PCRE2_SPTR subject,
         PCRE2_SIZE length, PCRE2_SIZE startoffset,
         uint32_t options, pcre2_match_data *match_data,
         pcre2_match_context *mcontext);

       The function pcre2_match() is called to match a subject string  against
       a  compiled pattern, which is passed in the code argument. You can call
       pcre2_match() with the same code argument as many times as you like, in
       order  to  find multiple matches in the subject string or to match dif-
       ferent subject strings with the same pattern.

       This function is the main matching facility  of  the  library,  and  it
       operates  in  a  Perl-like  manner. For specialist use there is also an
       alternative matching function, which is described below in the  section
       about the pcre2_dfa_match() function.

       Here is an example of a simple call to pcre2_match():

         pcre2_match_data *md = pcre2_match_data_create(4, NULL);
         int rc = pcre2_match(
           re,             /* result of pcre2_compile() */
           "some string",  /* the subject string */
           11,             /* the length of the subject string */
           0,              /* start at offset 0 in the subject */
           0,              /* default options */
           match_data,     /* the match data block */
           NULL);          /* a match context; NULL means use defaults */

       If  the  subject  string is zero-terminated, the length can be given as
       PCRE2_ZERO_TERMINATED. A match context must be provided if certain less
       common matching parameters are to be changed. For details, see the sec-
       tion on the match context above.

   The string to be matched by pcre2_match()

       The subject string is passed to pcre2_match() as a pointer in  subject,
       a  length  in  length, and a starting offset in startoffset. The length
       and offset are in code units, not characters.  That  is,  they  are  in
       bytes  for the 8-bit library, 16-bit code units for the 16-bit library,
       and 32-bit code units for the 32-bit library, whether or not  UTF  pro-
       cessing is enabled.

       If startoffset is greater than the length of the subject, pcre2_match()
       returns PCRE2_ERROR_BADOFFSET. When the starting offset  is  zero,  the
       search  for a match starts at the beginning of the subject, and this is
       by far the most common case. In UTF-8 or UTF-16 mode, the starting off-
       set  must  point to the start of a character, or to the end of the sub-
       ject (in UTF-32 mode, one code unit equals one character, so  all  off-
       sets  are  valid).  Like  the  pattern  string, the subject may contain
       binary zeroes.

       A non-zero starting offset is useful when searching for  another  match
       in  the  same  subject  by calling pcre2_match() again after a previous
       success.  Setting startoffset differs from  passing  over  a  shortened
       string  and  setting  PCRE2_NOTBOL in the case of a pattern that begins
       with any kind of lookbehind. For example, consider the pattern

         \Biss\B

       which finds occurrences of "iss" in the middle of  words.  (\B  matches
       only  if  the  current position in the subject is not a word boundary.)
       When applied to the string "Mississipi" the first call to pcre2_match()
       finds  the first occurrence. If pcre2_match() is called again with just
       the remainder of the subject,  namely  "issipi",  it  does  not  match,
       because \B is always false at the start of the subject, which is deemed
       to be a word boundary. However, if pcre2_match() is passed  the  entire
       string again, but with startoffset set to 4, it finds the second occur-
       rence of "iss" because it is able to look behind the starting point  to
       discover that it is preceded by a letter.

       Finding  all  the  matches  in a subject is tricky when the pattern can
       match an empty string. It is possible to emulate Perl's /g behaviour by
       first   trying   the   match   again  at  the  same  offset,  with  the
       PCRE2_NOTEMPTY_ATSTART and PCRE2_ANCHORED options,  and  then  if  that
       fails,  advancing  the  starting  offset  and  trying an ordinary match
       again. There is some code that demonstrates  how  to  do  this  in  the
       pcre2demo  sample  program. In the most general case, you have to check
       to see if the newline convention recognizes CRLF as a newline,  and  if
       so,  and the current character is CR followed by LF, advance the start-
       ing offset by two characters instead of one.

       If a non-zero starting offset is passed when the pattern  is  anchored,
       one attempt to match at the given offset is made. This can only succeed
       if the pattern does not require the match to be at  the  start  of  the
       subject.

   Option bits for pcre2_match()

       The unused bits of the options argument for pcre2_match() must be zero.
       The only  bits  that  may  be  set  are  PCRE2_ANCHORED,  PCRE2_NOTBOL,
       PCRE2_NOTEOL,   PCRE2_NOTEMPTY,  PCRE2_NOTEMPTY_ATSTART,  PCRE2_NO_JIT,
       PCRE2_NO_UTF_CHECK, PCRE2_PARTIAL_HARD, and  PCRE2_PARTIAL_SOFT.  Their
       action is described below.

       Setting  PCRE2_ANCHORED  at match time is not supported by the just-in-
       time (JIT) compiler. If it is set, JIT matching  is  disabled  and  the
       normal   interpretive   code   in  pcre2_match()  is  run.  Apart  from
       PCRE2_NO_JIT (obviously), the remaining options are supported  for  JIT
       matching.

         PCRE2_ANCHORED

       The PCRE2_ANCHORED option limits pcre2_match() to matching at the first
       matching position. If a pattern was compiled  with  PCRE2_ANCHORED,  or
       turned  out to be anchored by virtue of its contents, it cannot be made
       unachored at matching time. Note that setting the option at match  time
       disables JIT matching.

         PCRE2_NOTBOL

       This option specifies that first character of the subject string is not
       the beginning of a line, so the  circumflex  metacharacter  should  not
       match  before  it.  Setting  this without having set PCRE2_MULTILINE at
       compile time causes circumflex never to match. This option affects only
       the behaviour of the circumflex metacharacter. It does not affect \A.

         PCRE2_NOTEOL

       This option specifies that the end of the subject string is not the end
       of a line, so the dollar metacharacter should not match it nor  (except
       in  multiline mode) a newline immediately before it. Setting this with-
       out having set PCRE2_MULTILINE at compile time causes dollar  never  to
       match. This option affects only the behaviour of the dollar metacharac-
       ter. It does not affect \Z or \z.

         PCRE2_NOTEMPTY

       An empty string is not considered to be a valid match if this option is
       set.  If  there are alternatives in the pattern, they are tried. If all
       the alternatives match the empty string, the entire  match  fails.  For
       example, if the pattern

         a?b?

       is  applied  to  a  string not beginning with "a" or "b", it matches an
       empty string at the start of the subject. With PCRE2_NOTEMPTY set, this
       match  is  not valid, so pcre2_match() searches further into the string
       for occurrences of "a" or "b".

         PCRE2_NOTEMPTY_ATSTART

       This is like PCRE2_NOTEMPTY, except that it locks out an  empty  string
       match only at the first matching position, that is, at the start of the
       subject plus the starting offset. An empty string match  later  in  the
       subject  is  permitted.   If  the pattern is anchored, such a match can
       occur only if the pattern contains \K.

         PCRE2_NO_JIT

       By  default,  if  a  pattern  has  been   successfully   processed   by
       pcre2_jit_compile(),  JIT  is  automatically used when pcre2_match() is
       called with options that JIT supports.  Setting  PCRE2_NO_JIT  disables
       the use of JIT; it forces matching to be done by the interpreter.

         PCRE2_NO_UTF_CHECK

       When PCRE2_UTF is set at compile time, the validity of the subject as a
       UTF string is checked by default  when  pcre2_match()  is  subsequently
       called.   If  a non-zero starting offset is given, the check is applied
       only to that part of the subject that could be inspected during  match-
       ing,  and there is a check that the starting offset points to the first
       code unit of a character or to the end of the subject. If there are  no
       lookbehind  assertions in the pattern, the check starts at the starting
       offset. Otherwise, it starts at the length of  the  longest  lookbehind
       before the starting offset, or at the start of the subject if there are
       not that many characters before the  starting  offset.  Note  that  the
       sequences \b and \B are one-character lookbehinds.

       The check is carried out before any other processing takes place, and a
       negative error code is returned if the check fails. There  are  several
       UTF  error  codes  for each code unit width, corresponding to different
       problems with the code unit sequence. There are discussions  about  the
       validity  of  UTF-8  strings, UTF-16 strings, and UTF-32 strings in the
       pcre2unicode page.

       If you know that your subject is valid, and  you  want  to  skip  these
       checks  for  performance  reasons,  you  can set the PCRE2_NO_UTF_CHECK
       option when calling pcre2_match(). You might want to do  this  for  the
       second and subsequent calls to pcre2_match() if you are making repeated
       calls to find all the matches in a single subject string.

       NOTE: When PCRE2_NO_UTF_CHECK is set, the effect of passing an  invalid
       string  as a subject, or an invalid value of startoffset, is undefined.
       Your program may crash or loop indefinitely.

         PCRE2_PARTIAL_HARD
         PCRE2_PARTIAL_SOFT

       These options turn on the partial matching  feature.  A  partial  match
       occurs  if  the  end of the subject string is reached successfully, but
       there are not enough subject characters to complete the match. If  this
       happens  when  PCRE2_PARTIAL_SOFT  (but not PCRE2_PARTIAL_HARD) is set,
       matching continues by testing any remaining alternatives.  Only  if  no
       complete  match can be found is PCRE2_ERROR_PARTIAL returned instead of
       PCRE2_ERROR_NOMATCH. In other words, PCRE2_PARTIAL_SOFT specifies  that
       the  caller  is prepared to handle a partial match, but only if no com-
       plete match can be found.

       If PCRE2_PARTIAL_HARD is set, it overrides PCRE2_PARTIAL_SOFT. In  this
       case,  if  a  partial match is found, pcre2_match() immediately returns
       PCRE2_ERROR_PARTIAL, without considering  any  other  alternatives.  In
       other words, when PCRE2_PARTIAL_HARD is set, a partial match is consid-
       ered to be more important that an alternative complete match.

       There is a more detailed discussion of partial and multi-segment match-
       ing, with examples, in the pcre2partial documentation.


NEWLINE HANDLING WHEN MATCHING

       When  PCRE2 is built, a default newline convention is set; this is usu-
       ally the standard convention for the operating system. The default  can
       be  overridden  in a compile context by calling pcre2_set_newline(). It
       can also be overridden by starting a pattern string with, for  example,
       (*CRLF),  as  described  in  the  section on newline conventions in the
       pcre2pattern page. During matching, the newline choice affects the  be-
       haviour  of the dot, circumflex, and dollar metacharacters. It may also
       alter the way the match starting position is  advanced  after  a  match
       failure for an unanchored pattern.

       When PCRE2_NEWLINE_CRLF, PCRE2_NEWLINE_ANYCRLF, or PCRE2_NEWLINE_ANY is
       set as the newline convention, and a match attempt  for  an  unanchored
       pattern fails when the current starting position is at a CRLF sequence,
       and the pattern contains no explicit matches for CR or  LF  characters,
       the  match  position  is  advanced by two characters instead of one, in
       other words, to after the CRLF.

       The above rule is a compromise that makes the most common cases work as
       expected.  For  example,  if  the  pattern is .+A (and the PCRE2_DOTALL
       option is not set), it does not match the string "\r\nA" because, after
       failing  at the start, it skips both the CR and the LF before retrying.
       However, the pattern [\r\n]A does match that string,  because  it  con-
       tains an explicit CR or LF reference, and so advances only by one char-
       acter after the first failure.

       An explicit match for CR of LF is either a literal appearance of one of
       those  characters  in  the  pattern,  or  one  of  the  \r or \n escape
       sequences. Implicit matches such as [^X] do not  count,  nor  does  \s,
       even though it includes CR and LF in the characters that it matches.

       Notwithstanding  the above, anomalous effects may still occur when CRLF
       is a valid newline sequence and explicit \r or \n escapes appear in the
       pattern.


HOW PCRE2_MATCH() RETURNS A STRING AND CAPTURED SUBSTRINGS

       uint32_t pcre2_get_ovector_count(pcre2_match_data *match_data);

       PCRE2_SIZE *pcre2_get_ovector_pointer(pcre2_match_data *match_data);

       In  general, a pattern matches a certain portion of the subject, and in
       addition, further substrings from the subject  may  be  picked  out  by
       parenthesized  parts  of  the  pattern.  Following the usage in Jeffrey
       Friedl's book, this is called "capturing"  in  what  follows,  and  the
       phrase  "capturing subpattern" or "capturing group" is used for a frag-
       ment of a pattern that picks out a substring.  PCRE2  supports  several
       other kinds of parenthesized subpattern that do not cause substrings to
       be captured. The pcre2_pattern_info() function can be used to find  out
       how many capturing subpatterns there are in a compiled pattern.

       You  can  use  auxiliary functions for accessing captured substrings by
       number or by name, as described in sections below.

       Alternatively, you can make direct use of the vector of PCRE2_SIZE val-
       ues,  called  the  ovector,  which  contains  the  offsets  of captured
       strings.  It  is  part  of  the  match  data   block.    The   function
       pcre2_get_ovector_pointer()  returns  the  address  of the ovector, and
       pcre2_get_ovector_count() returns the number of pairs of values it con-
       tains.

       Within the ovector, the first in each pair of values is set to the off-
       set of the first code unit of a substring, and the second is set to the
       offset  of the first code unit after the end of a substring. These val-
       ues are always code unit offsets, not character offsets. That is,  they
       are  byte  offsets  in  the 8-bit library, 16-bit offsets in the 16-bit
       library, and 32-bit offsets in the 32-bit library.

       After a partial match  (error  return  PCRE2_ERROR_PARTIAL),  only  the
       first  pair  of  offsets  (that is, ovector[0] and ovector[1]) are set.
       They identify the part of the subject that was partially  matched.  See
       the pcre2partial documentation for details of partial matching.

       After a successful match, the first pair of offsets identifies the por-
       tion of the subject string that was matched by the entire pattern.  The
       next  pair  is  used for the first capturing subpattern, and so on. The
       value returned by pcre2_match() is one more than the  highest  numbered
       pair  that  has been set. For example, if two substrings have been cap-
       tured, the returned value is 3. If there are no capturing  subpatterns,
       the return value from a successful match is 1, indicating that just the
       first pair of offsets has been set.

       If a pattern uses the \K escape sequence within a  positive  assertion,
       the reported start of a successful match can be greater than the end of
       the match.  For example, if the pattern  (?=ab\K)  is  matched  against
       "ab", the start and end offset values for the match are 2 and 0.

       If  a  capturing subpattern group is matched repeatedly within a single
       match operation, it is the last portion of the subject that it  matched
       that is returned.

       If the ovector is too small to hold all the captured substring offsets,
       as much as possible is filled in, and the function returns a  value  of
       zero.  If captured substrings are not of interest, pcre2_match() may be
       called with a match data block whose ovector is of minimum length (that
       is, one pair). However, if the pattern contains back references and the
       ovector is not big enough to remember the related substrings, PCRE2 has
       to  get  additional  memory for use during matching. Thus it is usually
       advisable to set up a match data block containing an ovector of reason-
       able size.

       It  is  possible for capturing subpattern number n+1 to match some part
       of the subject when subpattern n has not been used at all. For example,
       if  the  string  "abc"  is  matched against the pattern (a|(z))(bc) the
       return from the function is 4, and subpatterns 1 and 3 are matched, but
       2  is  not.  When  this happens, both values in the offset pairs corre-
       sponding to unused subpatterns are set to PCRE2_UNSET.

       Offset values that correspond to unused subpatterns at the end  of  the
       expression  are  also  set  to  PCRE2_UNSET. For example, if the string
       "abc" is matched against the pattern (abc)(x(yz)?)? subpatterns 2 and 3
       are  not matched.  The return from the function is 2, because the high-
       est used capturing subpattern number is 1. The offsets for for the sec-
       ond  and  third  capturing  subpatterns  (assuming  the vector is large
       enough, of course) are set to PCRE2_UNSET.

       Elements in the ovector that do not correspond to capturing parentheses
       in the pattern are never changed. That is, if a pattern contains n cap-
       turing parentheses, no more than ovector[0] to ovector[2n+1] are set by
       pcre2_match().  The  other  elements retain whatever values they previ-
       ously had.


OTHER INFORMATION ABOUT A MATCH

       PCRE2_SPTR pcre2_get_mark(pcre2_match_data *match_data);

       PCRE2_SIZE pcre2_get_startchar(pcre2_match_data *match_data);

       As well as the offsets in the ovector, other information about a  match
       is  retained  in the match data block and can be retrieved by the above
       functions in appropriate circumstances. If they  are  called  at  other
       times, the result is undefined.

       After  a  successful match, a partial match (PCRE2_ERROR_PARTIAL), or a
       failure to match (PCRE2_ERROR_NOMATCH), a (*MARK) name  may  be  avail-
       able,  and  pcre2_get_mark() can be called. It returns a pointer to the
       zero-terminated name, which is within the compiled  pattern.  Otherwise
       NULL  is returned. The length of the (*MARK) name (excluding the termi-
       nating zero) is stored in the code unit that  preceeds  the  name.  You
       should  use  this  instead  of  relying  on the terminating zero if the
       (*MARK) name might contain a binary zero.

       After a successful match, the (*MARK) name that is returned is the last
       one  encountered  on the matching path through the pattern. After a "no
       match" or a  partial  match,  the  last  encountered  (*MARK)  name  is
       returned. For example, consider this pattern:

         ^(*MARK:A)((*MARK:B)a|b)c

       When  it  matches "bc", the returned mark is A. The B mark is "seen" in
       the first branch of the group, but it is not on the matching  path.  On
       the  other  hand,  when  this pattern fails to match "bx", the returned
       mark is B.

       After a successful match, a partial match, or one of  the  invalid  UTF
       errors  (for example, PCRE2_ERROR_UTF8_ERR5), pcre2_get_startchar() can
       be called. After a successful or partial match it returns the code unit
       offset  of  the character at which the match started. For a non-partial
       match, this can be different to the value of ovector[0] if the  pattern
       contains  the  \K escape sequence. After a partial match, however, this
       value is always the same as ovector[0] because \K does not  affect  the
       result of a partial match.

       After  a UTF check failure, pcre2_get_startchar() can be used to obtain
       the code unit offset of the invalid UTF character. Details are given in
       the pcre2unicode page.


ERROR RETURNS FROM pcre2_match()

       If  pcre2_match() fails, it returns a negative number. This can be con-
       verted to a text string by calling the pcre2_get_error_message()  func-
       tion  (see  "Obtaining a textual error message" below).  Negative error
       codes are also returned by other functions,  and  are  documented  with
       them.  The codes are given names in the header file. If UTF checking is
       in force and an invalid UTF subject string is detected, one of a number
       of  UTF-specific negative error codes is returned. Details are given in
       the pcre2unicode page. The following are the other errors that  may  be
       returned by pcre2_match():

         PCRE2_ERROR_NOMATCH

       The subject string did not match the pattern.

         PCRE2_ERROR_PARTIAL

       The  subject  string did not match, but it did match partially. See the
       pcre2partial documentation for details of partial matching.

         PCRE2_ERROR_BADMAGIC

       PCRE2 stores a 4-byte "magic number" at the start of the compiled code,
       to  catch  the case when it is passed a junk pointer. This is the error
       that is returned when the magic number is not present.

         PCRE2_ERROR_BADMODE

       This error is given when a pattern  that  was  compiled  by  the  8-bit
       library  is  passed  to  a  16-bit  or 32-bit library function, or vice
       versa.

         PCRE2_ERROR_BADOFFSET

       The value of startoffset was greater than the length of the subject.

         PCRE2_ERROR_BADOPTION

       An unrecognized bit was set in the options argument.

         PCRE2_ERROR_BADUTFOFFSET

       The UTF code unit sequence that was passed as a subject was checked and
       found  to be valid (the PCRE2_NO_UTF_CHECK option was not set), but the
       value of startoffset did not point to the beginning of a UTF  character
       or the end of the subject.

         PCRE2_ERROR_CALLOUT

       This  error  is never generated by pcre2_match() itself. It is provided
       for use by callout  functions  that  want  to  cause  pcre2_match()  or
       pcre2_callout_enumerate()  to  return a distinctive error code. See the
       pcre2callout documentation for details.

         PCRE2_ERROR_INTERNAL

       An unexpected internal error has occurred. This error could  be  caused
       by a bug in PCRE2 or by overwriting of the compiled pattern.

         PCRE2_ERROR_JIT_BADOPTION

       This  error  is  returned  when a pattern that was successfully studied
       using JIT is being matched, but the matching mode (partial or  complete
       match)  does  not  correspond to any JIT compilation mode. When the JIT
       fast path function is used, this error may be also  given  for  invalid
       options. See the pcre2jit documentation for more details.

         PCRE2_ERROR_JIT_STACKLIMIT

       This  error  is  returned  when a pattern that was successfully studied
       using JIT is being matched, but the memory available for  the  just-in-
       time  processing stack is not large enough. See the pcre2jit documenta-
       tion for more details.

         PCRE2_ERROR_MATCHLIMIT

       The backtracking limit was reached.

         PCRE2_ERROR_NOMEMORY

       If a pattern contains back references,  but  the  ovector  is  not  big
       enough  to  remember  the  referenced substrings, PCRE2 gets a block of
       memory at the start of matching to use for this purpose. There are some
       other  special cases where extra memory is needed during matching. This
       error is given when memory cannot be obtained.

         PCRE2_ERROR_NULL

       Either the code, subject, or match_data argument was passed as NULL.

         PCRE2_ERROR_RECURSELOOP

       This error is returned when  pcre2_match()  detects  a  recursion  loop
       within  the  pattern. Specifically, it means that either the whole pat-
       tern or a subpattern has been called recursively for the second time at
       the  same  position  in  the  subject string. Some simple patterns that
       might do this are detected and faulted at compile time, but  more  com-
       plicated  cases,  in particular mutual recursions between two different
       subpatterns, cannot be detected until matching is attempted.

         PCRE2_ERROR_RECURSIONLIMIT

       The internal recursion limit was reached.


OBTAINING A TEXTUAL ERROR MESSAGE

       int pcre2_get_error_message(int errorcode, PCRE2_UCHAR *buffer,
         PCRE2_SIZE bufflen);

       A text message for an error code  from  any  PCRE2  function  (compile,
       match,  or  auxiliary)  can be obtained by calling pcre2_get_error_mes-
       sage(). The code is passed as the first argument,  with  the  remaining
       two  arguments specifying a code unit buffer and its length, into which
       the text message is placed. Note that the message is returned  in  code
       units of the appropriate width for the library that is being used.

       The  returned message is terminated with a trailing zero, and the func-
       tion returns the number of code  units  used,  excluding  the  trailing
       zero.  If  the  error  number  is  unknown,  the  negative  error  code
       PCRE2_ERROR_BADDATA is returned. If the buffer is too small,  the  mes-
       sage  is  truncated  (but still with a trailing zero), and the negative
       error code PCRE2_ERROR_NOMEMORY is returned.  None of the messages  are
       very long; a buffer size of 120 code units is ample.


EXTRACTING CAPTURED SUBSTRINGS BY NUMBER

       int pcre2_substring_length_bynumber(pcre2_match_data *match_data,
         uint32_t number, PCRE2_SIZE *length);

       int pcre2_substring_copy_bynumber(pcre2_match_data *match_data,
         uint32_t number, PCRE2_UCHAR *buffer,
         PCRE2_SIZE *bufflen);

       int pcre2_substring_get_bynumber(pcre2_match_data *match_data,
         uint32_t number, PCRE2_UCHAR **bufferptr,
         PCRE2_SIZE *bufflen);

       void pcre2_substring_free(PCRE2_UCHAR *buffer);

       Captured  substrings  can  be accessed directly by using the ovector as
       described above.  For convenience, auxiliary functions are provided for
       extracting   captured  substrings  as  new,  separate,  zero-terminated
       strings. A substring that contains a binary zero is correctly extracted
       and  has  a  further  zero  added on the end, but the result is not, of
       course, a C string.

       The functions in this section identify substrings by number. The number
       zero refers to the entire matched substring, with higher numbers refer-
       ring to substrings captured by parenthesized groups.  After  a  partial
       match,  only  substring  zero  is  available. An attempt to extract any
       other substring gives the error PCRE2_ERROR_PARTIAL. The  next  section
       describes similar functions for extracting captured substrings by name.

       If  a  pattern uses the \K escape sequence within a positive assertion,
       the reported start of a successful match can be greater than the end of
       the  match.   For  example,  if the pattern (?=ab\K) is matched against
       "ab", the start and end offset values for the match are  2  and  0.  In
       this  situation,  calling  these functions with a zero substring number
       extracts a zero-length empty string.

       You can find the length in code units of a captured  substring  without
       extracting  it  by calling pcre2_substring_length_bynumber(). The first
       argument is a pointer to the match data block, the second is the  group
       number,  and the third is a pointer to a variable into which the length
       is placed. If you just want to know whether or not  the  substring  has
       been captured, you can pass the third argument as NULL.

       The  pcre2_substring_copy_bynumber()  function  copies  a captured sub-
       string into a supplied buffer,  whereas  pcre2_substring_get_bynumber()
       copies  it  into  new memory, obtained using the same memory allocation
       function that was used for the match data block. The  first  two  argu-
       ments  of  these  functions are a pointer to the match data block and a
       capturing group number.

       The final arguments of pcre2_substring_copy_bynumber() are a pointer to
       the buffer and a pointer to a variable that contains its length in code
       units.  This is updated to contain the actual number of code units used
       for the extracted substring, excluding the terminating zero.

       For pcre2_substring_get_bynumber() the third and fourth arguments point
       to variables that are updated with a pointer to the new memory and  the
       number  of  code units that comprise the substring, again excluding the
       terminating zero. When the substring is no longer  needed,  the  memory
       should be freed by calling pcre2_substring_free().

       The  return  value  from  all these functions is zero for success, or a
       negative error code. If the pattern match  failed,  the  match  failure
       code  is  returned.   If  a  substring number greater than zero is used
       after a partial match, PCRE2_ERROR_PARTIAL is returned. Other  possible
       error codes are:

         PCRE2_ERROR_NOMEMORY

       The  buffer  was  too small for pcre2_substring_copy_bynumber(), or the
       attempt to get memory failed for pcre2_substring_get_bynumber().

         PCRE2_ERROR_NOSUBSTRING

       There is no substring with that number in the  pattern,  that  is,  the
       number is greater than the number of capturing parentheses.

         PCRE2_ERROR_UNAVAILABLE

       The substring number, though not greater than the number of captures in
       the pattern, is greater than the number of slots in the ovector, so the
       substring could not be captured.

         PCRE2_ERROR_UNSET

       The  substring  did  not  participate in the match. For example, if the
       pattern is (abc)|(def) and the subject is "def", and the  ovector  con-
       tains at least two capturing slots, substring number 1 is unset.


EXTRACTING A LIST OF ALL CAPTURED SUBSTRINGS

       int pcre2_substring_list_get(pcre2_match_data *match_data,
         PCRE2_UCHAR ***listptr, PCRE2_SIZE **lengthsptr);

       void pcre2_substring_list_free(PCRE2_SPTR *list);

       The  pcre2_substring_list_get()  function  extracts  all available sub-
       strings and builds a list of pointers to  them.  It  also  (optionally)
       builds  a  second  list  that  contains  their lengths (in code units),
       excluding a terminating zero that is added to each of them. All this is
       done in a single block of memory that is obtained using the same memory
       allocation function that was used to get the match data block.

       This function must be called only after a successful match.  If  called
       after a partial match, the error code PCRE2_ERROR_PARTIAL is returned.

       The  address of the memory block is returned via listptr, which is also
       the start of the list of string pointers. The end of the list is marked
       by  a  NULL pointer. The address of the list of lengths is returned via
       lengthsptr. If your strings do not contain binary zeros and you do  not
       therefore need the lengths, you may supply NULL as the lengthsptr argu-
       ment to disable the creation of a list of lengths.  The  yield  of  the
       function  is zero if all went well, or PCRE2_ERROR_NOMEMORY if the mem-
       ory block could not be obtained. When the list is no longer needed,  it
       should be freed by calling pcre2_substring_list_free().

       If this function encounters a substring that is unset, which can happen
       when capturing subpattern number n+1 matches some part of the  subject,
       but  subpattern n has not been used at all, it returns an empty string.
       This can be distinguished  from  a  genuine  zero-length  substring  by
       inspecting  the  appropriate  offset  in  the  ovector,  which  contain
       PCRE2_UNSET  for   unset   substrings,   or   by   calling   pcre2_sub-
       string_length_bynumber().


EXTRACTING CAPTURED SUBSTRINGS BY NAME

       int pcre2_substring_number_from_name(const pcre2_code *code,
         PCRE2_SPTR name);

       int pcre2_substring_length_byname(pcre2_match_data *match_data,
         PCRE2_SPTR name, PCRE2_SIZE *length);

       int pcre2_substring_copy_byname(pcre2_match_data *match_data,
         PCRE2_SPTR name, PCRE2_UCHAR *buffer, PCRE2_SIZE *bufflen);

       int pcre2_substring_get_byname(pcre2_match_data *match_data,
         PCRE2_SPTR name, PCRE2_UCHAR **bufferptr, PCRE2_SIZE *bufflen);

       void pcre2_substring_free(PCRE2_UCHAR *buffer);

       To  extract a substring by name, you first have to find associated num-
       ber.  For example, for this pattern:

         (a+)b(?<xxx>\d+)...

       the number of the subpattern called "xxx" is 2. If the name is known to
       be  unique  (PCRE2_DUPNAMES  was not set), you can find the number from
       the name by calling pcre2_substring_number_from_name(). The first argu-
       ment  is the compiled pattern, and the second is the name. The yield of
       the function is the subpattern number, PCRE2_ERROR_NOSUBSTRING if there
       is  no  subpattern  of  that  name, or PCRE2_ERROR_NOUNIQUESUBSTRING if
       there is more than one subpattern of that name. Given the  number,  you
       can  extract  the  substring  directly,  or  use  one  of the functions
       described above.

       For convenience, there are also "byname" functions that  correspond  to
       the  "bynumber"  functions,  the  only difference being that the second
       argument is a name instead of a number. If PCRE2_DUPNAMES  is  set  and
       there are duplicate names, these functions scan all the groups with the
       given name, and return the first named string that is set.

       If there are no groups with the given name, PCRE2_ERROR_NOSUBSTRING  is
       returned.  If  all  groups  with the name have numbers that are greater
       than the number of slots in  the  ovector,  PCRE2_ERROR_UNAVAILABLE  is
       returned.  If  there  is at least one group with a slot in the ovector,
       but no group is found to be set, PCRE2_ERROR_UNSET is returned.

       Warning: If the pattern uses the (?| feature to set up multiple subpat-
       terns  with  the  same number, as described in the section on duplicate
       subpattern numbers in the pcre2pattern page, you cannot  use  names  to
       distinguish  the  different subpatterns, because names are not included
       in the compiled code. The matching process uses only numbers. For  this
       reason,  the  use of different names for subpatterns of the same number
       causes an error at compile time.


CREATING A NEW STRING WITH SUBSTITUTIONS

       int pcre2_substitute(const pcre2_code *code, PCRE2_SPTR subject,
         PCRE2_SIZE length, PCRE2_SIZE startoffset,
         uint32_t options, pcre2_match_data *match_data,
         pcre2_match_context *mcontext, PCRE2_SPTR replacement,
         PCRE2_SIZE rlength, PCRE2_UCHAR *outputbufferP,
         PCRE2_SIZE *outlengthptr);

       This function calls pcre2_match() and then makes a copy of the  subject
       string  in  outputbuffer,  replacing the part that was matched with the
       replacement string, whose length is supplied in rlength.  This  can  be
       given as PCRE2_ZERO_TERMINATED for a zero-terminated string. Matches in
       which a \K item in a lookahead in the pattern causes the match  to  end
       before it starts are not supported, and give rise to an error return.

       The  first  seven  arguments  of pcre2_substitute() are the same as for
       pcre2_match(), except that the partial matching options are not permit-
       ted,  and  match_data may be passed as NULL, in which case a match data
       block is obtained and freed within this function, using memory  manage-
       ment  functions from the match context, if provided, or else those that
       were used to allocate memory for the compiled code.

       The outlengthptr argument must point to a variable  that  contains  the
       length,  in  code  units, of the output buffer. If the function is suc-
       cessful, the value is updated to contain the length of the new  string,
       excluding the trailing zero that is automatically added.

       If  the  function  is  not  successful,  the value set via outlengthptr
       depends on the type of error. For  syntax  errors  in  the  replacement
       string,  the  value  is  the offset in the replacement string where the
       error was detected. For other  errors,  the  value  is  PCRE2_UNSET  by
       default.  This  includes the case of the output buffer being too small,
       unless PCRE2_SUBSTITUTE_OVERFLOW_LENGTH is set (see  below),  in  which
       case  the  value  is the minimum length needed, including space for the
       trailing zero. Note that in  order  to  compute  the  required  length,
       pcre2_substitute()  has  to  simulate  all  the  matching  and copying,
       instead of giving an error return as soon as the buffer overflows. Note
       also that the length is in code units, not bytes.

       In  the replacement string, which is interpreted as a UTF string in UTF
       mode, and is checked for UTF  validity  unless  the  PCRE2_NO_UTF_CHECK
       option is set, a dollar character is an escape character that can spec-
       ify the insertion of characters from capturing groups or (*MARK)  items
       in the pattern. The following forms are always recognized:

         $$                  insert a dollar character
         $<n> or ${<n>}      insert the contents of group <n>
         $*MARK or ${*MARK}  insert the name of the last (*MARK) encountered

       Either  a  group  number  or  a  group name can be given for <n>. Curly
       brackets are required only if the following character would  be  inter-
       preted as part of the number or name. The number may be zero to include
       the entire matched string.   For  example,  if  the  pattern  a(b)c  is
       matched  with "=abc=" and the replacement string "+$1$0$1+", the result
       is "=+babcb+=".

       The facility for inserting a (*MARK) name can be used to perform simple
       simultaneous substitutions, as this pcre2test example shows:

         /(*:pear)apple|(*:orange)lemon/g,replace=${*MARK}
             apple lemon
          2: pear orange

       As  well as the usual options for pcre2_match(), a number of additional
       options can be set in the options argument.

       PCRE2_SUBSTITUTE_GLOBAL causes the function to iterate over the subject
       string,  replacing  every  matching substring. If this is not set, only
       the first matching substring is replaced. If any matched substring  has
       zero  length, after the substitution has happened, an attempt to find a
       non-empty match at the same position is performed. If this is not  suc-
       cessful,  the current position is advanced by one character except when
       CRLF is a valid newline sequence and the next two  characters  are  CR,
       LF. In this case, the current position is advanced by two characters.

       PCRE2_SUBSTITUTE_OVERFLOW_LENGTH  changes  what happens when the output
       buffer is too small. The default action is to return PCRE2_ERROR_NOMEM-
       ORY  immediately.  If  this  option is set, however, pcre2_substitute()
       continues to go through the motions of matching and substituting (with-
       out,  of course, writing anything) in order to compute the size of buf-
       fer that is needed. This value is  passed  back  via  the  outlengthptr
       variable,    with    the   result   of   the   function   still   being
       PCRE2_ERROR_NOMEMORY.

       Passing a buffer size of zero is a permitted way  of  finding  out  how
       much  memory  is needed for given substitution. However, this does mean
       that the entire operation is carried out twice. Depending on the appli-
       cation,  it  may  be more efficient to allocate a large buffer and free
       the  excess  afterwards,  instead   of   using   PCRE2_SUBSTITUTE_OVER-
       FLOW_LENGTH.

       PCRE2_SUBSTITUTE_UNKNOWN_UNSET  causes  references  to capturing groups
       that do not appear in the pattern to be treated as unset  groups.  This
       option  should  be  used  with  care, because it means that a typo in a
       group name or  number  no  longer  causes  the  PCRE2_ERROR_NOSUBSTRING
       error.

       PCRE2_SUBSTITUTE_UNSET_EMPTY  causes  unset capturing groups (including
       unknown  groups  when  PCRE2_SUBSTITUTE_UNKNOWN_UNSET  is  set)  to  be
       treated  as  empty  strings  when  inserted as described above. If this
       option is not set, an attempt to  insert  an  unset  group  causes  the
       PCRE2_ERROR_UNSET  error.  This  option does not influence the extended
       substitution syntax described below.

       PCRE2_SUBSTITUTE_EXTENDED causes extra processing to be applied to  the
       replacement  string.  Without this option, only the dollar character is
       special, and only the group insertion forms  listed  above  are  valid.
       When PCRE2_SUBSTITUTE_EXTENDED is set, two things change:

       Firstly,  backslash in a replacement string is interpreted as an escape
       character. The usual forms such as \n or \x{ddd} can be used to specify
       particular  character codes, and backslash followed by any non-alphanu-
       meric character quotes that character. Extended quoting  can  be  coded
       using \Q...\E, exactly as in pattern strings.

       There  are  also four escape sequences for forcing the case of inserted
       letters.  The insertion mechanism has three states:  no  case  forcing,
       force upper case, and force lower case. The escape sequences change the
       current state: \U and \L change to upper or lower case forcing, respec-
       tively,  and  \E (when not terminating a \Q quoted sequence) reverts to
       no case forcing. The sequences \u and \l force the next  character  (if
       it  is  a  letter)  to  upper or lower case, respectively, and then the
       state automatically reverts to no case forcing. Case forcing applies to
       all inserted  characters, including those from captured groups and let-
       ters within \Q...\E quoted sequences.

       Note that case forcing sequences such as \U...\E do not nest. For exam-
       ple,  the  result of processing "\Uaa\LBB\Ecc\E" is "AAbbcc"; the final
       \E has no effect.

       The second effect of setting PCRE2_SUBSTITUTE_EXTENDED is to  add  more
       flexibility  to  group substitution. The syntax is similar to that used
       by Bash:

         ${<n>:-<string>}
         ${<n>:+<string1>:<string2>}

       As before, <n> may be a group number or a name. The first  form  speci-
       fies  a  default  value. If group <n> is set, its value is inserted; if
       not, <string> is expanded and the  result  inserted.  The  second  form
       specifies  strings that are expanded and inserted when group <n> is set
       or unset, respectively. The first form is just a  convenient  shorthand
       for

         ${<n>:+${<n>}:<string>}

       Backslash  can  be  used to escape colons and closing curly brackets in
       the replacement strings. A change of the case forcing  state  within  a
       replacement  string  remains  in  force  afterwards,  as  shown in this
       pcre2test example:

         /(some)?(body)/substitute_extended,replace=${1:+\U:\L}HeLLo
             body
          1: hello
             somebody
          1: HELLO

       The PCRE2_SUBSTITUTE_UNSET_EMPTY option does not affect these  extended
       substitutions.   However,   PCRE2_SUBSTITUTE_UNKNOWN_UNSET  does  cause
       unknown groups in the extended syntax forms to be treated as unset.

       If successful, pcre2_substitute() returns the  number  of  replacements
       that were made. This may be zero if no matches were found, and is never
       greater than 1 unless PCRE2_SUBSTITUTE_GLOBAL is set.

       In the event of an error, a negative error code is returned. Except for
       PCRE2_ERROR_NOMATCH    (which   is   never   returned),   errors   from
       pcre2_match() are passed straight back.

       PCRE2_ERROR_NOSUBSTRING is returned for a non-existent substring inser-
       tion, unless PCRE2_SUBSTITUTE_UNKNOWN_UNSET is set.

       PCRE2_ERROR_UNSET is returned for an unset substring insertion (includ-
       ing an unknown substring when  PCRE2_SUBSTITUTE_UNKNOWN_UNSET  is  set)
       when  the  simple  (non-extended)  syntax  is  used  and  PCRE2_SUBSTI-
       TUTE_UNSET_EMPTY is not set.

       PCRE2_ERROR_NOMEMORY is returned  if  the  output  buffer  is  not  big
       enough. If the PCRE2_SUBSTITUTE_OVERFLOW_LENGTH option is set, the size
       of buffer that is needed is returned via outlengthptr. Note  that  this
       does not happen by default.

       PCRE2_ERROR_BADREPLACEMENT  is  used for miscellaneous syntax errors in
       the   replacement   string,   with   more   particular   errors   being
       PCRE2_ERROR_BADREPESCAPE  (invalid  escape  sequence), PCRE2_ERROR_REP-
       MISSING_BRACE (closing curly bracket not found),  PCRE2_BADSUBSTITUTION
       (syntax  error in extended group substitution), and PCRE2_BADSUBPATTERN
       (the pattern match ended before it started, which can happen if  \K  is
       used in an assertion).

       As for all PCRE2 errors, a text message that describes the error can be
       obtained  by  calling  the  pcre2_get_error_message()   function   (see
       "Obtaining a textual error message" above).


DUPLICATE SUBPATTERN NAMES

       int pcre2_substring_nametable_scan(const pcre2_code *code,
         PCRE2_SPTR name, PCRE2_SPTR *first, PCRE2_SPTR *last);

       When  a  pattern  is compiled with the PCRE2_DUPNAMES option, names for
       subpatterns are not required to be unique. Duplicate names  are  always
       allowed  for subpatterns with the same number, created by using the (?|
       feature. Indeed, if such subpatterns are named, they  are  required  to
       use the same names.

       Normally, patterns with duplicate names are such that in any one match,
       only one of the named subpatterns participates. An example is shown  in
       the pcre2pattern documentation.

       When   duplicates   are   present,   pcre2_substring_copy_byname()  and
       pcre2_substring_get_byname() return the first  substring  corresponding
       to   the   given   name   that   is  set.  Only  if  none  are  set  is
       PCRE2_ERROR_UNSET is returned.  The  pcre2_substring_number_from_name()
       function returns the error PCRE2_ERROR_NOUNIQUESUBSTRING when there are
       duplicate names.

       If you want to get full details of all captured substrings for a  given
       name,  you  must use the pcre2_substring_nametable_scan() function. The
       first argument is the compiled pattern, and the second is the name.  If
       the  third  and fourth arguments are NULL, the function returns a group
       number for a unique name, or PCRE2_ERROR_NOUNIQUESUBSTRING otherwise.

       When the third and fourth arguments are not NULL, they must be pointers
       to  variables  that are updated by the function. After it has run, they
       point to the first and last entries in the name-to-number table for the
       given  name,  and the function returns the length of each entry in code
       units. In both cases, PCRE2_ERROR_NOSUBSTRING is returned if there  are
       no entries for the given name.

       The format of the name table is described above in the section entitled
       Information about a pattern. Given all the  relevant  entries  for  the
       name,  you  can  extract  each of their numbers, and hence the captured
       data.


FINDING ALL POSSIBLE MATCHES AT ONE POSITION

       The traditional matching function uses a  similar  algorithm  to  Perl,
       which  stops when it finds the first match at a given point in the sub-
       ject. If you want to find all possible matches, or the longest possible
       match  at  a  given  position,  consider using the alternative matching
       function (see below) instead. If you cannot use the  alternative  func-
       tion, you can kludge it up by making use of the callout facility, which
       is described in the pcre2callout documentation.

       What you have to do is to insert a callout right at the end of the pat-
       tern.   When your callout function is called, extract and save the cur-
       rent matched substring. Then return 1, which  forces  pcre2_match()  to
       backtrack  and  try other alternatives. Ultimately, when it runs out of
       matches, pcre2_match() will yield PCRE2_ERROR_NOMATCH.


MATCHING A PATTERN: THE ALTERNATIVE FUNCTION

       int pcre2_dfa_match(const pcre2_code *code, PCRE2_SPTR subject,
         PCRE2_SIZE length, PCRE2_SIZE startoffset,
         uint32_t options, pcre2_match_data *match_data,
         pcre2_match_context *mcontext,
         int *workspace, PCRE2_SIZE wscount);

       The function pcre2_dfa_match() is called  to  match  a  subject  string
       against  a  compiled pattern, using a matching algorithm that scans the
       subject string just once, and does not backtrack.  This  has  different
       characteristics  to  the  normal  algorithm, and is not compatible with
       Perl. Some of the features of PCRE2 patterns are not supported.  Never-
       theless,  there are times when this kind of matching can be useful. For
       a discussion of the two matching algorithms, and  a  list  of  features
       that pcre2_dfa_match() does not support, see the pcre2matching documen-
       tation.

       The arguments for the pcre2_dfa_match() function are the  same  as  for
       pcre2_match(), plus two extras. The ovector within the match data block
       is used in a different way, and this is described below. The other com-
       mon  arguments  are used in the same way as for pcre2_match(), so their
       description is not repeated here.

       The two additional arguments provide workspace for  the  function.  The
       workspace  vector  should  contain at least 20 elements. It is used for
       keeping  track  of  multiple  paths  through  the  pattern  tree.  More
       workspace  is needed for patterns and subjects where there are a lot of
       potential matches.

       Here is an example of a simple call to pcre2_dfa_match():

         int wspace[20];
         pcre2_match_data *md = pcre2_match_data_create(4, NULL);
         int rc = pcre2_dfa_match(
           re,             /* result of pcre2_compile() */
           "some string",  /* the subject string */
           11,             /* the length of the subject string */
           0,              /* start at offset 0 in the subject */
           0,              /* default options */
           match_data,     /* the match data block */
           NULL,           /* a match context; NULL means use defaults */
           wspace,         /* working space vector */
           20);            /* number of elements (NOT size in bytes) */

   Option bits for pcre_dfa_match()

       The unused bits of the options argument for pcre2_dfa_match()  must  be
       zero.  The  only bits that may be set are PCRE2_ANCHORED, PCRE2_NOTBOL,
       PCRE2_NOTEOL,          PCRE2_NOTEMPTY,          PCRE2_NOTEMPTY_ATSTART,
       PCRE2_NO_UTF_CHECK,       PCRE2_PARTIAL_HARD,       PCRE2_PARTIAL_SOFT,
       PCRE2_DFA_SHORTEST, and PCRE2_DFA_RESTART. All but  the  last  four  of
       these  are  exactly the same as for pcre2_match(), so their description
       is not repeated here.

         PCRE2_PARTIAL_HARD
         PCRE2_PARTIAL_SOFT

       These have the same general effect as they do  for  pcre2_match(),  but
       the  details are slightly different. When PCRE2_PARTIAL_HARD is set for
       pcre2_dfa_match(), it returns PCRE2_ERROR_PARTIAL if  the  end  of  the
       subject is reached and there is still at least one matching possibility
       that requires additional characters. This happens even if some complete
       matches  have  already  been found. When PCRE2_PARTIAL_SOFT is set, the
       return code PCRE2_ERROR_NOMATCH is converted  into  PCRE2_ERROR_PARTIAL
       if  the  end  of  the  subject  is reached, there have been no complete
       matches, but there is still at least one matching possibility. The por-
       tion  of  the  string that was inspected when the longest partial match
       was found is set as the first matching string in both cases. There is a
       more  detailed  discussion  of partial and multi-segment matching, with
       examples, in the pcre2partial documentation.

         PCRE2_DFA_SHORTEST

       Setting the PCRE2_DFA_SHORTEST option causes the matching algorithm  to
       stop as soon as it has found one match. Because of the way the alterna-
       tive algorithm works, this is necessarily the shortest  possible  match
       at the first possible matching point in the subject string.

         PCRE2_DFA_RESTART

       When  pcre2_dfa_match() returns a partial match, it is possible to call
       it again, with additional subject characters, and have it continue with
       the same match. The PCRE2_DFA_RESTART option requests this action; when
       it is set, the workspace and wscount options must  reference  the  same
       vector  as  before  because data about the match so far is left in them
       after a partial match. There is more discussion of this facility in the
       pcre2partial documentation.

   Successful returns from pcre2_dfa_match()

       When pcre2_dfa_match() succeeds, it may have matched more than one sub-
       string in the subject. Note, however, that all the matches from one run
       of  the  function  start  at the same point in the subject. The shorter
       matches are all initial substrings of the longer matches. For  example,
       if the pattern

         <.*>

       is matched against the string

         This is <something> <something else> <something further> no more

       the three matched strings are

         <something> <something else> <something further>
         <something> <something else>
         <something>

       On  success,  the  yield of the function is a number greater than zero,
       which is the number of matched substrings.  The  offsets  of  the  sub-
       strings  are returned in the ovector, and can be extracted by number in
       the same way as for pcre2_match(), but the numbers bear no relation  to
       any  capturing groups that may exist in the pattern, because DFA match-
       ing does not support group capture.

       Calls to the convenience functions  that  extract  substrings  by  name
       return  the  error PCRE2_ERROR_DFA_UFUNC (unsupported function) if used
       after a DFA match. The convenience functions that extract substrings by
       number  never  return PCRE2_ERROR_NOSUBSTRING, and the meanings of some
       other errors are slightly different:

         PCRE2_ERROR_UNAVAILABLE

       The ovector is not big enough to include a slot for the given substring
       number.

         PCRE2_ERROR_UNSET

       There  is  a  slot  in  the  ovector for this substring, but there were
       insufficient matches to fill it.

       The matched strings are stored in  the  ovector  in  reverse  order  of
       length;  that  is,  the longest matching string is first. If there were
       too many matches to fit into the ovector, the yield of the function  is
       zero, and the vector is filled with the longest matches.

       NOTE:  PCRE2's  "auto-possessification" optimization usually applies to
       character repeats at the end of a pattern (as well as internally).  For
       example,  the pattern "a\d+" is compiled as if it were "a\d++". For DFA
       matching, this means that only one possible  match  is  found.  If  you
       really  do  want multiple matches in such cases, either use an ungreedy
       repeat auch as "a\d+?" or set  the  PCRE2_NO_AUTO_POSSESS  option  when
       compiling.

   Error returns from pcre2_dfa_match()

       The pcre2_dfa_match() function returns a negative number when it fails.
       Many of the errors are the same  as  for  pcre2_match(),  as  described
       above.  There are in addition the following errors that are specific to
       pcre2_dfa_match():

         PCRE2_ERROR_DFA_UITEM

       This return is given if pcre2_dfa_match() encounters  an  item  in  the
       pattern  that it does not support, for instance, the use of \C in a UTF
       mode or a back reference.

         PCRE2_ERROR_DFA_UCOND

       This return is given if pcre2_dfa_match() encounters a  condition  item
       that  uses  a back reference for the condition, or a test for recursion
       in a specific group. These are not supported.

         PCRE2_ERROR_DFA_WSSIZE

       This return is given if pcre2_dfa_match() runs  out  of  space  in  the
       workspace vector.

         PCRE2_ERROR_DFA_RECURSE

       When  a  recursive subpattern is processed, the matching function calls
       itself recursively, using private memory for the ovector and workspace.
       This  error  is given if the internal ovector is not large enough. This
       should be extremely rare, as a vector of size 1000 is used.

         PCRE2_ERROR_DFA_BADRESTART

       When pcre2_dfa_match() is called  with  the  PCRE2_DFA_RESTART  option,
       some  plausibility  checks  are  made on the contents of the workspace,
       which should contain data about the previous partial match. If  any  of
       these checks fail, this error is given.


SEE ALSO

       pcre2build(3),    pcre2callout(3),    pcre2demo(3),   pcre2matching(3),
       pcre2partial(3),    pcre2posix(3),    pcre2sample(3),    pcre2stack(3),
       pcre2unicode(3).


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge, England.


REVISION

       Last updated: 17 June 2016
       Copyright (c) 1997-2016 University of Cambridge.
------------------------------------------------------------------------------


PCRE2BUILD(3)              Library Functions Manual              PCRE2BUILD(3)



NAME
       PCRE2 - Perl-compatible regular expressions (revised API)

BUILDING PCRE2

       PCRE2  is distributed with a configure script that can be used to build
       the library in Unix-like environments using the applications  known  as
       Autotools. Also in the distribution are files to support building using
       CMake instead of configure.  The  text  file  README  contains  general
       information  about  building  with Autotools (some of which is repeated
       below), and also has some comments about building on various  operating
       systems.  There  is a lot more information about building PCRE2 without
       using Autotools (including information about using CMake  and  building
       "by  hand")  in  the  text file called NON-AUTOTOOLS-BUILD.  You should
       consult this file as well as the README file if you are building  in  a
       non-Unix-like environment.


PCRE2 BUILD-TIME OPTIONS

       The rest of this document describes the optional features of PCRE2 that
       can be selected when the library is compiled. It  assumes  use  of  the
       configure  script,  where  the  optional features are selected or dese-
       lected by providing options to configure before running the  make  com-
       mand.  However,  the same options can be selected in both Unix-like and
       non-Unix-like environments if you are using CMake instead of  configure
       to build PCRE2.

       If  you  are not using Autotools or CMake, option selection can be done
       by editing the config.h file, or by passing parameter settings  to  the
       compiler, as described in NON-AUTOTOOLS-BUILD.

       The complete list of options for configure (which includes the standard
       ones such as the  selection  of  the  installation  directory)  can  be
       obtained by running

         ./configure --help

       The  following  sections  include  descriptions  of options whose names
       begin with --enable or --disable. These settings specify changes to the
       defaults  for  the configure command. Because of the way that configure
       works, --enable and --disable always come in pairs, so  the  complemen-
       tary  option always exists as well, but as it specifies the default, it
       is not described.


BUILDING 8-BIT, 16-BIT AND 32-BIT LIBRARIES

       By default, a library called libpcre2-8 is built, containing  functions
       that  take  string arguments contained in vectors of bytes, interpreted
       either as single-byte characters, or UTF-8 strings. You can also  build
       two  other libraries, called libpcre2-16 and libpcre2-32, which process
       strings that are contained in vectors of 16-bit and 32-bit code  units,
       respectively. These can be interpreted either as single-unit characters
       or UTF-16/UTF-32 strings. To build these additional libraries, add  one
       or both of the following to the configure command:

         --enable-pcre2-16
         --enable-pcre2-32

       If you do not want the 8-bit library, add

         --disable-pcre2-8

       as  well.  At least one of the three libraries must be built. Note that
       the POSIX wrapper is for the 8-bit library only, and that pcre2grep  is
       an  8-bit  program.  Neither  of these are built if you select only the
       16-bit or 32-bit libraries.


BUILDING SHARED AND STATIC LIBRARIES

       The Autotools PCRE2 building process uses libtool to build both  shared
       and  static  libraries by default. You can suppress an unwanted library
       by adding one of

         --disable-shared
         --disable-static

       to the configure command.


UNICODE AND UTF SUPPORT

       By default, PCRE2 is built with support for Unicode and  UTF  character
       strings.  To build it without Unicode support, add

         --disable-unicode

       to  the configure command. This setting applies to all three libraries.
       It is not possible to build  one  library  with  Unicode  support,  and
       another without, in the same configuration.

       Of  itself, Unicode support does not make PCRE2 treat strings as UTF-8,
       UTF-16 or UTF-32. To do that, applications that use the library can set
       the  PCRE2_UTF  option when they call pcre2_compile() to compile a pat-
       tern.  Alternatively, patterns may be started with  (*UTF)  unless  the
       application has locked this out by setting PCRE2_NEVER_UTF.

       UTF support allows the libraries to process character code points up to
       0x10ffff in the strings that they handle. It also provides support  for
       accessing  the  Unicode  properties  of  such characters, using pattern
       escapes such as \P, \p, and \X. Only the  general  category  properties
       such  as Lu and Nd are supported. Details are given in the pcre2pattern
       documentation.

       Pattern escapes such as \d and \w do not by default make use of Unicode
       properties.  The  application  can  request that they do by setting the
       PCRE2_UCP option. Unless the application  has  set  PCRE2_NEVER_UCP,  a
       pattern may also request this by starting with (*UCP).


DISABLING THE USE OF \C

       The \C escape sequence, which matches a single code unit, even in a UTF
       mode, can cause unpredictable behaviour because it may leave  the  cur-
       rent  matching  point in the middle of a multi-code-unit character. The
       application can lock it  out  by  setting  the  PCRE2_NEVER_BACKSLASH_C
       option when calling pcre2_compile(). There is also a build-time option

         --enable-never-backslash-C

       (note the upper case C) which locks out the use of \C entirely.


JUST-IN-TIME COMPILER SUPPORT

       Just-in-time compiler support is included in the build by specifying

         --enable-jit

       This  support  is available only for certain hardware architectures. If
       this option is set for an unsupported architecture,  a  building  error
       occurs.   See the pcre2jit documentation for a discussion of JIT usage.
       When JIT support is enabled, pcre2grep automatically makes use  of  it,
       unless you add

         --disable-pcre2grep-jit

       to the "configure" command.


NEWLINE RECOGNITION

       By  default, PCRE2 interprets the linefeed (LF) character as indicating
       the end of a line. This is the normal newline  character  on  Unix-like
       systems.  You can compile PCRE2 to use carriage return (CR) instead, by
       adding

         --enable-newline-is-cr

       to the configure  command.  There  is  also  an  --enable-newline-is-lf
       option, which explicitly specifies linefeed as the newline character.

       Alternatively, you can specify that line endings are to be indicated by
       the two-character sequence CRLF (CR immediately followed by LF). If you
       want this, add

         --enable-newline-is-crlf

       to the configure command. There is a fourth option, specified by

         --enable-newline-is-anycrlf

       which  causes  PCRE2 to recognize any of the three sequences CR, LF, or
       CRLF as indicating a line ending. Finally, a fifth option, specified by

         --enable-newline-is-any

       causes PCRE2 to recognize any Unicode  newline  sequence.  The  Unicode
       newline sequences are the three just mentioned, plus the single charac-
       ters VT (vertical tab, U+000B), FF (form feed, U+000C), NEL (next line,
       U+0085),  LS  (line  separator,  U+2028),  and PS (paragraph separator,
       U+2029).

       Whatever default line ending convention is selected when PCRE2 is built
       can  be  overridden by applications that use the library. At build time
       it is conventional to use the standard for your operating system.


WHAT \R MATCHES

       By default, the sequence \R in a pattern matches  any  Unicode  newline
       sequence,  independently  of  what has been selected as the line ending
       sequence. If you specify

         --enable-bsr-anycrlf

       the default is changed so that \R matches only CR, LF, or  CRLF.  What-
       ever  is selected when PCRE2 is built can be overridden by applications
       that use the called.


HANDLING VERY LARGE PATTERNS

       Within a compiled pattern, offset values are used  to  point  from  one
       part  to another (for example, from an opening parenthesis to an alter-
       nation metacharacter). By default, in the 8-bit and  16-bit  libraries,
       two-byte  values  are used for these offsets, leading to a maximum size
       for a compiled pattern of around 64K code units. This is sufficient  to
       handle all but the most gigantic patterns. Nevertheless, some people do
       want to process truly enormous patterns, so it is possible  to  compile
       PCRE2  to  use three-byte or four-byte offsets by adding a setting such
       as

         --with-link-size=3

       to the configure command. The value given must be 2, 3, or 4.  For  the
       16-bit  library,  a  value of 3 is rounded up to 4. In these libraries,
       using longer offsets slows down the operation of PCRE2 because  it  has
       to  load additional data when handling them. For the 32-bit library the
       value is always 4 and cannot be overridden; the value  of  --with-link-
       size is ignored.


AVOIDING EXCESSIVE STACK USAGE

       When  matching  with the pcre2_match() function, PCRE2 implements back-
       tracking by making recursive  calls  to  an  internal  function  called
       match().  In  environments where the size of the stack is limited, this
       can severely limit PCRE2's operation. (The Unix  environment  does  not
       usually  suffer from this problem, but it may sometimes be necessary to
       increase  the  maximum  stack  size.  There  is  a  discussion  in  the
       pcre2stack  documentation.)  An  alternative approach to recursion that
       uses memory from the heap to remember data, instead of using  recursive
       function  calls, has been implemented to work round the problem of lim-
       ited stack size. If you want to build a version  of  PCRE2  that  works
       this way, add

         --disable-stack-for-recursion

       to the configure command. By default, the system functions malloc() and
       free() are called to manage the heap memory that is required, but  cus-
       tom  memory  management  functions  can  be  called instead. PCRE2 runs
       noticeably more slowly when built in this way. This option affects only
       the pcre2_match() function; it is not relevant for pcre2_dfa_match().


LIMITING PCRE2 RESOURCE USAGE

       Internally, PCRE2 has a function called match(), which it calls repeat-
       edly  (sometimes  recursively)  when  matching  a  pattern   with   the
       pcre2_match() function. By controlling the maximum number of times this
       function may be called during a single matching operation, a limit  can
       be  placed on the resources used by a single call to pcre2_match(). The
       limit can be changed at run time, as described in the pcre2api documen-
       tation.  The default is 10 million, but this can be changed by adding a
       setting such as

         --with-match-limit=500000

       to  the  configure  command.  This  setting  has  no  effect   on   the
       pcre2_dfa_match() matching function.

       In  some  environments  it is desirable to limit the depth of recursive
       calls of match() more strictly than the total number of calls, in order
       to  restrict  the maximum amount of stack (or heap, if --disable-stack-
       for-recursion is specified) that is used. A second limit controls this;
       it  defaults  to  the  value  that is set for --with-match-limit, which
       imposes no additional constraints. However, you can set a  lower  limit
       by adding, for example,

         --with-match-limit-recursion=10000

       to  the  configure  command.  This  value can also be overridden at run
       time.


CREATING CHARACTER TABLES AT BUILD TIME

       PCRE2 uses fixed tables for processing characters whose code points are
       less than 256. By default, PCRE2 is built with a set of tables that are
       distributed in the file src/pcre2_chartables.c.dist. These  tables  are
       for ASCII codes only. If you add

         --enable-rebuild-chartables

       to  the  configure  command, the distributed tables are no longer used.
       Instead, a program called dftables is compiled and  run.  This  outputs
       the source for new set of tables, created in the default locale of your
       C run-time system. (This method of replacing the tables does  not  work
       if  you are cross compiling, because dftables is run on the local host.
       If you need to create alternative tables when cross compiling, you will
       have to do so "by hand".)


USING EBCDIC CODE

       PCRE2  assumes  by default that it will run in an environment where the
       character code is ASCII or Unicode, which is a superset of ASCII.  This
       is the case for most computer operating systems. PCRE2 can, however, be
       compiled to run in an 8-bit EBCDIC environment by adding

         --enable-ebcdic --disable-unicode

       to the configure command. This setting implies --enable-rebuild-charta-
       bles.  You  should  only  use  it if you know that you are in an EBCDIC
       environment (for example, an IBM mainframe operating system).

       It is not possible to support both EBCDIC and UTF-8 codes in  the  same
       version  of  the  library. Consequently, --enable-unicode and --enable-
       ebcdic are mutually exclusive.

       The EBCDIC character that corresponds to an ASCII LF is assumed to have
       the  value  0x15 by default. However, in some EBCDIC environments, 0x25
       is used. In such an environment you should use

         --enable-ebcdic-nl25

       as well as, or instead of, --enable-ebcdic. The EBCDIC character for CR
       has  the  same  value  as in ASCII, namely, 0x0d. Whichever of 0x15 and
       0x25 is not chosen as LF is made to correspond to the Unicode NEL char-
       acter (which, in Unicode, is 0x85).

       The options that select newline behaviour, such as --enable-newline-is-
       cr, and equivalent run-time options, refer to these character values in
       an EBCDIC environment.


PCRE2GREP SUPPORT FOR EXTERNAL SCRIPTS

       By default, on non-Windows systems, pcre2grep supports the use of call-
       outs with string arguments within the patterns it is matching, in order
       to  run external scripts. For details, see the pcre2grep documentation.
       This support can be disabled by adding  --disable-pcre2grep-callout  to
       the configure command.


PCRE2GREP OPTIONS FOR COMPRESSED FILE SUPPORT

       By  default,  pcre2grep reads all files as plain text. You can build it
       so that it recognizes files whose names end in .gz or .bz2,  and  reads
       them with libz or libbz2, respectively, by adding one or both of

         --enable-pcre2grep-libz
         --enable-pcre2grep-libbz2

       to the configure command. These options naturally require that the rel-
       evant libraries are installed on your system. Configuration  will  fail
       if they are not.


PCRE2GREP BUFFER SIZE

       pcre2grep  uses an internal buffer to hold a "window" on the file it is
       scanning, in order to be able to output "before" and "after" lines when
       it  finds  a match. The size of the buffer is controlled by a parameter
       whose default value is 20K. The buffer itself is three times this size,
       but because of the way it is used for holding "before" lines, the long-
       est line that is guaranteed to be processable is  the  parameter  size.
       You can change the default parameter value by adding, for example,

         --with-pcre2grep-bufsize=50K

       to  the  configure  command.  The caller of pcre2grep can override this
       value by using --buffer-size on the command line.


PCRE2TEST OPTION FOR LIBREADLINE SUPPORT

       If you add one of

         --enable-pcre2test-libreadline
         --enable-pcre2test-libedit

       to the configure command, pcre2test  is  linked  with  the  libreadline
       orlibedit library, respectively, and when its input is from a terminal,
       it reads it using the readline() function. This  provides  line-editing
       and  history  facilities.  Note that libreadline is GPL-licensed, so if
       you distribute a binary of pcre2test linked in this way, there  may  be
       licensing issues. These can be avoided by linking instead with libedit,
       which has a BSD licence.

       Setting --enable-pcre2test-libreadline causes the -lreadline option  to
       be  added to the pcre2test build. In many operating environments with a
       sytem-installed readline library this is sufficient. However,  in  some
       environments (e.g. if an unmodified distribution version of readline is
       in use), some extra configuration may be necessary.  The  INSTALL  file
       for libreadline says this:

         "Readline uses the termcap functions, but does not link with
         the termcap or curses library itself, allowing applications
         which link with readline the to choose an appropriate library."

       If  your environment has not been set up so that an appropriate library
       is automatically included, you may need to add something like

         LIBS="-ncurses"

       immediately before the configure command.


INCLUDING DEBUGGING CODE

       If you add

         --enable-debug

       to the configure command, additional debugging code is included in  the
       build. This feature is intended for use by the PCRE2 maintainers.


DEBUGGING WITH VALGRIND SUPPORT

       If you add

         --enable-valgrind

       to  the  configure command, PCRE2 will use valgrind annotations to mark
       certain memory regions as  unaddressable.  This  allows  it  to  detect
       invalid  memory  accesses,  and  is  mostly  useful for debugging PCRE2
       itself.


CODE COVERAGE REPORTING

       If your C compiler is gcc, you can build a version of  PCRE2  that  can
       generate a code coverage report for its test suite. To enable this, you
       must install lcov version 1.6 or above. Then specify

         --enable-coverage

       to the configure command and build PCRE2 in the usual way.

       Note that using ccache (a caching C compiler) is incompatible with code
       coverage  reporting. If you have configured ccache to run automatically
       on your system, you must set the environment variable

         CCACHE_DISABLE=1

       before running make to build PCRE2, so that ccache is not used.

       When --enable-coverage is used,  the  following  addition  targets  are
       added to the Makefile:

         make coverage

       This  creates  a  fresh coverage report for the PCRE2 test suite. It is
       equivalent to running "make coverage-reset", "make  coverage-baseline",
       "make check", and then "make coverage-report".

         make coverage-reset

       This zeroes the coverage counters, but does nothing else.

         make coverage-baseline

       This captures baseline coverage information.

         make coverage-report

       This creates the coverage report.

         make coverage-clean-report

       This  removes the generated coverage report without cleaning the cover-
       age data itself.

         make coverage-clean-data

       This removes the captured coverage data without removing  the  coverage
       files created at compile time (*.gcno).

         make coverage-clean

       This  cleans all coverage data including the generated coverage report.
       For more information about code coverage, see the gcov and  lcov  docu-
       mentation.


SEE ALSO

       pcre2api(3), pcre2-config(3).


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge, England.


REVISION

       Last updated: 01 April 2016
       Copyright (c) 1997-2016 University of Cambridge.
------------------------------------------------------------------------------


PCRE2CALLOUT(3)            Library Functions Manual            PCRE2CALLOUT(3)



NAME
       PCRE2 - Perl-compatible regular expressions (revised API)

SYNOPSIS

       #include <pcre2.h>

       int (*pcre2_callout)(pcre2_callout_block *, void *);

       int pcre2_callout_enumerate(const pcre2_code *code,
         int (*callback)(pcre2_callout_enumerate_block *, void *),
         void *user_data);


DESCRIPTION

       PCRE2  provides  a feature called "callout", which is a means of tempo-
       rarily passing control to the caller of PCRE2 in the middle of  pattern
       matching.  The caller of PCRE2 provides an external function by putting
       its entry point in a match  context  (see  pcre2_set_callout()  in  the
       pcre2api documentation).

       Within  a  regular expression, (?C<arg>) indicates a point at which the
       external function is to be called.  Different  callout  points  can  be
       identified  by  putting  a number less than 256 after the letter C. The
       default value is zero.  Alternatively, the argument may be a  delimited
       string.  The  starting delimiter must be one of ` ' " ^ % # $ { and the
       ending delimiter is the same as the start, except for {, where the end-
       ing  delimiter  is  }.  If  the  ending  delimiter is needed within the
       string, it must be doubled. For example, this pattern has  two  callout
       points:

         (?C1)abc(?C"some ""arbitrary"" text")def

       If the PCRE2_AUTO_CALLOUT option bit is set when a pattern is compiled,
       PCRE2 automatically inserts callouts, all with number 255, before  each
       item  in  the  pattern. For example, if PCRE2_AUTO_CALLOUT is used with
       the pattern

         A(\d{2}|--)

       it is processed as if it were

       (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)

       Notice that there is a callout before and after  each  parenthesis  and
       alternation bar. If the pattern contains a conditional group whose con-
       dition is an assertion, an automatic callout  is  inserted  immediately
       before  the  condition. Such a callout may also be inserted explicitly,
       for example:

         (?(?C9)(?=a)ab|de)  (?(?C%text%)(?!=d)ab|de)

       This applies only to assertion conditions (because they are  themselves
       independent groups).

       Callouts  can  be useful for tracking the progress of pattern matching.
       The pcre2test program has a pattern qualifier (/auto_callout) that sets
       automatic  callouts.   When  any  callouts are present, the output from
       pcre2test indicates how the pattern is being matched.  This  is  useful
       information  when  you are trying to optimize the performance of a par-
       ticular pattern.


MISSING CALLOUTS

       You should be aware that, because of optimizations  in  the  way  PCRE2
       compiles and matches patterns, callouts sometimes do not happen exactly
       as you might expect.

   Auto-possessification

       At compile time, PCRE2 "auto-possessifies" repeated items when it knows
       that  what follows cannot be part of the repeat. For example, a+[bc] is
       compiled as if it were a++[bc]. The pcre2test output when this  pattern
       is compiled with PCRE2_ANCHORED and PCRE2_AUTO_CALLOUT and then applied
       to the string "aaaa" is:

         --->aaaa
          +0 ^        a+
          +2 ^   ^    [bc]
         No match

       This indicates that when matching [bc] fails, there is no  backtracking
       into  a+  and  therefore the callouts that would be taken for the back-
       tracks do not occur.  You can disable the  auto-possessify  feature  by
       passing  PCRE2_NO_AUTO_POSSESS to pcre2_compile(), or starting the pat-
       tern with (*NO_AUTO_POSSESS). In this case, the output changes to this:

         --->aaaa
          +0 ^        a+
          +2 ^   ^    [bc]
          +2 ^  ^     [bc]
          +2 ^ ^      [bc]
          +2 ^^       [bc]
         No match

       This time, when matching [bc] fails, the matcher backtracks into a+ and
       tries again, repeatedly, until a+ itself fails.

   Automatic .* anchoring

       By default, an optimization is applied when .* is the first significant
       item in a pattern. If PCRE2_DOTALL is set, so that the  dot  can  match
       any  character,  the pattern is automatically anchored. If PCRE2_DOTALL
       is not set, a match can start only after an internal newline or at  the
       beginning  of  the  subject,  and  pcre2_compile() remembers this. This
       optimization is disabled, however, if .* is in an atomic  group  or  if
       there  is  a back reference to the capturing group in which it appears.
       It is also disabled if the pattern contains (*PRUNE) or  (*SKIP).  How-
       ever, the presence of callouts does not affect it.

       For  example,  if  the pattern .*\d is compiled with PCRE2_AUTO_CALLOUT
       and applied to the string "aa", the pcre2test output is:

         --->aa
          +0 ^      .*
          +2 ^ ^    \d
          +2 ^^     \d
          +2 ^      \d
         No match

       This shows that all match attempts start at the beginning of  the  sub-
       ject.  In  other  words,  the pattern is anchored. You can disable this
       optimization by passing PCRE2_NO_DOTSTAR_ANCHOR to pcre2_compile(),  or
       starting  the pattern with (*NO_DOTSTAR_ANCHOR). In this case, the out-
       put changes to:

         --->aa
          +0 ^      .*
          +2 ^ ^    \d
          +2 ^^     \d
          +2 ^      \d
          +0  ^     .*
          +2  ^^    \d
          +2  ^     \d
         No match

       This shows more match attempts, starting at the second subject  charac-
       ter.   Another  optimization, described in the next section, means that
       there is no subsequent attempt to match with an empty subject.

       If a pattern has more than one top-level  branch,  automatic  anchoring
       occurs if all branches are anchorable.

   Other optimizations

       Other  optimizations  that  provide fast "no match" results also affect
       callouts.  For example, if the pattern is

         ab(?C4)cd

       PCRE2 knows that any matching string must contain the  letter  "d".  If
       the  subject  string  is  "abyz",  the  lack of "d" means that matching
       doesn't ever start, and the callout is  never  reached.  However,  with
       "abyd", though the result is still no match, the callout is obeyed.

       PCRE2  also  knows  the  minimum  length of a matching string, and will
       immediately give a "no match" return without actually running  a  match
       if  the  subject is not long enough, or, for unanchored patterns, if it
       has been scanned far enough.

       You can disable these optimizations by passing the PCRE2_NO_START_OPTI-
       MIZE  option  to  pcre2_compile(),  or  by  starting  the  pattern with
       (*NO_START_OPT). This slows down the matching process, but does  ensure
       that callouts such as the example above are obeyed.


THE CALLOUT INTERFACE

       During  matching,  when  PCRE2  reaches a callout point, if an external
       function is set in the match context, it is  called.  This  applies  to
       both  normal  and DFA matching. The first argument to the callout func-
       tion is a pointer to a pcre2_callout block. The second argument is  the
       void  *  callout  data that was supplied when the callout was set up by
       calling pcre2_set_callout() (see the pcre2api documentation). The call-
       out block structure contains the following fields:

         uint32_t      version;
         uint32_t      callout_number;
         uint32_t      capture_top;
         uint32_t      capture_last;
         PCRE2_SIZE   *offset_vector;
         PCRE2_SPTR    mark;
         PCRE2_SPTR    subject;
         PCRE2_SIZE    subject_length;
         PCRE2_SIZE    start_match;
         PCRE2_SIZE    current_position;
         PCRE2_SIZE    pattern_position;
         PCRE2_SIZE    next_item_length;
         PCRE2_SIZE    callout_string_offset;
         PCRE2_SIZE    callout_string_length;
         PCRE2_SPTR    callout_string;

       The  version field contains the version number of the block format. The
       current version is 1; the three callout string fields  were  added  for
       this  version. If you are writing an application that might use an ear-
       lier release of PCRE2, you  should  check  the  version  number  before
       accessing  any  of  these  fields.  The version number will increase in
       future if more fields are added, but the intention is never  to  remove
       any of the existing fields.

   Fields for numerical callouts

       For  a  numerical  callout,  callout_string is NULL, and callout_number
       contains the number of the callout, in the range  0-255.  This  is  the
       number  that  follows  (?C for manual callouts; it is 255 for automati-
       cally generated callouts.

   Fields for string callouts

       For callouts with string arguments, callout_number is always zero,  and
       callout_string  points  to the string that is contained within the com-
       piled pattern. Its length is given by callout_string_length. Duplicated
       ending delimiters that were present in the original pattern string have
       been turned into single characters, but there is no other processing of
       the  callout string argument. An additional code unit containing binary
       zero is present after the string, but is not included  in  the  length.
       The  delimiter  that was used to start the string is also stored within
       the pattern, immediately before the string itself. You can access  this
       delimiter as callout_string[-1] if you need it.

       The callout_string_offset field is the code unit offset to the start of
       the callout argument string within the original pattern string. This is
       provided  for the benefit of applications such as script languages that
       might need to report errors in the callout string within the pattern.

   Fields for all callouts

       The remaining fields in the callout block are the same for  both  kinds
       of callout.

       The offset_vector field is a pointer to the vector of capturing offsets
       (the "ovector") that was passed to the matching function in  the  match
       data  block.  When pcre2_match() is used, the contents can be inspected
       in order to extract substrings that have been matched so  far,  in  the
       same  way as for extracting substrings after a match has completed. For
       the DFA matching function, this field is not useful.

       The subject and subject_length fields contain copies of the values that
       were passed to the matching function.

       The  start_match  field normally contains the offset within the subject
       at which the current match attempt  started.  However,  if  the  escape
       sequence  \K has been encountered, this value is changed to reflect the
       modified starting point. If the pattern is not  anchored,  the  callout
       function may be called several times from the same point in the pattern
       for different starting points in the subject.

       The current_position field contains the offset within  the  subject  of
       the current match pointer.

       When the pcre2_match() is used, the capture_top field contains one more
       than the number of the highest numbered captured substring so  far.  If
       no substrings have been captured, the value of capture_top is one. This
       is always the case when the DFA functions are used, because they do not
       support captured substrings.

       The  capture_last  field  contains the number of the most recently cap-
       tured substring. However, when a recursion exits, the value reverts  to
       what  it  was  outside  the recursion, as do the values of all captured
       substrings. If no substrings have been  captured,  the  value  of  cap-
       ture_last is 0. This is always the case for the DFA matching functions.

       The pattern_position field contains the offset in the pattern string to
       the next item to be matched.

       The next_item_length field contains the length of the next item  to  be
       matched in the pattern string. When the callout immediately precedes an
       alternation bar, a closing parenthesis, or the end of the pattern,  the
       length  is  zero. When the callout precedes an opening parenthesis, the
       length is that of the entire subpattern.

       The pattern_position and next_item_length fields are intended  to  help
       in  distinguishing between different automatic callouts, which all have
       the same callout number. However, they are set for  all  callouts,  and
       are used by pcre2test to show the next item to be matched when display-
       ing callout information.

       In callouts from pcre2_match() the mark field contains a pointer to the
       zero-terminated  name of the most recently passed (*MARK), (*PRUNE), or
       (*THEN) item in the match, or NULL if no such items have  been  passed.
       Instances  of  (*PRUNE)  or  (*THEN) without a name do not obliterate a
       previous (*MARK). In callouts from the DFA matching function this field
       always contains NULL.


RETURN VALUES FROM CALLOUTS

       The external callout function returns an integer to PCRE2. If the value
       is zero, matching proceeds as normal. If  the  value  is  greater  than
       zero,  matching  fails  at  the current point, but the testing of other
       matching possibilities goes ahead, just as if a lookahead assertion had
       failed. If the value is less than zero, the match is abandoned, and the
       matching function returns the negative value.

       Negative  values  should  normally  be   chosen   from   the   set   of
       PCRE2_ERROR_xxx  values.  In  particular,  PCRE2_ERROR_NOMATCH forces a
       standard "no match" failure. The error  number  PCRE2_ERROR_CALLOUT  is
       reserved  for  use by callout functions; it will never be used by PCRE2
       itself.


CALLOUT ENUMERATION

       int pcre2_callout_enumerate(const pcre2_code *code,
         int (*callback)(pcre2_callout_enumerate_block *, void *),
         void *user_data);

       A script language that supports the use of string arguments in callouts
       might  like  to  scan  all the callouts in a pattern before running the
       match. This can be done by calling pcre2_callout_enumerate(). The first
       argument  is  a  pointer  to a compiled pattern, the second points to a
       callback function, and the third is arbitrary user data.  The  callback
       function  is  called  for  every callout in the pattern in the order in
       which they appear. Its first argument is a pointer to a callout enumer-
       ation  block,  and  its second argument is the user_data value that was
       passed to pcre2_callout_enumerate(). The data block contains  the  fol-
       lowing fields:

         version                Block version number
         pattern_position       Offset to next item in pattern
         next_item_length       Length of next item in pattern
         callout_number         Number for numbered callouts
         callout_string_offset  Offset to string within pattern
         callout_string_length  Length of callout string
         callout_string         Points to callout string or is NULL

       The  version  number is currently 0. It will increase if new fields are
       ever added to the block. The remaining fields are  the  same  as  their
       namesakes  in  the pcre2_callout block that is used for callouts during
       matching, as described above.

       Note that the value of pattern_position is  unique  for  each  callout.
       However,  if  a callout occurs inside a group that is quantified with a
       non-zero minimum or a fixed maximum, the group is replicated inside the
       compiled  pattern.  For example, a pattern such as /(a){2}/ is compiled
       as if it were /(a)(a)/. This means that the callout will be  enumerated
       more  than  once,  but with the same value for pattern_position in each
       case.

       The callback function should normally return zero. If it returns a non-
       zero value, scanning the pattern stops, and that value is returned from
       pcre2_callout_enumerate().


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge, England.


REVISION

       Last updated: 23 March 2015
       Copyright (c) 1997-2015 University of Cambridge.
------------------------------------------------------------------------------


PCRE2COMPAT(3)             Library Functions Manual             PCRE2COMPAT(3)



NAME
       PCRE2 - Perl-compatible regular expressions (revised API)

DIFFERENCES BETWEEN PCRE2 AND PERL

       This document describes the differences in the ways that PCRE2 and Perl
       handle regular expressions. The differences  described  here  are  with
       respect to Perl versions 5.10 and above.

       1.  PCRE2  has only a subset of Perl's Unicode support. Details of what
       it does have are given in the pcre2unicode page.

       2. PCRE2 allows repeat quantifiers only  on  parenthesized  assertions,
       but  they  do not mean what you might think. For example, (?!a){3} does
       not assert that the next three characters are not "a". It just  asserts
       that  the  next  character  is not "a" three times (in principle: PCRE2
       optimizes this to run the assertion  just  once).  Perl  allows  repeat
       quantifiers  on  other  assertions such as \b, but these do not seem to
       have any use.

       3. Capturing subpatterns that occur inside  negative  lookahead  asser-
       tions  are  counted,  but their entries in the offsets vector are never
       set. Perl sometimes (but not always) sets its numerical variables  from
       inside negative assertions.

       4.  The  following Perl escape sequences are not supported: \l, \u, \L,
       \U, and \N when followed by a character name or Unicode value.  (\N  on
       its own, matching a non-newline character, is supported.) In fact these
       are implemented by Perl's general string-handling and are not  part  of
       its  pattern matching engine. If any of these are encountered by PCRE2,
       an error is generated by default. However, if the PCRE2_ALT_BSUX option
       is set, \U and \u are interpreted as ECMAScript interprets them.

       5. The Perl escape sequences \p, \P, and \X are supported only if PCRE2
       is built with Unicode support. The properties that can be  tested  with
       \p and \P are limited to the general category properties such as Lu and
       Nd, script names such as Greek or Han, and the derived  properties  Any
       and L&. PCRE2 does support the Cs (surrogate) property, which Perl does
       not; the Perl documentation says "Because Perl hides the need  for  the
       user  to  understand the internal representation of Unicode characters,
       there is no need to implement the  somewhat  messy  concept  of  surro-
       gates."

       6.  PCRE2 does support the \Q...\E escape for quoting substrings. Char-
       acters in between are treated as literals. This is  slightly  different
       from  Perl  in  that  $  and  @ are also handled as literals inside the
       quotes. In Perl, they cause variable interpolation (but of course PCRE2
       does not have variables).  Note the following examples:

           Pattern            PCRE2 matches      Perl matches

           \Qabc$xyz\E        abc$xyz           abc followed by the
                                                  contents of $xyz
           \Qabc\$xyz\E       abc\$xyz          abc\$xyz
           \Qabc\E\$\Qxyz\E   abc$xyz           abc$xyz

       The  \Q...\E  sequence  is recognized both inside and outside character
       classes.

       7.  Fairly  obviously,  PCRE2  does  not  support  the  (?{code})   and
       (??{code})  constructions. However, there is support for recursive pat-
       terns. This is not available in Perl 5.8, but it is in Perl 5.10. Also,
       the  PCRE2  "callout"  feature allows an external function to be called
       during  pattern  matching.  See  the  pcre2callout  documentation   for
       details.

       8.  Subroutine  calls  (whether recursive or not) are treated as atomic
       groups.  Atomic recursion is like Python,  but  unlike  Perl.  Captured
       values  that  are  set outside a subroutine call can be referenced from
       inside in PCRE2, but not in Perl. There is a discussion  that  explains
       these  differences  in  more detail in the section on recursion differ-
       ences from Perl in the pcre2pattern page.

       9. If any of the backtracking control verbs are used  in  a  subpattern
       that  is  called  as  a  subroutine (whether or not recursively), their
       effect is confined to that subpattern; it does not extend to  the  sur-
       rounding  pattern.  This is not always the case in Perl. In particular,
       if (*THEN) is present in a group that is called as  a  subroutine,  its
       action is limited to that group, even if the group does not contain any
       | characters. Note that such subpatterns are processed as  anchored  at
       the point where they are tested.

       10.  If a pattern contains more than one backtracking control verb, the
       first one that is backtracked onto acts. For example,  in  the  pattern
       A(*COMMIT)B(*PRUNE)C  a  failure in B triggers (*COMMIT), but a failure
       in C triggers (*PRUNE). Perl's behaviour is more complex; in many cases
       it is the same as PCRE2, but there are examples where it differs.

       11.  Most  backtracking  verbs in assertions have their normal actions.
       They are not confined to the assertion.

       12. There are some differences that are concerned with the settings  of
       captured  strings  when  part  of  a  pattern is repeated. For example,
       matching "aba" against the  pattern  /^(a(b)?)+$/  in  Perl  leaves  $2
       unset, but in PCRE2 it is set to "b".

       13. PCRE2's handling of duplicate subpattern numbers and duplicate sub-
       pattern names is not as general as Perl's. This is a consequence of the
       fact  the  PCRE2  works internally just with numbers, using an external
       table to translate between numbers and names. In particular, a  pattern
       such  as  (?|(?<a>A)|(?<b)B),  where the two capturing parentheses have
       the same number but different names, is not supported,  and  causes  an
       error  at compile time. If it were allowed, it would not be possible to
       distinguish which parentheses matched, because both names map  to  cap-
       turing subpattern number 1. To avoid this confusing situation, an error
       is given at compile time.

       14. Perl recognizes comments in some places that PCRE2  does  not,  for
       example,  between  the  ( and ? at the start of a subpattern. If the /x
       modifier is set, Perl allows white space between ( and ?  (though  cur-
       rent  Perls warn that this is deprecated) but PCRE2 never does, even if
       the PCRE2_EXTENDED option is set.

       15. Perl, when in warning mode, gives warnings  for  character  classes
       such  as  [A-\d] or [a-[:digit:]]. It then treats the hyphens as liter-
       als. PCRE2 has no warning features, so it gives an error in these cases
       because they are almost certainly user mistakes.

       16.  In  PCRE2, the upper/lower case character properties Lu and Ll are
       not affected when case-independent matching is specified. For  example,
       \p{Lu} always matches an upper case letter. I think Perl has changed in
       this respect; in the release at the time of writing (5.16), \p{Lu}  and
       \p{Ll} match all letters, regardless of case, when case independence is
       specified.

       17. PCRE2 provides some  extensions  to  the  Perl  regular  expression
       facilities.   Perl  5.10  includes new features that are not in earlier
       versions of Perl, some of which (such as named parentheses)  have  been
       in PCRE2 for some time. This list is with respect to Perl 5.10:

       (a)  Although  lookbehind  assertions  in PCRE2 must match fixed length
       strings, each alternative branch of a lookbehind assertion can match  a
       different  length  of  string.  Perl requires them all to have the same
       length.

       (b) If PCRE2_DOLLAR_ENDONLY is set and PCRE2_MULTILINE is not set,  the
       $ meta-character matches only at the very end of the string.

       (c)  A  backslash  followed  by  a  letter  with  no special meaning is
       faulted. (Perl can be made to issue a warning.)

       (d) If PCRE2_UNGREEDY is set, the greediness of the repetition  quanti-
       fiers is inverted, that is, by default they are not greedy, but if fol-
       lowed by a question mark they are.

       (e) PCRE2_ANCHORED can be used at matching time to force a  pattern  to
       be tried only at the first matching position in the subject string.

       (f)      The      PCRE2_NOTBOL,      PCRE2_NOTEOL,      PCRE2_NOTEMPTY,
       PCRE2_NOTEMPTY_ATSTART, and PCRE2_NO_AUTO_CAPTURE options have no  Perl
       equivalents.

       (g)  The  \R escape sequence can be restricted to match only CR, LF, or
       CRLF by the PCRE2_BSR_ANYCRLF option.

       (h) The callout facility is PCRE2-specific.

       (i) The partial matching facility is PCRE2-specific.

       (j) The alternative matching function (pcre2_dfa_match() matches  in  a
       different way and is not Perl-compatible.

       (k)  PCRE2 recognizes some special sequences such as (*CR) at the start
       of a pattern that set overall options that cannot be changed within the
       pattern.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge, England.


REVISION

       Last updated: 15 March 2015
       Copyright (c) 1997-2015 University of Cambridge.
------------------------------------------------------------------------------


PCRE2JIT(3)                Library Functions Manual                PCRE2JIT(3)



NAME
       PCRE2 - Perl-compatible regular expressions (revised API)

PCRE2 JUST-IN-TIME COMPILER SUPPORT

       Just-in-time  compiling  is a heavyweight optimization that can greatly
       speed up pattern matching. However, it comes at the cost of extra  pro-
       cessing  before  the  match is performed, so it is of most benefit when
       the same pattern is going to be matched many times. This does not  nec-
       essarily  mean many calls of a matching function; if the pattern is not
       anchored, matching attempts may take place many times at various  posi-
       tions in the subject, even for a single call. Therefore, if the subject
       string is very long, it may still pay  to  use  JIT  even  for  one-off
       matches.  JIT  support  is  available  for all of the 8-bit, 16-bit and
       32-bit PCRE2 libraries.

       JIT support applies only to the  traditional  Perl-compatible  matching
       function.   It  does  not apply when the DFA matching function is being
       used. The code for this support was written by Zoltan Herczeg.


AVAILABILITY OF JIT SUPPORT

       JIT support is an optional feature of  PCRE2.  The  "configure"  option
       --enable-jit  (or  equivalent  CMake  option) must be set when PCRE2 is
       built if you want to use JIT. The support is limited to  the  following
       hardware platforms:

         ARM 32-bit (v5, v7, and Thumb2)
         ARM 64-bit
         Intel x86 32-bit and 64-bit
         MIPS 32-bit and 64-bit
         Power PC 32-bit and 64-bit
         SPARC 32-bit

       If --enable-jit is set on an unsupported platform, compilation fails.

       A  program  can  tell if JIT support is available by calling pcre2_con-
       fig() with the PCRE2_CONFIG_JIT option. The result is  1  when  JIT  is
       available,  and 0 otherwise. However, a simple program does not need to
       check this in order to use JIT. The API is implemented in  a  way  that
       falls  back  to the interpretive code if JIT is not available. For pro-
       grams that need the best possible performance, there is  also  a  "fast
       path" API that is JIT-specific.


SIMPLE USE OF JIT

       To  make use of the JIT support in the simplest way, all you have to do
       is to call pcre2_jit_compile() after successfully compiling  a  pattern
       with pcre2_compile(). This function has two arguments: the first is the
       compiled pattern pointer that was returned by pcre2_compile(), and  the
       second  is  zero  or  more of the following option bits: PCRE2_JIT_COM-
       PLETE, PCRE2_JIT_PARTIAL_HARD, or PCRE2_JIT_PARTIAL_SOFT.

       If JIT support is not available, a  call  to  pcre2_jit_compile()  does
       nothing  and returns PCRE2_ERROR_JIT_BADOPTION. Otherwise, the compiled
       pattern is passed to the JIT compiler, which turns it into machine code
       that executes much faster than the normal interpretive code, but yields
       exactly the same results. The returned value  from  pcre2_jit_compile()
       is zero on success, or a negative error code.

       There  is  a limit to the size of pattern that JIT supports, imposed by
       the size of machine stack that it uses. The exact rules are  not  docu-
       mented  because  they  may  change at any time, in particular, when new
       optimizations are introduced.  If a pattern  is  too  big,  a  call  to
       pcre2_jit_compile() returns PCRE2_ERROR_NOMEMORY.

       PCRE2_JIT_COMPLETE  requests the JIT compiler to generate code for com-
       plete matches. If you want to run partial matches using the  PCRE2_PAR-
       TIAL_HARD  or  PCRE2_PARTIAL_SOFT  options of pcre2_match(), you should
       set one or both of  the  other  options  as  well  as,  or  instead  of
       PCRE2_JIT_COMPLETE. The JIT compiler generates different optimized code
       for each of the three modes (normal, soft partial, hard partial).  When
       pcre2_match()  is  called,  the appropriate code is run if it is avail-
       able. Otherwise, the pattern is matched using interpretive code.

       You can call pcre2_jit_compile() multiple times for the  same  compiled
       pattern.  It does nothing if it has previously compiled code for any of
       the option bits. For example, you can call it once with  PCRE2_JIT_COM-
       PLETE  and  (perhaps  later,  when  you find you need partial matching)
       again with PCRE2_JIT_COMPLETE and PCRE2_JIT_PARTIAL_HARD. This time  it
       will ignore PCRE2_JIT_COMPLETE and just compile code for partial match-
       ing. If pcre2_jit_compile() is called with no option bits set, it imme-
       diately returns zero. This is an alternative way of testing whether JIT
       is available.

       At present, it is not possible to free JIT compiled  code  except  when
       the entire compiled pattern is freed by calling pcre2_code_free().

       In  some circumstances you may need to call additional functions. These
       are described in the  section  entitled  "Controlling  the  JIT  stack"
       below.

       There are some pcre2_match() options that are not supported by JIT, and
       there are also some pattern items that JIT cannot handle.  Details  are
       given  below.  In  both cases, matching automatically falls back to the
       interpretive code. If you want to know whether JIT  was  actually  used
       for  a particular match, you should arrange for a JIT callback function
       to be set up as described in the section entitled "Controlling the  JIT
       stack"  below,  even  if  you  do  not need to supply a non-default JIT
       stack. Such a callback function is called whenever JIT code is about to
       be  obeyed.  If the match-time options are not right for JIT execution,
       the callback function is not obeyed.

       If the JIT compiler finds an unsupported item, no JIT  data  is  gener-
       ated.  You  can find out if JIT matching is available after compiling a
       pattern by calling  pcre2_pattern_info()  with  the  PCRE2_INFO_JITSIZE
       option.  A non-zero result means that JIT compilation was successful. A
       result of 0 means that JIT support is not available, or the pattern was
       not  processed by pcre2_jit_compile(), or the JIT compiler was not able
       to handle the pattern.


UNSUPPORTED OPTIONS AND PATTERN ITEMS

       The pcre2_match() options that  are  supported  for  JIT  matching  are
       PCRE2_NOTBOL,   PCRE2_NOTEOL,  PCRE2_NOTEMPTY,  PCRE2_NOTEMPTY_ATSTART,
       PCRE2_NO_UTF_CHECK,  PCRE2_PARTIAL_HARD,  and  PCRE2_PARTIAL_SOFT.  The
       PCRE2_ANCHORED option is not supported at match time.

       If  the  PCRE2_NO_JIT option is passed to pcre2_match() it disables the
       use of JIT, forcing matching by the interpreter code.

       The only unsupported pattern items are \C (match a  single  data  unit)
       when  running in a UTF mode, and a callout immediately before an asser-
       tion condition in a conditional group.


RETURN VALUES FROM JIT MATCHING

       When a pattern is matched using JIT matching, the return values are the
       same  as  those  given by the interpretive pcre2_match() code, with the
       addition of one new error code: PCRE2_ERROR_JIT_STACKLIMIT. This  means
       that  the memory used for the JIT stack was insufficient. See "Control-
       ling the JIT stack" below for a discussion of JIT stack usage.

       The error code PCRE2_ERROR_MATCHLIMIT is returned by the  JIT  code  if
       searching  a  very large pattern tree goes on for too long, as it is in
       the same circumstance when JIT is not used, but the details of  exactly
       what  is counted are not the same. The PCRE2_ERROR_RECURSIONLIMIT error
       code is never returned when JIT matching is used.


CONTROLLING THE JIT STACK

       When the compiled JIT code runs, it needs a block of memory to use as a
       stack.   By  default,  it  uses 32K on the machine stack. However, some
       large  or  complicated  patterns  need  more  than  this.   The   error
       PCRE2_ERROR_JIT_STACKLIMIT  is  given  when  there is not enough stack.
       Three functions are provided for managing blocks of memory for  use  as
       JIT  stacks. There is further discussion about the use of JIT stacks in
       the section entitled "JIT stack FAQ" below.

       The pcre2_jit_stack_create() function creates a JIT  stack.  Its  argu-
       ments  are  a starting size, a maximum size, and a general context (for
       memory allocation functions, or NULL for standard  memory  allocation).
       It returns a pointer to an opaque structure of type pcre2_jit_stack, or
       NULL if there is an error. The pcre2_jit_stack_free() function is  used
       to  free a stack that is no longer needed. (For the technically minded:
       the address space is allocated by mmap or VirtualAlloc.)

       JIT uses far less memory for recursion than the interpretive code,  and
       a  maximum  stack size of 512K to 1M should be more than enough for any
       pattern.

       The pcre2_jit_stack_assign() function specifies which  stack  JIT  code
       should use. Its arguments are as follows:

         pcre2_match_context  *mcontext
         pcre2_jit_callback    callback
         void                 *data

       The first argument is a pointer to a match context. When this is subse-
       quently passed to a matching function, its information determines which
       JIT  stack  is  used. There are three cases for the values of the other
       two options:

         (1) If callback is NULL and data is NULL, an internal 32K block
             on the machine stack is used. This is the default when a match
             context is created.

         (2) If callback is NULL and data is not NULL, data must be
             a pointer to a valid JIT stack, the result of calling
             pcre2_jit_stack_create().

         (3) If callback is not NULL, it must point to a function that is
             called with data as an argument at the start of matching, in
             order to set up a JIT stack. If the return from the callback
             function is NULL, the internal 32K stack is used; otherwise the
             return value must be a valid JIT stack, the result of calling
             pcre2_jit_stack_create().

       A callback function is obeyed whenever JIT code is about to be run;  it
       is not obeyed when pcre2_match() is called with options that are incom-
       patible for JIT matching. A callback function can therefore be used  to
       determine  whether  a  match  operation  was  executed by JIT or by the
       interpreter.

       You may safely use the same JIT stack for more than one pattern (either
       by  assigning  directly  or  by  callback), as long as the patterns are
       matched sequentially in the same thread. Currently, the only way to set
       up  non-sequential matches in one thread is to use callouts: if a call-
       out function starts another match, that match must use a different  JIT
       stack to the one used for currently suspended match(es).

       In  a multithread application, if you do not specify a JIT stack, or if
       you assign or pass back NULL from  a  callback,  that  is  thread-safe,
       because  each  thread has its own machine stack. However, if you assign
       or pass back a non-NULL JIT stack, this must be a different  stack  for
       each thread so that the application is thread-safe.

       Strictly  speaking,  even more is allowed. You can assign the same non-
       NULL stack to a match context that is used by any number  of  patterns,
       as  long  as  they are not used for matching by multiple threads at the
       same time. For example, you could use the same stack  in  all  compiled
       patterns,  with  a global mutex in the callback to wait until the stack
       is available for use. However, this is an inefficient solution, and not
       recommended.

       This  is a suggestion for how a multithreaded program that needs to set
       up non-default JIT stacks might operate:

         During thread initalization
           thread_local_var = pcre2_jit_stack_create(...)

         During thread exit
           pcre2_jit_stack_free(thread_local_var)

         Use a one-line callback function
           return thread_local_var

       All the functions described in this section do nothing if  JIT  is  not
       available.


JIT STACK FAQ

       (1) Why do we need JIT stacks?

       PCRE2 (and JIT) is a recursive, depth-first engine, so it needs a stack
       where the local data of the current node is pushed before checking  its
       child nodes.  Allocating real machine stack on some platforms is diffi-
       cult. For example, the stack chain needs to be updated every time if we
       extend  the  stack  on  PowerPC.  Although it is possible, its updating
       time overhead decreases performance. So we do the recursion in memory.

       (2) Why don't we simply allocate blocks of memory with malloc()?

       Modern operating systems have a  nice  feature:  they  can  reserve  an
       address space instead of allocating memory. We can safely allocate mem-
       ory pages inside this address space, so the stack  could  grow  without
       moving memory data (this is important because of pointers). Thus we can
       allocate 1M address space, and use only a single memory  page  (usually
       4K)  if  that is enough. However, we can still grow up to 1M anytime if
       needed.

       (3) Who "owns" a JIT stack?

       The owner of the stack is the user program, not the JIT studied pattern
       or anything else. The user program must ensure that if a stack is being
       used by pcre2_match(), (that is, it is assigned to a match context that
       is  passed  to  the  pattern currently running), that stack must not be
       used by any other threads (to avoid overwriting the same memory  area).
       The best practice for multithreaded programs is to allocate a stack for
       each thread, and return this stack through the JIT callback function.

       (4) When should a JIT stack be freed?

       You can free a JIT stack at any time, as long as it will not be used by
       pcre2_match() again. When you assign the stack to a match context, only
       a pointer is set. There is no reference counting or  any  other  magic.
       You can free compiled patterns, contexts, and stacks in any order, any-
       time. Just do not call pcre2_match() with a match context  pointing  to
       an already freed stack, as that will cause SEGFAULT. (Also, do not free
       a stack currently used by pcre2_match() in  another  thread).  You  can
       also  replace the stack in a context at any time when it is not in use.
       You should free the previous stack before assigning a replacement.

       (5) Should I allocate/free a  stack  every  time  before/after  calling
       pcre2_match()?

       No,  because  this  is  too  costly in terms of resources. However, you
       could implement some clever idea which release the stack if it  is  not
       used  in  let's  say  two minutes. The JIT callback can help to achieve
       this without keeping a list of patterns.

       (6) OK, the stack is for long term memory allocation. But what  happens
       if  a pattern causes stack overflow with a stack of 1M? Is that 1M kept
       until the stack is freed?

       Especially on embedded sytems, it might be a good idea to release  mem-
       ory  sometimes  without  freeing the stack. There is no API for this at
       the moment.  Probably a function call which returns with the  currently
       allocated  memory for any stack and another which allows releasing mem-
       ory (shrinking the stack) would be a good idea if someone needs this.

       (7) This is too much of a headache. Isn't there any better solution for
       JIT stack handling?

       No,  thanks to Windows. If POSIX threads were used everywhere, we could
       throw out this complicated API.


FREEING JIT SPECULATIVE MEMORY

       void pcre2_jit_free_unused_memory(pcre2_general_context *gcontext);

       The JIT executable allocator does not free all memory when it is possi-
       ble.   It expects new allocations, and keeps some free memory around to
       improve allocation speed. However, in low memory conditions,  it  might
       be  better to free all possible memory. You can cause this to happen by
       calling pcre2_jit_free_unused_memory(). Its argument is a general  con-
       text, for custom memory management, or NULL for standard memory manage-
       ment.


EXAMPLE CODE

       This is a single-threaded example that specifies a  JIT  stack  without
       using  a  callback.  A real program should include error checking after
       all the function calls.

         int rc;
         pcre2_code *re;
         pcre2_match_data *match_data;
         pcre2_match_context *mcontext;
         pcre2_jit_stack *jit_stack;

         re = pcre2_compile(pattern, PCRE2_ZERO_TERMINATED, 0,
           &errornumber, &erroffset, NULL);
         rc = pcre2_jit_compile(re, PCRE2_JIT_COMPLETE);
         mcontext = pcre2_match_context_create(NULL);
         jit_stack = pcre2_jit_stack_create(32*1024, 512*1024, NULL);
         pcre2_jit_stack_assign(mcontext, NULL, jit_stack);
         match_data = pcre2_match_data_create(re, 10);
         rc = pcre2_match(re, subject, length, 0, 0, match_data, mcontext);
         /* Process result */

         pcre2_code_free(re);
         pcre2_match_data_free(match_data);
         pcre2_match_context_free(mcontext);
         pcre2_jit_stack_free(jit_stack);


JIT FAST PATH API

       Because the API described above falls back to interpreted matching when
       JIT  is  not  available, it is convenient for programs that are written
       for  general  use  in  many  environments.  However,  calling  JIT  via
       pcre2_match() does have a performance impact. Programs that are written
       for use where JIT is known to be available, and  which  need  the  best
       possible  performance,  can  instead  use a "fast path" API to call JIT
       matching directly instead of calling pcre2_match() (obviously only  for
       patterns that have been successfully processed by pcre2_jit_compile()).

       The  fast  path  function  is  called  pcre2_jit_match(),  and it takes
       exactly the same arguments as pcre2_match(). The return values are also
       the same, plus PCRE2_ERROR_JIT_BADOPTION if a matching mode (partial or
       complete) is requested that was not compiled. Unsupported  option  bits
       (for  example,  PCRE2_ANCHORED)  are  ignored,  as  is the PCRE2_NO_JIT
       option.

       When you call pcre2_match(), as well as testing for invalid options,  a
       number of other sanity checks are performed on the arguments. For exam-
       ple, if the subject pointer is NULL, an immediate error is given. Also,
       unless  PCRE2_NO_UTF_CHECK  is  set, a UTF subject string is tested for
       validity. In the interests of speed, these checks do not happen on  the
       JIT fast path, and if invalid data is passed, the result is undefined.

       Bypassing  the  sanity  checks  and the pcre2_match() wrapping can give
       speedups of more than 10%.


SEE ALSO

       pcre2api(3)


AUTHOR

       Philip Hazel (FAQ by Zoltan Herczeg)
       University Computing Service
       Cambridge, England.


REVISION

       Last updated: 05 June 2016
       Copyright (c) 1997-2016 University of Cambridge.
------------------------------------------------------------------------------


PCRE2LIMITS(3)             Library Functions Manual             PCRE2LIMITS(3)



NAME
       PCRE2 - Perl-compatible regular expressions (revised API)

SIZE AND OTHER LIMITATIONS

       There are some size limitations in PCRE2 but it is hoped that they will
       never in practice be relevant.

       The maximum size of a compiled pattern is approximately 64K code  units
       for  the  8-bit  and  16-bit  libraries  if  PCRE2 is compiled with the
       default internal linkage size, which is 2 bytes for these libraries. If
       you  want  to  process regular expressions that are truly enormous, you
       can compile PCRE2 with an internal linkage size of 3 or 4 (when  build-
       ing  the  16-bit library, 3 is rounded up to 4). See the README file in
       the source distribution and the pcre2build documentation  for  details.
       In  these  cases the limit is substantially larger.  However, the speed
       of execution is slower. In the 32-bit  library,  the  internal  linkage
       size is always 4.

       The maximum length of a source pattern string is essentially unlimited;
       it is the largest number a PCRE2_SIZE variable can hold.  However,  the
       program that calls pcre2_compile() can specify a smaller limit.

       The maximum length (in code units) of a subject string is one less than
       the largest number a PCRE2_SIZE variable can  hold.  PCRE2_SIZE  is  an
       unsigned  integer  type,  usually  defined as size_t. Its maximum value
       (that is ~(PCRE2_SIZE)0) is reserved as a special indicator  for  zero-
       terminated strings and unset offsets.

       Note  that  when  using  the  traditional matching function, PCRE2 uses
       recursion to handle subpatterns and indefinite repetition.  This  means
       that  the  available stack space may limit the size of a subject string
       that can be processed by certain patterns. For a  discussion  of  stack
       issues, see the pcre2stack documentation.

       All values in repeating quantifiers must be less than 65536.

       The maximum length of a lookbehind assertion is 65535 characters.

       There is no limit to the number of parenthesized subpatterns, but there
       can be no more than 65535 capturing subpatterns. There is,  however,  a
       limit  to  the  depth  of  nesting  of parenthesized subpatterns of all
       kinds. This is imposed in order to limit the  amount  of  system  stack
       used  at  compile time. The limit can be specified when PCRE2 is built;
       the default is 250.

       There is a limit to the number of forward references to subsequent sub-
       patterns  of  around  200,000.  Repeated  forward references with fixed
       upper limits, for example, (?2){0,100} when subpattern number 2  is  to
       the  right,  are included in the count. There is no limit to the number
       of backward references.

       The maximum length of name for a named subpattern is 32 code units, and
       the maximum number of named subpatterns is 10000.

       The  maximum  length  of  a  name  in  a (*MARK), (*PRUNE), (*SKIP), or
       (*THEN) verb is 255 for the 8-bit library and 65535 for the 16-bit  and
       32-bit libraries.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge, England.


REVISION

       Last updated: 05 November 2015
       Copyright (c) 1997-2015 University of Cambridge.
------------------------------------------------------------------------------


PCRE2MATCHING(3)           Library Functions Manual           PCRE2MATCHING(3)



NAME
       PCRE2 - Perl-compatible regular expressions (revised API)

PCRE2 MATCHING ALGORITHMS

       This document describes the two different algorithms that are available
       in PCRE2 for matching a compiled regular  expression  against  a  given
       subject  string.  The  "standard"  algorithm is the one provided by the
       pcre2_match() function. This works in the same as  as  Perl's  matching
       function,  and  provide a Perl-compatible matching operation. The just-
       in-time (JIT) optimization that is described in the pcre2jit documenta-
       tion is compatible with this function.

       An alternative algorithm is provided by the pcre2_dfa_match() function;
       it operates in a different way, and is not Perl-compatible. This alter-
       native  has  advantages  and  disadvantages  compared with the standard
       algorithm, and these are described below.

       When there is only one possible way in which a given subject string can
       match  a pattern, the two algorithms give the same answer. A difference
       arises, however, when there are multiple possibilities. For example, if
       the pattern

         ^<.*>

       is matched against the string

         <something> <something else> <something further>

       there are three possible answers. The standard algorithm finds only one
       of them, whereas the alternative algorithm finds all three.


REGULAR EXPRESSIONS AS TREES

       The set of strings that are matched by a regular expression can be rep-
       resented  as  a  tree structure. An unlimited repetition in the pattern
       makes the tree of infinite size, but it is still a tree.  Matching  the
       pattern  to a given subject string (from a given starting point) can be
       thought of as a search of the tree.  There are two  ways  to  search  a
       tree:  depth-first  and  breadth-first, and these correspond to the two
       matching algorithms provided by PCRE2.


THE STANDARD MATCHING ALGORITHM

       In the terminology of Jeffrey Friedl's book "Mastering Regular  Expres-
       sions",  the  standard  algorithm  is an "NFA algorithm". It conducts a
       depth-first search of the pattern tree. That is, it  proceeds  along  a
       single path through the tree, checking that the subject matches what is
       required. When there is a mismatch, the algorithm  tries  any  alterna-
       tives  at  the  current point, and if they all fail, it backs up to the
       previous branch point in the  tree,  and  tries  the  next  alternative
       branch  at  that  level.  This often involves backing up (moving to the
       left) in the subject string as well.  The  order  in  which  repetition
       branches  are  tried  is controlled by the greedy or ungreedy nature of
       the quantifier.

       If a leaf node is reached, a matching string has  been  found,  and  at
       that  point the algorithm stops. Thus, if there is more than one possi-
       ble match, this algorithm returns the first one that it finds.  Whether
       this  is the shortest, the longest, or some intermediate length depends
       on the way the greedy and ungreedy repetition quantifiers are specified
       in the pattern.

       Because  it  ends  up  with a single path through the tree, it is rela-
       tively straightforward for this algorithm to keep  track  of  the  sub-
       strings  that  are  matched  by portions of the pattern in parentheses.
       This provides support for capturing parentheses and back references.


THE ALTERNATIVE MATCHING ALGORITHM

       This algorithm conducts a breadth-first search of  the  tree.  Starting
       from  the  first  matching  point  in the subject, it scans the subject
       string from left to right, once, character by character, and as it does
       this,  it remembers all the paths through the tree that represent valid
       matches. In Friedl's terminology, this is a kind  of  "DFA  algorithm",
       though  it is not implemented as a traditional finite state machine (it
       keeps multiple states active simultaneously).

       Although the general principle of this matching algorithm  is  that  it
       scans  the subject string only once, without backtracking, there is one
       exception: when a lookaround assertion is encountered,  the  characters
       following  or  preceding  the  current  point  have to be independently
       inspected.

       The scan continues until either the end of the subject is  reached,  or
       there  are  no more unterminated paths. At this point, terminated paths
       represent the different matching possibilities (if there are none,  the
       match  has  failed).   Thus,  if there is more than one possible match,
       this algorithm finds all of them, and in particular, it finds the long-
       est.  The  matches are returned in decreasing order of length. There is
       an option to stop the algorithm after the first match (which is  neces-
       sarily the shortest) is found.

       Note that all the matches that are found start at the same point in the
       subject. If the pattern

         cat(er(pillar)?)?

       is matched against the string "the caterpillar catchment",  the  result
       is  the  three  strings "caterpillar", "cater", and "cat" that start at
       the fifth character of the subject. The algorithm  does  not  automati-
       cally move on to find matches that start at later positions.

       PCRE2's "auto-possessification" optimization usually applies to charac-
       ter repeats at the end of a pattern (as well as internally). For  exam-
       ple, the pattern "a\d+" is compiled as if it were "a\d++" because there
       is no point even considering the possibility of backtracking  into  the
       repeated  digits.  For  DFA matching, this means that only one possible
       match is found. If you really do want multiple matches in  such  cases,
       either  use  an ungreedy repeat ("a\d+?") or set the PCRE2_NO_AUTO_POS-
       SESS option when compiling.

       There are a number of features of PCRE2 regular  expressions  that  are
       not  supported  by the alternative matching algorithm. They are as fol-
       lows:

       1. Because the algorithm finds all  possible  matches,  the  greedy  or
       ungreedy  nature  of  repetition quantifiers is not relevant (though it
       may affect auto-possessification, as just described). During  matching,
       greedy  and  ungreedy  quantifiers are treated in exactly the same way.
       However, possessive quantifiers can make a difference when what follows
       could  also  match  what  is  quantified, for example in a pattern like
       this:

         ^a++\w!

       This pattern matches "aaab!" but not "aaa!", which would be matched  by
       a  non-possessive quantifier. Similarly, if an atomic group is present,
       it is matched as if it were a standalone pattern at the current  point,
       and  the  longest match is then "locked in" for the rest of the overall
       pattern.

       2. When dealing with multiple paths through the tree simultaneously, it
       is  not  straightforward  to  keep track of captured substrings for the
       different matching possibilities, and PCRE2's  implementation  of  this
       algorithm does not attempt to do this. This means that no captured sub-
       strings are available.

       3. Because no substrings are captured, back references within the  pat-
       tern are not supported, and cause errors if encountered.

       4.  For  the same reason, conditional expressions that use a backrefer-
       ence as the condition or test for a specific group  recursion  are  not
       supported.

       5.  Because  many  paths  through the tree may be active, the \K escape
       sequence, which resets the start of the match when encountered (but may
       be  on  some  paths  and not on others), is not supported. It causes an
       error if encountered.

       6. Callouts are supported, but the value of the  capture_top  field  is
       always 1, and the value of the capture_last field is always 0.

       7.  The  \C  escape  sequence, which (in the standard algorithm) always
       matches a single code unit, even in a UTF mode,  is  not  supported  in
       these  modes,  because the alternative algorithm moves through the sub-
       ject string one character (not code unit) at a  time,  for  all  active
       paths through the tree.

       8.  Except for (*FAIL), the backtracking control verbs such as (*PRUNE)
       are not supported. (*FAIL) is supported, and  behaves  like  a  failing
       negative assertion.


ADVANTAGES OF THE ALTERNATIVE ALGORITHM

       Using  the alternative matching algorithm provides the following advan-
       tages:

       1. All possible matches (at a single point in the subject) are automat-
       ically  found,  and  in particular, the longest match is found. To find
       more than one match using the standard algorithm, you have to do kludgy
       things with callouts.

       2.  Because  the  alternative  algorithm  scans the subject string just
       once, and never needs to backtrack (except for lookbehinds), it is pos-
       sible  to  pass  very  long subject strings to the matching function in
       several pieces, checking for partial matching each time. Although it is
       also  possible  to  do  multi-segment matching using the standard algo-
       rithm, by retaining partially matched substrings, it  is  more  compli-
       cated. The pcre2partial documentation gives details of partial matching
       and discusses multi-segment matching.


DISADVANTAGES OF THE ALTERNATIVE ALGORITHM

       The alternative algorithm suffers from a number of disadvantages:

       1. It is substantially slower than  the  standard  algorithm.  This  is
       partly  because  it has to search for all possible matches, but is also
       because it is less susceptible to optimization.

       2. Capturing parentheses and back references are not supported.

       3. Although atomic groups are supported, their use does not provide the
       performance advantage that it does for the standard algorithm.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge, England.


REVISION

       Last updated: 29 September 2014
       Copyright (c) 1997-2014 University of Cambridge.
------------------------------------------------------------------------------


PCRE2PARTIAL(3)            Library Functions Manual            PCRE2PARTIAL(3)



NAME
       PCRE2 - Perl-compatible regular expressions

PARTIAL MATCHING IN PCRE2

       In  normal  use  of  PCRE2,  if  the subject string that is passed to a
       matching function matches as far as it goes, but is too short to  match
       the  entire pattern, PCRE2_ERROR_NOMATCH is returned. There are circum-
       stances where it might be helpful to distinguish this case  from  other
       cases in which there is no match.

       Consider, for example, an application where a human is required to type
       in data for a field with specific formatting requirements.  An  example
       might be a date in the form ddmmmyy, defined by this pattern:

         ^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$

       If the application sees the user's keystrokes one by one, and can check
       that what has been typed so far is potentially valid,  it  is  able  to
       raise  an  error  as  soon  as  a  mistake  is made, by beeping and not
       reflecting the character that has been typed, for example. This immedi-
       ate  feedback is likely to be a better user interface than a check that
       is delayed until the entire string has been entered.  Partial  matching
       can  also be useful when the subject string is very long and is not all
       available at once.

       PCRE2 supports partial matching by means of the PCRE2_PARTIAL_SOFT  and
       PCRE2_PARTIAL_HARD  options,  which  can be set when calling a matching
       function.  The difference between the two options is whether or  not  a
       partial match is preferred to an alternative complete match, though the
       details differ between the two types  of  matching  function.  If  both
       options are set, PCRE2_PARTIAL_HARD takes precedence.

       If  you  want to use partial matching with just-in-time optimized code,
       you must call pcre2_jit_compile() with one or both of these options:

         PCRE2_JIT_PARTIAL_SOFT
         PCRE2_JIT_PARTIAL_HARD

       PCRE2_JIT_COMPLETE should also be set if you are going to run  non-par-
       tial  matches  on the same pattern. If the appropriate JIT mode has not
       been compiled, interpretive matching code is used.

       Setting a partial matching option  disables  two  of  PCRE2's  standard
       optimizations. PCRE2 remembers the last literal code unit in a pattern,
       and abandons matching immediately if it is not present in  the  subject
       string.  This  optimization  cannot  be  used for a subject string that
       might match only partially. PCRE2 also knows the minimum  length  of  a
       matching  string,  and  does not bother to run the matching function on
       shorter strings. This optimization is also disabled for partial  match-
       ing.


PARTIAL MATCHING USING pcre2_match()

       A  partial  match occurs during a call to pcre2_match() when the end of
       the subject string is reached successfully, but  matching  cannot  con-
       tinue because more characters are needed. However, at least one charac-
       ter in the subject must have been inspected. This  character  need  not
       form part of the final matched string; lookbehind assertions and the \K
       escape sequence provide ways of inspecting characters before the  start
       of  a matched string. The requirement for inspecting at least one char-
       acter exists because an empty string can  always  be  matched;  without
       such  a  restriction  there would always be a partial match of an empty
       string at the end of the subject.

       When a partial match is returned, the first two elements in the ovector
       point to the portion of the subject that was matched, but the values in
       the rest of the ovector are undefined. The appearance of \K in the pat-
       tern has no effect for a partial match. Consider this pattern:

         /abc\K123/

       If it is matched against "456abc123xyz" the result is a complete match,
       and the ovector defines the matched string as "123", because \K  resets
       the  "start  of  match" point. However, if a partial match is requested
       and the subject string is "456abc12", a partial match is found for  the
       string  "abc12",  because  all these characters are needed for a subse-
       quent re-match with additional characters.

       What happens when a partial match is identified depends on which of the
       two partial matching options are set.

   PCRE2_PARTIAL_SOFT WITH pcre2_match()

       If  PCRE2_PARTIAL_SOFT  is  set when pcre2_match() identifies a partial
       match, the partial match is remembered, but matching continues as  nor-
       mal,  and  other  alternatives in the pattern are tried. If no complete
       match  can  be  found,  PCRE2_ERROR_PARTIAL  is  returned  instead   of
       PCRE2_ERROR_NOMATCH.

       This  option  is "soft" because it prefers a complete match over a par-
       tial match.  All the various matching items in a pattern behave  as  if
       the  subject string is potentially complete. For example, \z, \Z, and $
       match at the end of the subject, as normal, and for \b and \B  the  end
       of the subject is treated as a non-alphanumeric.

       If  there  is more than one partial match, the first one that was found
       provides the data that is returned. Consider this pattern:

         /123\w+X|dogY/

       If this is matched against the subject string "abc123dog", both  alter-
       natives  fail  to  match,  but the end of the subject is reached during
       matching, so PCRE2_ERROR_PARTIAL is returned. The offsets are set to  3
       and  9, identifying "123dog" as the first partial match that was found.
       (In this example, there are two partial matches, because "dog"  on  its
       own partially matches the second alternative.)

   PCRE2_PARTIAL_HARD WITH pcre2_match()

       If  PCRE2_PARTIAL_HARD is set for pcre2_match(), PCRE2_ERROR_PARTIAL is
       returned as soon as a partial match is  found,  without  continuing  to
       search  for possible complete matches. This option is "hard" because it
       prefers an earlier partial match over a later complete match. For  this
       reason,  the  assumption  is  made that the end of the supplied subject
       string may not be the true end of the available data, and  so,  if  \z,
       \Z,  \b, \B, or $ are encountered at the end of the subject, the result
       is PCRE2_ERROR_PARTIAL, provided that at least  one  character  in  the
       subject has been inspected.

   Comparing hard and soft partial matching

       The  difference  between the two partial matching options can be illus-
       trated by a pattern such as:

         /dog(sbody)?/

       This matches either "dog" or "dogsbody", greedily (that is, it  prefers
       the  longer  string  if  possible). If it is matched against the string
       "dog" with PCRE2_PARTIAL_SOFT, it yields a complete  match  for  "dog".
       However,  if  PCRE2_PARTIAL_HARD is set, the result is PCRE2_ERROR_PAR-
       TIAL. On the other hand, if the pattern is made ungreedy the result  is
       different:

         /dog(sbody)??/

       In  this  case  the  result  is always a complete match because that is
       found first, and matching never  continues  after  finding  a  complete
       match. It might be easier to follow this explanation by thinking of the
       two patterns like this:

         /dog(sbody)?/    is the same as  /dogsbody|dog/
         /dog(sbody)??/   is the same as  /dog|dogsbody/

       The second pattern will never match "dogsbody", because it will  always
       find the shorter match first.


PARTIAL MATCHING USING pcre2_dfa_match()

       The DFA functions move along the subject string character by character,
       without backtracking, searching for  all  possible  matches  simultane-
       ously.  If the end of the subject is reached before the end of the pat-
       tern, there is the possibility of a partial match, again provided  that
       at least one character has been inspected.

       When PCRE2_PARTIAL_SOFT is set, PCRE2_ERROR_PARTIAL is returned only if
       there have been no complete matches. Otherwise,  the  complete  matches
       are  returned.   However, if PCRE2_PARTIAL_HARD is set, a partial match
       takes precedence over any complete matches. The portion of  the  string
       that was matched when the longest partial match was found is set as the
       first matching string.

       Because the DFA functions always search for all possible  matches,  and
       there  is  no  difference between greedy and ungreedy repetition, their
       behaviour is different from  the  standard  functions  when  PCRE2_PAR-
       TIAL_HARD  is  set.  Consider  the  string  "dog"  matched  against the
       ungreedy pattern shown above:

         /dog(sbody)??/

       Whereas the standard function stops as soon as it  finds  the  complete
       match  for  "dog",  the  DFA  function also finds the partial match for
       "dogsbody", and so returns that when PCRE2_PARTIAL_HARD is set.


PARTIAL MATCHING AND WORD BOUNDARIES

       If a pattern ends with one of sequences \b or \B, which test  for  word
       boundaries,  partial matching with PCRE2_PARTIAL_SOFT can give counter-
       intuitive results. Consider this pattern:

         /\bcat\b/

       This matches "cat", provided there is a word boundary at either end. If
       the subject string is "the cat", the comparison of the final "t" with a
       following character cannot take place, so a  partial  match  is  found.
       However,  normal  matching carries on, and \b matches at the end of the
       subject when the last character is a letter, so  a  complete  match  is
       found.   The  result,  therefore,  is  not  PCRE2_ERROR_PARTIAL.  Using
       PCRE2_PARTIAL_HARD in this case does yield PCRE2_ERROR_PARTIAL, because
       then the partial match takes precedence.


EXAMPLE OF PARTIAL MATCHING USING PCRE2TEST

       If  the  partial_soft  (or  ps) modifier is present on a pcre2test data
       line, the PCRE2_PARTIAL_SOFT option is used for the match.  Here  is  a
       run of pcre2test that uses the date example quoted above:

           re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
         data> 25jun04\=ps
          0: 25jun04
          1: jun
         data> 25dec3\=ps
         Partial match: 23dec3
         data> 3ju\=ps
         Partial match: 3ju
         data> 3juj\=ps
         No match
         data> j\=ps
         No match

       The  first  data  string  is matched completely, so pcre2test shows the
       matched substrings. The remaining four strings do not  match  the  com-
       plete pattern, but the first two are partial matches. Similar output is
       obtained if DFA matching is used.

       If the partial_hard (or ph) modifier is present  on  a  pcre2test  data
       line, the PCRE2_PARTIAL_HARD option is set for the match.


MULTI-SEGMENT MATCHING WITH pcre2_dfa_match()

       When  a  partial match has been found using a DFA matching function, it
       is possible to continue the match by providing additional subject  data
       and  calling  the function again with the same compiled regular expres-
       sion, this time setting the PCRE2_DFA_RESTART option. You must pass the
       same working space as before, because this is where details of the pre-
       vious partial match are stored. Here is an example using pcre2test:

           re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
         data> 23ja\=dfa,ps
         Partial match: 23ja
         data> n05\=dfa,dfa_restart
          0: n05

       The first call has "23ja" as the subject, and requests  partial  match-
       ing;  the  second  call  has  "n05"  as  the  subject for the continued
       (restarted) match.  Notice that when the match is  complete,  only  the
       last  part  is  shown;  PCRE2 does not retain the previously partially-
       matched string. It is up to the calling program to do that if it  needs
       to.

       That means that, for an unanchored pattern, if a continued match fails,
       it is not possible to try again at  a  new  starting  point.  All  this
       facility  is  capable  of  doing  is continuing with the previous match
       attempt. In the previous example, if the second set of data  is  "ug23"
       the  result is no match, even though there would be a match for "aug23"
       if the entire string were given at once. Depending on the  application,
       this may or may not be what you want.  The only way to allow for start-
       ing again at the next character is to retain the matched  part  of  the
       subject and try a new complete match.

       You  can  set the PCRE2_PARTIAL_SOFT or PCRE2_PARTIAL_HARD options with
       PCRE2_DFA_RESTART to continue partial matching over multiple  segments.
       This  facility can be used to pass very long subject strings to the DFA
       matching functions.


MULTI-SEGMENT MATCHING WITH pcre2_match()

       Unlike the DFA function, it is not possible  to  restart  the  previous
       match with a new segment of data when using pcre2_match(). Instead, new
       data must be added to the previous subject string, and the entire match
       re-run,  starting from the point where the partial match occurred. Ear-
       lier data can be discarded.

       It is best to use PCRE2_PARTIAL_HARD in this situation, because it does
       not  treat the end of a segment as the end of the subject when matching
       \z, \Z, \b, \B, and $. Consider  an  unanchored  pattern  that  matches
       dates:

           re> /\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d/
         data> The date is 23ja\=ph
         Partial match: 23ja

       At  this stage, an application could discard the text preceding "23ja",
       add on text from the next  segment,  and  call  the  matching  function
       again.  Unlike  the  DFA  matching function, the entire matching string
       must always be available, and the complete matching process occurs  for
       each call, so more memory and more processing time is needed.


ISSUES WITH MULTI-SEGMENT MATCHING

       Certain types of pattern may give problems with multi-segment matching,
       whichever matching function is used.

       1. If the pattern contains a test for the beginning of a line, you need
       to  pass  the  PCRE2_NOTBOL option when the subject string for any call
       does start at the beginning of a line. There  is  also  a  PCRE2_NOTEOL
       option, but in practice when doing multi-segment matching you should be
       using PCRE2_PARTIAL_HARD, which includes the effect of PCRE2_NOTEOL.

       2. If a pattern contains a lookbehind assertion, characters  that  pre-
       cede  the start of the partial match may have been inspected during the
       matching process.  When using pcre2_match(), sufficient characters must
       be  retained  for  the  next  match attempt. You can ensure that enough
       characters are retained by doing the following:

       Before doing any matching, find the length of the longest lookbehind in
       the     pattern    by    calling    pcre2_pattern_info()    with    the
       PCRE2_INFO_MAXLOOKBEHIND option. Note that the resulting  count  is  in
       characters, not code units. After a partial match, moving back from the
       ovector[0] offset in the subject by the number of characters given  for
       the  maximum lookbehind gets you to the earliest character that must be
       retained. In a non-UTF or a 32-bit situation, moving  back  is  just  a
       subtraction,  but in UTF-8 or UTF-16 you have to count characters while
       moving back through the code units.

       Characters before the point you have now reached can be discarded,  and
       after  the  next segment has been added to what is retained, you should
       run the next match with the startoffset argument set so that the  match
       begins at the same point as before.

       For  example, if the pattern "(?<=123)abc" is partially matched against
       the string "xx123ab", the ovector offsets are 5 and 7 ("ab"). The maxi-
       mum  lookbehind  count  is  3, so all characters before offset 2 can be
       discarded. The value of startoffset for the next  match  should  be  3.
       When  pcre2test  displays  a partial match, it indicates the lookbehind
       characters with '<' characters:

           re> "(?<=123)abc"
         data> xx123ab\=ph
         Partial match: 123ab
                        <<<

       3. Because a partial match must always contain at least one  character,
       what  might  be  considered a partial match of an empty string actually
       gives a "no match" result. For example:

           re> /c(?<=abc)x/
         data> ab\=ps
         No match

       If the next segment begins "cx", a match should be found, but this will
       only  happen  if characters from the previous segment are retained. For
       this reason, a "no match" result  should  be  interpreted  as  "partial
       match of an empty string" when the pattern contains lookbehinds.

       4.  Matching  a subject string that is split into multiple segments may
       not always produce exactly the same result as matching over one  single
       long  string,  especially  when PCRE2_PARTIAL_SOFT is used. The section
       "Partial Matching and Word Boundaries" above describes  an  issue  that
       arises  if  the  pattern ends with \b or \B. Another kind of difference
       may occur when there are multiple matching possibilities, because  (for
       PCRE2_PARTIAL_SOFT) a partial match result is given only when there are
       no completed matches. This means that as soon as the shortest match has
       been  found,  continuation to a new subject segment is no longer possi-
       ble. Consider this pcre2test example:

           re> /dog(sbody)?/
         data> dogsb\=ps
          0: dog
         data> do\=ps,dfa
         Partial match: do
         data> gsb\=ps,dfa,dfa_restart
          0: g
         data> dogsbody\=dfa
          0: dogsbody
          1: dog

       The first data line passes the string "dogsb" to  a  standard  matching
       function, setting the PCRE2_PARTIAL_SOFT option. Although the string is
       a partial match for "dogsbody", the result is not  PCRE2_ERROR_PARTIAL,
       because  the  shorter string "dog" is a complete match. Similarly, when
       the subject is presented to a DFA matching function  in  several  parts
       ("do"  and  "gsb"  being  the first two) the match stops when "dog" has
       been found, and it is not possible to continue.  On the other hand,  if
       "dogsbody"  is  presented  as  a single string, a DFA matching function
       finds both matches.

       Because of these problems, it is best to  use  PCRE2_PARTIAL_HARD  when
       matching  multi-segment  data.  The  example above then behaves differ-
       ently:

           re> /dog(sbody)?/
         data> dogsb\=ph
         Partial match: dogsb
         data> do\=ps,dfa
         Partial match: do
         data> gsb\=ph,dfa,dfa_restart
         Partial match: gsb

       5. Patterns that contain alternatives at the top level which do not all
       start  with  the  same  pattern  item  may  not  work  as expected when
       PCRE2_DFA_RESTART is used. For example, consider this pattern:

         1234|3789

       If the first part of the subject is "ABC123", a partial  match  of  the
       first  alternative  is found at offset 3. There is no partial match for
       the second alternative, because such a match does not start at the same
       point  in  the  subject  string. Attempting to continue with the string
       "7890" does not yield a match  because  only  those  alternatives  that
       match  at  one  point in the subject are remembered. The problem arises
       because the start of the second alternative matches  within  the  first
       alternative.  There  is  no  problem with anchored patterns or patterns
       such as:

         1234|ABCD

       where no string can be a partial match for both alternatives.  This  is
       not  a  problem  if  a  standard matching function is used, because the
       entire match has to be rerun each time:

           re> /1234|3789/
         data> ABC123\=ph
         Partial match: 123
         data> 1237890
          0: 3789

       Of course, instead of using PCRE2_DFA_RESTART, the  same  technique  of
       re-running  the  entire  match  can  also be used with the DFA matching
       function. Another possibility is to work with two buffers. If a partial
       match  at  offset  n in the first buffer is followed by "no match" when
       PCRE2_DFA_RESTART is used on the second buffer, you can then try a  new
       match starting at offset n+1 in the first buffer.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge, England.


REVISION

       Last updated: 22 December 2014
       Copyright (c) 1997-2014 University of Cambridge.
------------------------------------------------------------------------------


PCRE2PATTERN(3)            Library Functions Manual            PCRE2PATTERN(3)



NAME
       PCRE2 - Perl-compatible regular expressions (revised API)

PCRE2 REGULAR EXPRESSION DETAILS

       The  syntax and semantics of the regular expressions that are supported
       by PCRE2 are described in detail below. There is a quick-reference syn-
       tax  summary  in the pcre2syntax page. PCRE2 tries to match Perl syntax
       and semantics as closely as it can.  PCRE2 also supports some  alterna-
       tive  regular  expression syntax (which does not conflict with the Perl
       syntax) in order to provide some compatibility with regular expressions
       in Python, .NET, and Oniguruma.

       Perl's  regular expressions are described in its own documentation, and
       regular expressions in general are covered in a number of  books,  some
       of  which  have  copious  examples. Jeffrey Friedl's "Mastering Regular
       Expressions", published by  O'Reilly,  covers  regular  expressions  in
       great  detail.  This  description  of  PCRE2's  regular  expressions is
       intended as reference material.

       This document discusses the patterns that are supported by  PCRE2  when
       its  main  matching function, pcre2_match(), is used. PCRE2 also has an
       alternative matching function, pcre2_dfa_match(), which matches using a
       different  algorithm  that is not Perl-compatible. Some of the features
       discussed below are not available when DFA matching is used. The advan-
       tages and disadvantages of the alternative function, and how it differs
       from the normal function, are discussed in the pcre2matching page.


SPECIAL START-OF-PATTERN ITEMS

       A number of options that can be passed to pcre2_compile() can  also  be
       set by special items at the start of a pattern. These are not Perl-com-
       patible, but are provided to make these options accessible  to  pattern
       writers  who are not able to change the program that processes the pat-
       tern. Any number of these items  may  appear,  but  they  must  all  be
       together right at the start of the pattern string, and the letters must
       be in upper case.

   UTF support

       In the 8-bit and 16-bit PCRE2 libraries, characters may be coded either
       as single code units, or as multiple UTF-8 or UTF-16 code units. UTF-32
       can be specified for the 32-bit library, in which  case  it  constrains
       the  character  values  to  valid  Unicode  code points. To process UTF
       strings, PCRE2 must be built to include Unicode support (which  is  the
       default).  When  using  UTF  strings you must either call the compiling
       function with the PCRE2_UTF option, or the pattern must start with  the
       special  sequence  (*UTF),  which is equivalent to setting the relevant
       option. How setting a UTF mode affects pattern matching is mentioned in
       several  places  below.  There  is  also  a  summary of features in the
       pcre2unicode page.

       Some applications that allow their users to supply patterns may wish to
       restrict   them   to   non-UTF   data  for  security  reasons.  If  the
       PCRE2_NEVER_UTF option is passed  to  pcre2_compile(),  (*UTF)  is  not
       allowed, and its appearance in a pattern causes an error.

   Unicode property support

       Another  special  sequence that may appear at the start of a pattern is
       (*UCP).  This has the same effect as setting the PCRE2_UCP  option:  it
       causes  sequences such as \d and \w to use Unicode properties to deter-
       mine character types, instead of recognizing only characters with codes
       less than 256 via a lookup table.

       Some applications that allow their users to supply patterns may wish to
       restrict them for security reasons. If the  PCRE2_NEVER_UCP  option  is
       passed to pcre2_compile(), (*UCP) is not allowed, and its appearance in
       a pattern causes an error.

   Locking out empty string matching

       Starting a pattern with (*NOTEMPTY) or (*NOTEMPTY_ATSTART) has the same
       effect  as  passing the PCRE2_NOTEMPTY or PCRE2_NOTEMPTY_ATSTART option
       to whichever matching function is subsequently called to match the pat-
       tern.  These  options  lock  out  the matching of empty strings, either
       entirely, or only at the start of the subject.

   Disabling auto-possessification

       If a pattern starts with (*NO_AUTO_POSSESS), it has the same effect  as
       setting  the PCRE2_NO_AUTO_POSSESS option. This stops PCRE2 from making
       quantifiers possessive when what  follows  cannot  match  the  repeated
       item. For example, by default a+b is treated as a++b. For more details,
       see the pcre2api documentation.

   Disabling start-up optimizations

       If a pattern starts with (*NO_START_OPT), it has  the  same  effect  as
       setting the PCRE2_NO_START_OPTIMIZE option. This disables several opti-
       mizations for quickly reaching "no match" results.  For  more  details,
       see the pcre2api documentation.

   Disabling automatic anchoring

       If  a  pattern starts with (*NO_DOTSTAR_ANCHOR), it has the same effect
       as setting the PCRE2_NO_DOTSTAR_ANCHOR option. This disables  optimiza-
       tions that apply to patterns whose top-level branches all start with .*
       (match any number of arbitrary characters). For more details,  see  the
       pcre2api documentation.

   Disabling JIT compilation

       If  a  pattern  that starts with (*NO_JIT) is successfully compiled, an
       attempt by the application to apply the  JIT  optimization  by  calling
       pcre2_jit_compile() is ignored.

   Setting match and recursion limits

       The  caller of pcre2_match() can set a limit on the number of times the
       internal match() function is called and on the maximum depth of  recur-
       sive calls. These facilities are provided to catch runaway matches that
       are provoked by patterns with huge matching trees (a typical example is
       a  pattern  with  nested unlimited repeats) and to avoid running out of
       system stack by too  much  recursion.  When  one  of  these  limits  is
       reached,  pcre2_match()  gives  an error return. The limits can also be
       set by items at the start of the pattern of the form

         (*LIMIT_MATCH=d)
         (*LIMIT_RECURSION=d)

       where d is any number of decimal digits. However, the value of the set-
       ting  must  be  less than the value set (or defaulted) by the caller of
       pcre2_match() for it to have any effect. In other  words,  the  pattern
       writer  can lower the limits set by the programmer, but not raise them.
       If there is more than one setting of one of  these  limits,  the  lower
       value is used.

   Newline conventions

       PCRE2 supports five different conventions for indicating line breaks in
       strings: a single CR (carriage return) character, a  single  LF  (line-
       feed) character, the two-character sequence CRLF, any of the three pre-
       ceding, or any Unicode newline sequence. The pcre2api page has  further
       discussion  about newlines, and shows how to set the newline convention
       when calling pcre2_compile().

       It is also possible to specify a newline convention by starting a  pat-
       tern string with one of the following five sequences:

         (*CR)        carriage return
         (*LF)        linefeed
         (*CRLF)      carriage return, followed by linefeed
         (*ANYCRLF)   any of the three above
         (*ANY)       all Unicode newline sequences

       These override the default and the options given to the compiling func-
       tion. For example, on a Unix system where LF  is  the  default  newline
       sequence, the pattern

         (*CR)a.b

       changes the convention to CR. That pattern matches "a\nb" because LF is
       no longer a newline. If more than one of these settings is present, the
       last one is used.

       The  newline  convention affects where the circumflex and dollar asser-
       tions are true. It also affects the interpretation of the dot metachar-
       acter  when  PCRE2_DOTALL is not set, and the behaviour of \N. However,
       it does not affect what the \R escape  sequence  matches.  By  default,
       this  is any Unicode newline sequence, for Perl compatibility. However,
       this can be changed; see the description of \R in the section  entitled
       "Newline  sequences" below. A change of \R setting can be combined with
       a change of newline convention.

   Specifying what \R matches

       It is possible to restrict \R to match only CR, LF, or CRLF (instead of
       the  complete  set  of  Unicode  line  endings)  by  setting the option
       PCRE2_BSR_ANYCRLF at compile time. This effect can also be achieved  by
       starting  a  pattern  with (*BSR_ANYCRLF). For completeness, (*BSR_UNI-
       CODE) is also recognized, corresponding to PCRE2_BSR_UNICODE.


EBCDIC CHARACTER CODES

       PCRE2 can be compiled to run in an environment that uses EBCDIC as  its
       character code rather than ASCII or Unicode (typically a mainframe sys-
       tem). In the sections below, character code values are  ASCII  or  Uni-
       code; in an EBCDIC environment these characters may have different code
       values, and there are no code points greater than 255.


CHARACTERS AND METACHARACTERS

       A regular expression is a pattern that is  matched  against  a  subject
       string  from  left  to right. Most characters stand for themselves in a
       pattern, and match the corresponding characters in the  subject.  As  a
       trivial example, the pattern

         The quick brown fox

       matches a portion of a subject string that is identical to itself. When
       caseless matching is specified (the PCRE2_CASELESS option), letters are
       matched independently of case.

       The  power  of  regular  expressions  comes from the ability to include
       alternatives and repetitions in the pattern. These are encoded  in  the
       pattern by the use of metacharacters, which do not stand for themselves
       but instead are interpreted in some special way.

       There are two different sets of metacharacters: those that  are  recog-
       nized  anywhere in the pattern except within square brackets, and those
       that are recognized within square brackets.  Outside  square  brackets,
       the metacharacters are as follows:

         \      general escape character with several uses
         ^      assert start of string (or line, in multiline mode)
         $      assert end of string (or line, in multiline mode)
         .      match any character except newline (by default)
         [      start character class definition
         |      start of alternative branch
         (      start subpattern
         )      end subpattern
         ?      extends the meaning of (
                also 0 or 1 quantifier
                also quantifier minimizer
         *      0 or more quantifier
         +      1 or more quantifier
                also "possessive quantifier"
         {      start min/max quantifier

       Part  of  a  pattern  that is in square brackets is called a "character
       class". In a character class the only metacharacters are:

         \      general escape character
         ^      negate the class, but only if the first character
         -      indicates character range
         [      POSIX character class (only if followed by POSIX
                  syntax)
         ]      terminates the character class

       The following sections describe the use of each of the metacharacters.


BACKSLASH

       The backslash character has several uses. Firstly, if it is followed by
       a character that is not a number or a letter, it takes away any special
       meaning that character may have. This use of  backslash  as  an  escape
       character applies both inside and outside character classes.

       For  example,  if  you want to match a * character, you write \* in the
       pattern.  This escaping action applies whether  or  not  the  following
       character  would  otherwise be interpreted as a metacharacter, so it is
       always safe to precede a non-alphanumeric  with  backslash  to  specify
       that  it stands for itself. In particular, if you want to match a back-
       slash, you write \\.

       In a UTF mode, only ASCII numbers and letters have any special  meaning
       after  a  backslash.  All  other characters (in particular, those whose
       codepoints are greater than 127) are treated as literals.

       If a pattern is compiled with the  PCRE2_EXTENDED  option,  most  white
       space  in the pattern (other than in a character class), and characters
       between a # outside a character class and the next newline,  inclusive,
       are ignored. An escaping backslash can be used to include a white space
       or # character as part of the pattern.

       If you want to remove the special meaning from a  sequence  of  charac-
       ters,  you can do so by putting them between \Q and \E. This is differ-
       ent from Perl in that $ and  @  are  handled  as  literals  in  \Q...\E
       sequences  in PCRE2, whereas in Perl, $ and @ cause variable interpola-
       tion. Note the following examples:

         Pattern            PCRE2 matches   Perl matches

         \Qabc$xyz\E        abc$xyz        abc followed by the
                                             contents of $xyz
         \Qabc\$xyz\E       abc\$xyz       abc\$xyz
         \Qabc\E\$\Qxyz\E   abc$xyz        abc$xyz

       The \Q...\E sequence is recognized both inside  and  outside  character
       classes.   An  isolated \E that is not preceded by \Q is ignored. If \Q
       is not followed by \E later in the pattern, the literal  interpretation
       continues  to  the  end  of  the pattern (that is, \E is assumed at the
       end). If the isolated \Q is inside a character class,  this  causes  an
       error, because the character class is not terminated.

   Non-printing characters

       A second use of backslash provides a way of encoding non-printing char-
       acters in patterns in a visible manner. There is no restriction on  the
       appearance  of non-printing characters in a pattern, but when a pattern
       is being prepared by text editing, it is often easier to use one of the
       following  escape sequences than the binary character it represents. In
       an ASCII or Unicode environment, these escapes are as follows:

         \a        alarm, that is, the BEL character (hex 07)
         \cx       "control-x", where x is any printable ASCII character
         \e        escape (hex 1B)
         \f        form feed (hex 0C)
         \n        linefeed (hex 0A)
         \r        carriage return (hex 0D)
         \t        tab (hex 09)
         \0dd      character with octal code 0dd
         \ddd      character with octal code ddd, or back reference
         \o{ddd..} character with octal code ddd..
         \xhh      character with hex code hh
         \x{hhh..} character with hex code hhh.. (default mode)
         \uhhhh    character with hex code hhhh (when PCRE2_ALT_BSUX is set)

       The precise effect of \cx on ASCII characters is as follows: if x is  a
       lower  case  letter,  it  is converted to upper case. Then bit 6 of the
       character (hex 40) is inverted. Thus \cA to \cZ become hex 01 to hex 1A
       (A  is  41, Z is 5A), but \c{ becomes hex 3B ({ is 7B), and \c; becomes
       hex 7B (; is 3B). If the code unit following \c has a value  less  than
       32  or  greater  than  126, a compile-time error occurs. This locks out
       non-printable ASCII characters in all modes.

       When PCRE2 is compiled in EBCDIC mode, \a, \e, \f, \n, \r, and \t  gen-
       erate the appropriate EBCDIC code values. The \c escape is processed as
       specified for Perl in the perlebcdic document. The only characters that
       are  allowed  after  \c are A-Z, a-z, or one of @, [, \, ], ^, _, or ?.
       Any other character provokes a  compile-time  error.  The  sequence  \@
       encodes  character  code 0; the letters (in either case) encode charac-
       ters 1-26 (hex 01 to hex 1A); [, \, ], ^, and _ encode characters 27-31
       (hex 1B to hex 1F), and \? becomes either 255 (hex FF) or 95 (hex 5F).

       Thus,  apart  from  \?,  these escapes generate the same character code
       values as they do in an ASCII environment, though the meanings  of  the
       values  mostly  differ.  For example, \G always generates code value 7,
       which is BEL in ASCII but DEL in EBCDIC.

       The sequence \? generates DEL (127, hex 7F) in  an  ASCII  environment,
       but  because  127  is  not a control character in EBCDIC, Perl makes it
       generate the APC character. Unfortunately, there are  several  variants
       of  EBCDIC.  In  most  of them the APC character has the value 255 (hex
       FF), but in the one Perl calls POSIX-BC its value is 95  (hex  5F).  If
       certain  other characters have POSIX-BC values, PCRE2 makes \? generate
       95; otherwise it generates 255.

       After \0 up to two further octal digits are read. If  there  are  fewer
       than  two  digits,  just  those  that  are  present  are used. Thus the
       sequence \0\x\015 specifies two binary zeros followed by a CR character
       (code value 13). Make sure you supply two digits after the initial zero
       if the pattern character that follows is itself an octal digit.

       The escape \o must be followed by a sequence of octal digits,  enclosed
       in  braces.  An  error occurs if this is not the case. This escape is a
       recent addition to Perl; it provides way of specifying  character  code
       points  as  octal  numbers  greater than 0777, and it also allows octal
       numbers and back references to be unambiguously specified.

       For greater clarity and unambiguity, it is best to avoid following \ by
       a digit greater than zero. Instead, use \o{} or \x{} to specify charac-
       ter numbers, and \g{} to specify back references. The  following  para-
       graphs describe the old, ambiguous syntax.

       The handling of a backslash followed by a digit other than 0 is compli-
       cated, and Perl has changed over time, causing PCRE2 also to change.

       Outside a character class, PCRE2 reads the digit and any following dig-
       its as a decimal number. If the number is less than 10, begins with the
       digit 8 or 9, or if there are at least  that  many  previous  capturing
       left  parentheses  in the expression, the entire sequence is taken as a
       back reference. A description of how this works is given later, follow-
       ing  the  discussion  of  parenthesized  subpatterns.  Otherwise, up to
       three octal digits are read to form a character code.

       Inside a character class, PCRE2 handles \8 and \9 as the literal  char-
       acters  "8"  and "9", and otherwise reads up to three octal digits fol-
       lowing the backslash, using them to generate a data character. Any sub-
       sequent  digits  stand for themselves. For example, outside a character
       class:

         \040   is another way of writing an ASCII space
         \40    is the same, provided there are fewer than 40
                   previous capturing subpatterns
         \7     is always a back reference
         \11    might be a back reference, or another way of
                   writing a tab
         \011   is always a tab
         \0113  is a tab followed by the character "3"
         \113   might be a back reference, otherwise the
                   character with octal code 113
         \377   might be a back reference, otherwise
                   the value 255 (decimal)
         \81    is always a back reference

       Note that octal values of 100 or greater that are specified using  this
       syntax  must  not be introduced by a leading zero, because no more than
       three octal digits are ever read.

       By default, after \x that is not followed by {, from zero to two  hexa-
       decimal  digits  are  read (letters can be in upper or lower case). Any
       number of hexadecimal digits may appear between \x{ and }. If a charac-
       ter  other  than  a  hexadecimal digit appears between \x{ and }, or if
       there is no terminating }, an error occurs.

       If the PCRE2_ALT_BSUX option is set, the interpretation  of  \x  is  as
       just described only when it is followed by two hexadecimal digits. Oth-
       erwise, it matches a literal "x" character. In this mode mode,  support
       for  code points greater than 256 is provided by \u, which must be fol-
       lowed by four hexadecimal digits; otherwise it matches  a  literal  "u"
       character.

       Characters whose value is less than 256 can be defined by either of the
       two syntaxes for \x (or by \u in PCRE2_ALT_BSUX mode). There is no dif-
       ference  in  the way they are handled. For example, \xdc is exactly the
       same as \x{dc} (or \u00dc in PCRE2_ALT_BSUX mode).

   Constraints on character values

       Characters that are specified using octal or  hexadecimal  numbers  are
       limited to certain values, as follows:

         8-bit non-UTF mode    less than 0x100
         8-bit UTF-8 mode      less than 0x10ffff and a valid codepoint
         16-bit non-UTF mode   less than 0x10000
         16-bit UTF-16 mode    less than 0x10ffff and a valid codepoint
         32-bit non-UTF mode   less than 0x100000000
         32-bit UTF-32 mode    less than 0x10ffff and a valid codepoint

       Invalid  Unicode  codepoints  are  the  range 0xd800 to 0xdfff (the so-
       called "surrogate" codepoints), and 0xffef.

   Escape sequences in character classes

       All the sequences that define a single character value can be used both
       inside  and  outside character classes. In addition, inside a character
       class, \b is interpreted as the backspace character (hex 08).

       \N is not allowed in a character class. \B, \R, and \X are not  special
       inside  a  character  class.  Like other unrecognized alphabetic escape
       sequences, they cause  an  error.  Outside  a  character  class,  these
       sequences have different meanings.

   Unsupported escape sequences

       In  Perl, the sequences \l, \L, \u, and \U are recognized by its string
       handler and used  to  modify  the  case  of  following  characters.  By
       default, PCRE2 does not support these escape sequences. However, if the
       PCRE2_ALT_BSUX option is set, \U matches a "U" character, and \u can be
       used  to define a character by code point, as described in the previous
       section.

   Absolute and relative back references

       The sequence \g followed by an unsigned or a negative  number,  option-
       ally  enclosed  in braces, is an absolute or relative back reference. A
       named back reference can be coded as \g{name}. Back references are dis-
       cussed later, following the discussion of parenthesized subpatterns.

   Absolute and relative subroutine calls

       For  compatibility with Oniguruma, the non-Perl syntax \g followed by a
       name or a number enclosed either in angle brackets or single quotes, is
       an  alternative  syntax for referencing a subpattern as a "subroutine".
       Details are discussed later.   Note  that  \g{...}  (Perl  syntax)  and
       \g<...>  (Oniguruma  syntax)  are  not synonymous. The former is a back
       reference; the latter is a subroutine call.

   Generic character types

       Another use of backslash is for specifying generic character types:

         \d     any decimal digit
         \D     any character that is not a decimal digit
         \h     any horizontal white space character
         \H     any character that is not a horizontal white space character
         \s     any white space character
         \S     any character that is not a white space character
         \v     any vertical white space character
         \V     any character that is not a vertical white space character
         \w     any "word" character
         \W     any "non-word" character

       There is also the single sequence \N, which matches a non-newline char-
       acter.   This is the same as the "." metacharacter when PCRE2_DOTALL is
       not set. Perl also uses \N to match characters by name; PCRE2 does  not
       support this.

       Each  pair of lower and upper case escape sequences partitions the com-
       plete set of characters into two disjoint  sets.  Any  given  character
       matches  one, and only one, of each pair. The sequences can appear both
       inside and outside character classes. They each match one character  of
       the  appropriate  type.  If the current matching point is at the end of
       the subject string, all of them fail, because there is no character  to
       match.

       The  default  \s  characters  are HT (9), LF (10), VT (11), FF (12), CR
       (13), and space (32), which are defined  as  white  space  in  the  "C"
       locale. This list may vary if locale-specific matching is taking place.
       For example, in some locales the "non-breaking space" character  (\xA0)
       is recognized as white space, and in others the VT character is not.

       A  "word"  character is an underscore or any character that is a letter
       or digit.  By default, the definition of letters  and  digits  is  con-
       trolled by PCRE2's low-valued character tables, and may vary if locale-
       specific matching is taking place (see "Locale support" in the pcre2api
       page).  For  example,  in  a French locale such as "fr_FR" in Unix-like
       systems, or "french" in Windows, some character codes greater than  127
       are  used  for  accented letters, and these are then matched by \w. The
       use of locales with Unicode is discouraged.

       By default, characters whose code points are  greater  than  127  never
       match \d, \s, or \w, and always match \D, \S, and \W, although this may
       be different for characters in the range 128-255  when  locale-specific
       matching  is  happening.   These escape sequences retain their original
       meanings from before Unicode support was available,  mainly  for  effi-
       ciency  reasons.  If  the  PCRE2_UCP  option  is  set, the behaviour is
       changed so that Unicode properties  are  used  to  determine  character
       types, as follows:

         \d  any character that matches \p{Nd} (decimal digit)
         \s  any character that matches \p{Z} or \h or \v
         \w  any character that matches \p{L} or \p{N}, plus underscore

       The  upper case escapes match the inverse sets of characters. Note that
       \d matches only decimal digits, whereas \w matches any  Unicode  digit,
       as well as any Unicode letter, and underscore. Note also that PCRE2_UCP
       affects \b, and \B because they are defined in  terms  of  \w  and  \W.
       Matching these sequences is noticeably slower when PCRE2_UCP is set.

       The  sequences  \h, \H, \v, and \V, in contrast to the other sequences,
       which match only ASCII characters by default, always match  a  specific
       list  of  code  points, whether or not PCRE2_UCP is set. The horizontal
       space characters are:

         U+0009     Horizontal tab (HT)
         U+0020     Space
         U+00A0     Non-break space
         U+1680     Ogham space mark
         U+180E     Mongolian vowel separator
         U+2000     En quad
         U+2001     Em quad
         U+2002     En space
         U+2003     Em space
         U+2004     Three-per-em space
         U+2005     Four-per-em space
         U+2006     Six-per-em space
         U+2007     Figure space
         U+2008     Punctuation space
         U+2009     Thin space
         U+200A     Hair space
         U+202F     Narrow no-break space
         U+205F     Medium mathematical space
         U+3000     Ideographic space

       The vertical space characters are:

         U+000A     Linefeed (LF)
         U+000B     Vertical tab (VT)
         U+000C     Form feed (FF)
         U+000D     Carriage return (CR)
         U+0085     Next line (NEL)
         U+2028     Line separator
         U+2029     Paragraph separator

       In 8-bit, non-UTF-8 mode, only the characters  with  code  points  less
       than 256 are relevant.

   Newline sequences

       Outside  a  character class, by default, the escape sequence \R matches
       any Unicode newline sequence. In 8-bit non-UTF-8 mode \R is  equivalent
       to the following:

         (?>\r\n|\n|\x0b|\f|\r|\x85)

       This  is  an  example  of an "atomic group", details of which are given
       below.  This particular group matches either the two-character sequence
       CR  followed  by  LF,  or  one  of  the single characters LF (linefeed,
       U+000A), VT (vertical tab, U+000B), FF (form feed,  U+000C),  CR  (car-
       riage  return,  U+000D), or NEL (next line, U+0085). Because this is an
       atomic group, the two-character sequence is treated as  a  single  unit
       that cannot be split.

       In  other modes, two additional characters whose codepoints are greater
       than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa-
       rator,  U+2029).  Unicode support is not needed for these characters to
       be recognized.

       It is possible to restrict \R to match only CR, LF, or CRLF (instead of
       the  complete  set  of  Unicode  line  endings)  by  setting the option
       PCRE2_BSR_ANYCRLF at compile time. (BSR is an  abbrevation  for  "back-
       slash R".) This can be made the default when PCRE2 is built; if this is
       the case, the other behaviour can be requested via  the  PCRE2_BSR_UNI-
       CODE  option. It is also possible to specify these settings by starting
       a pattern string with one of the following sequences:

         (*BSR_ANYCRLF)   CR, LF, or CRLF only
         (*BSR_UNICODE)   any Unicode newline sequence

       These override the default and the options given to the compiling func-
       tion.  Note that these special settings, which are not Perl-compatible,
       are recognized only at the very start of a pattern, and that they  must
       be  in upper case. If more than one of them is present, the last one is
       used. They can be combined with a change  of  newline  convention;  for
       example, a pattern can start with:

         (*ANY)(*BSR_ANYCRLF)

       They  can also be combined with the (*UTF) or (*UCP) special sequences.
       Inside a character class, \R  is  treated  as  an  unrecognized  escape
       sequence, and causes an error.

   Unicode character properties

       When  PCRE2  is  built  with Unicode support (the default), three addi-
       tional escape sequences that match characters with specific  properties
       are  available.  In 8-bit non-UTF-8 mode, these sequences are of course
       limited to testing characters whose codepoints are less than  256,  but
       they do work in this mode.  The extra escape sequences are:

         \p{xx}   a character with the xx property
         \P{xx}   a character without the xx property
         \X       a Unicode extended grapheme cluster

       The  property  names represented by xx above are limited to the Unicode
       script names, the general category properties, "Any", which matches any
       character  (including  newline),  and  some  special  PCRE2  properties
       (described in the next section).  Other Perl properties such as  "InMu-
       sicalSymbols"  are  not supported by PCRE2.  Note that \P{Any} does not
       match any characters, so always causes a match failure.

       Sets of Unicode characters are defined as belonging to certain scripts.
       A  character from one of these sets can be matched using a script name.
       For example:

         \p{Greek}
         \P{Han}

       Those that are not part of an identified script are lumped together  as
       "Common". The current list of scripts is:

       Ahom,   Anatolian_Hieroglyphs,  Arabic,  Armenian,  Avestan,  Balinese,
       Bamum, Bassa_Vah, Batak, Bengali, Bopomofo, Brahmi, Braille,  Buginese,
       Buhid,  Canadian_Aboriginal,  Carian, Caucasian_Albanian, Chakma, Cham,
       Cherokee,  Common,  Coptic,  Cuneiform,  Cypriot,  Cyrillic,   Deseret,
       Devanagari,  Duployan,  Egyptian_Hieroglyphs,  Elbasan, Ethiopic, Geor-
       gian, Glagolitic, Gothic,  Grantha,  Greek,  Gujarati,  Gurmukhi,  Han,
       Hangul, Hanunoo, Hatran, Hebrew, Hiragana, Imperial_Aramaic, Inherited,
       Inscriptional_Pahlavi, Inscriptional_Parthian, Javanese,  Kaithi,  Kan-
       nada,  Katakana,  Kayah_Li,  Kharoshthi, Khmer, Khojki, Khudawadi, Lao,
       Latin, Lepcha, Limbu, Linear_A, Linear_B, Lisu, Lycian,  Lydian,  Maha-
       jani,  Malayalam,  Mandaic,  Manichaean,  Meetei_Mayek,  Mende_Kikakui,
       Meroitic_Cursive, Meroitic_Hieroglyphs,  Miao,  Modi,  Mongolian,  Mro,
       Multani,   Myanmar,   Nabataean,  New_Tai_Lue,  Nko,  Ogham,  Ol_Chiki,
       Old_Hungarian, Old_Italic, Old_North_Arabian, Old_Permic,  Old_Persian,
       Old_South_Arabian, Old_Turkic, Oriya, Osmanya, Pahawh_Hmong, Palmyrene,
       Pau_Cin_Hau,  Phags_Pa,  Phoenician,  Psalter_Pahlavi,  Rejang,  Runic,
       Samaritan, Saurashtra, Sharada, Shavian, Siddham, SignWriting, Sinhala,
       Sora_Sompeng,  Sundanese,  Syloti_Nagri,  Syriac,  Tagalog,   Tagbanwa,
       Tai_Le,   Tai_Tham,  Tai_Viet,  Takri,  Tamil,  Telugu,  Thaana,  Thai,
       Tibetan, Tifinagh, Tirhuta, Ugaritic, Vai, Warang_Citi, Yi.

       Each character has exactly one Unicode general category property, spec-
       ified  by a two-letter abbreviation. For compatibility with Perl, nega-
       tion can be specified by including a  circumflex  between  the  opening
       brace  and  the  property  name.  For  example,  \p{^Lu} is the same as
       \P{Lu}.

       If only one letter is specified with \p or \P, it includes all the gen-
       eral  category properties that start with that letter. In this case, in
       the absence of negation, the curly brackets in the escape sequence  are
       optional; these two examples have the same effect:

         \p{L}
         \pL

       The following general category property codes are supported:

         C     Other
         Cc    Control
         Cf    Format
         Cn    Unassigned
         Co    Private use
         Cs    Surrogate

         L     Letter
         Ll    Lower case letter
         Lm    Modifier letter
         Lo    Other letter
         Lt    Title case letter
         Lu    Upper case letter

         M     Mark
         Mc    Spacing mark
         Me    Enclosing mark
         Mn    Non-spacing mark

         N     Number
         Nd    Decimal number
         Nl    Letter number
         No    Other number

         P     Punctuation
         Pc    Connector punctuation
         Pd    Dash punctuation
         Pe    Close punctuation
         Pf    Final punctuation
         Pi    Initial punctuation
         Po    Other punctuation
         Ps    Open punctuation

         S     Symbol
         Sc    Currency symbol
         Sk    Modifier symbol
         Sm    Mathematical symbol
         So    Other symbol

         Z     Separator
         Zl    Line separator
         Zp    Paragraph separator
         Zs    Space separator

       The  special property L& is also supported: it matches a character that
       has the Lu, Ll, or Lt property, in other words, a letter  that  is  not
       classified as a modifier or "other".

       The  Cs  (Surrogate)  property  applies only to characters in the range
       U+D800 to U+DFFF. Such characters are not valid in Unicode strings  and
       so  cannot  be  tested  by PCRE2, unless UTF validity checking has been
       turned off (see the discussion of PCRE2_NO_UTF_CHECK  in  the  pcre2api
       page). Perl does not support the Cs property.

       The  long  synonyms  for  property  names  that  Perl supports (such as
       \p{Letter}) are not supported by PCRE2, nor is it permitted  to  prefix
       any of these properties with "Is".

       No character that is in the Unicode table has the Cn (unassigned) prop-
       erty.  Instead, this property is assumed for any code point that is not
       in the Unicode table.

       Specifying  caseless  matching  does not affect these escape sequences.
       For example, \p{Lu} always matches only upper  case  letters.  This  is
       different from the behaviour of current versions of Perl.

       Matching  characters by Unicode property is not fast, because PCRE2 has
       to do a multistage table lookup in order to find  a  character's  prop-
       erty. That is why the traditional escape sequences such as \d and \w do
       not use Unicode properties in PCRE2 by default,  though  you  can  make
       them  do  so by setting the PCRE2_UCP option or by starting the pattern
       with (*UCP).

   Extended grapheme clusters

       The \X escape matches any number of Unicode  characters  that  form  an
       "extended grapheme cluster", and treats the sequence as an atomic group
       (see below).  Unicode supports various kinds of composite character  by
       giving  each  character  a grapheme breaking property, and having rules
       that use these properties to define the boundaries of extended grapheme
       clusters.  \X  always  matches  at least one character. Then it decides
       whether to add additional characters according to the  following  rules
       for ending a cluster:

       1. End at the end of the subject string.

       2.  Do not end between CR and LF; otherwise end after any control char-
       acter.

       3. Do not break Hangul (a Korean  script)  syllable  sequences.  Hangul
       characters  are of five types: L, V, T, LV, and LVT. An L character may
       be followed by an L, V, LV, or LVT character; an LV or V character  may
       be followed by a V or T character; an LVT or T character may be follwed
       only by a T character.

       4. Do not end before extending characters or spacing marks.  Characters
       with  the  "mark"  property  always have the "extend" grapheme breaking
       property.

       5. Do not end after prepend characters.

       6. Otherwise, end the cluster.

   PCRE2's additional properties

       As well as the standard Unicode properties described above, PCRE2  sup-
       ports  four  more  that  make it possible to convert traditional escape
       sequences such as \w and \s to use Unicode properties. PCRE2 uses these
       non-standard,  non-Perl  properties  internally  when PCRE2_UCP is set.
       However, they may also be used explicitly. These properties are:

         Xan   Any alphanumeric character
         Xps   Any POSIX space character
         Xsp   Any Perl space character
         Xwd   Any Perl "word" character

       Xan matches characters that have either the L (letter) or the  N  (num-
       ber)  property. Xps matches the characters tab, linefeed, vertical tab,
       form feed, or carriage return, and any other character that has  the  Z
       (separator)  property.   Xsp  is  the  same as Xps; in PCRE1 it used to
       exclude vertical tab, for Perl compatibility,  but  Perl  changed.  Xwd
       matches the same characters as Xan, plus underscore.

       There  is another non-standard property, Xuc, which matches any charac-
       ter that can be represented by a Universal Character Name  in  C++  and
       other  programming  languages.  These are the characters $, @, ` (grave
       accent), and all characters with Unicode code points  greater  than  or
       equal  to U+00A0, except for the surrogates U+D800 to U+DFFF. Note that
       most base (ASCII) characters are excluded. (Universal  Character  Names
       are  of  the  form \uHHHH or \UHHHHHHHH where H is a hexadecimal digit.
       Note that the Xuc property does not match these sequences but the char-
       acters that they represent.)

   Resetting the match start

       The  escape sequence \K causes any previously matched characters not to
       be included in the final matched sequence. For example, the pattern:

         foo\Kbar

       matches "foobar", but reports that it has matched "bar".  This  feature
       is  similar  to  a lookbehind assertion (described below).  However, in
       this case, the part of the subject before the real match does not  have
       to  be of fixed length, as lookbehind assertions do. The use of \K does
       not interfere with the setting of captured  substrings.   For  example,
       when the pattern

         (foo)\Kbar

       matches "foobar", the first substring is still set to "foo".

       Perl  documents  that  the  use  of  \K  within assertions is "not well
       defined". In PCRE2, \K is acted upon when  it  occurs  inside  positive
       assertions,  but  is  ignored  in negative assertions. Note that when a
       pattern such as (?=ab\K) matches, the reported start of the  match  can
       be greater than the end of the match.

   Simple assertions

       The  final use of backslash is for certain simple assertions. An asser-
       tion specifies a condition that has to be met at a particular point  in
       a  match, without consuming any characters from the subject string. The
       use of subpatterns for more complicated assertions is described  below.
       The backslashed assertions are:

         \b     matches at a word boundary
         \B     matches when not at a word boundary
         \A     matches at the start of the subject
         \Z     matches at the end of the subject
                 also matches before a newline at the end of the subject
         \z     matches only at the end of the subject
         \G     matches at the first matching position in the subject

       Inside  a  character  class, \b has a different meaning; it matches the
       backspace character. If any other of  these  assertions  appears  in  a
       character class, an "invalid escape sequence" error is generated.

       A  word  boundary is a position in the subject string where the current
       character and the previous character do not both match \w or  \W  (i.e.
       one  matches  \w  and the other matches \W), or the start or end of the
       string if the first or last character matches \w,  respectively.  In  a
       UTF  mode,  the  meanings  of  \w  and \W can be changed by setting the
       PCRE2_UCP option. When this is done, it also affects \b and \B. Neither
       PCRE2  nor Perl has a separate "start of word" or "end of word" metase-
       quence. However, whatever follows \b normally determines which  it  is.
       For example, the fragment \ba matches "a" at the start of a word.

       The  \A,  \Z,  and \z assertions differ from the traditional circumflex
       and dollar (described in the next section) in that they only ever match
       at  the  very start and end of the subject string, whatever options are
       set. Thus, they are independent of multiline mode. These  three  asser-
       tions  are  not  affected  by the PCRE2_NOTBOL or PCRE2_NOTEOL options,
       which affect only the behaviour of the circumflex and dollar  metachar-
       acters.  However,  if the startoffset argument of pcre2_match() is non-
       zero, indicating that matching is to start at a point  other  than  the
       beginning  of  the subject, \A can never match.  The difference between
       \Z and \z is that \Z matches before a newline at the end of the  string
       as well as at the very end, whereas \z matches only at the end.

       The  \G assertion is true only when the current matching position is at
       the start point of the match, as specified by the startoffset  argument
       of  pcre2_match().  It differs from \A when the value of startoffset is
       non-zero. By calling  pcre2_match()  multiple  times  with  appropriate
       arguments,  you  can  mimic Perl's /g option, and it is in this kind of
       implementation where \G can be useful.

       Note, however, that PCRE2's interpretation of \G, as the start  of  the
       current match, is subtly different from Perl's, which defines it as the
       end of the previous match. In Perl, these can  be  different  when  the
       previously  matched string was empty. Because PCRE2 does just one match
       at a time, it cannot reproduce this behaviour.

       If all the alternatives of a pattern begin with \G, the  expression  is
       anchored to the starting match position, and the "anchored" flag is set
       in the compiled regular expression.


CIRCUMFLEX AND DOLLAR

       The circumflex and dollar  metacharacters  are  zero-width  assertions.
       That  is,  they test for a particular condition being true without con-
       suming any characters from the subject string. These two metacharacters
       are  concerned  with matching the starts and ends of lines. If the new-
       line convention is set so that only the two-character sequence CRLF  is
       recognized  as  a newline, isolated CR and LF characters are treated as
       ordinary data characters, and are not recognized as newlines.

       Outside a character class, in the default matching mode, the circumflex
       character  is  an  assertion  that is true only if the current matching
       point is at the start of the subject string. If the  startoffset  argu-
       ment  of  pcre2_match() is non-zero, or if PCRE2_NOTBOL is set, circum-
       flex can never match if the PCRE2_MULTILINE option is unset.  Inside  a
       character  class,  circumflex  has  an  entirely different meaning (see
       below).

       Circumflex need not be the first character of the pattern if  a  number
       of  alternatives are involved, but it should be the first thing in each
       alternative in which it appears if the pattern is ever  to  match  that
       branch.  If all possible alternatives start with a circumflex, that is,
       if the pattern is constrained to match only at the start  of  the  sub-
       ject,  it  is  said  to be an "anchored" pattern. (There are also other
       constructs that can cause a pattern to be anchored.)

       The dollar character is an assertion that is true only if  the  current
       matching  point  is  at  the  end of the subject string, or immediately
       before a newline  at  the  end  of  the  string  (by  default),  unless
       PCRE2_NOTEOL is set. Note, however, that it does not actually match the
       newline. Dollar need not be the last character of the pattern if a num-
       ber of alternatives are involved, but it should be the last item in any
       branch in which it appears. Dollar has no special meaning in a  charac-
       ter class.

       The  meaning  of  dollar  can be changed so that it matches only at the
       very end of the string, by setting the PCRE2_DOLLAR_ENDONLY  option  at
       compile time. This does not affect the \Z assertion.

       The meanings of the circumflex and dollar metacharacters are changed if
       the PCRE2_MULTILINE option is set. When this  is  the  case,  a  dollar
       character  matches before any newlines in the string, as well as at the
       very end, and a circumflex matches immediately after internal  newlines
       as  well as at the start of the subject string. It does not match after
       a newline that ends the string, for compatibility with  Perl.  However,
       this can be changed by setting the PCRE2_ALT_CIRCUMFLEX option.

       For  example, the pattern /^abc$/ matches the subject string "def\nabc"
       (where \n represents a newline) in multiline mode, but  not  otherwise.
       Consequently,  patterns  that  are anchored in single line mode because
       all branches start with ^ are not anchored in  multiline  mode,  and  a
       match  for  circumflex  is  possible  when  the startoffset argument of
       pcre2_match() is non-zero. The PCRE2_DOLLAR_ENDONLY option  is  ignored
       if PCRE2_MULTILINE is set.

       When  the  newline  convention (see "Newline conventions" below) recog-
       nizes the two-character sequence CRLF as a newline, this is  preferred,
       even  if  the  single  characters CR and LF are also recognized as new-
       lines. For example, if the newline convention  is  "any",  a  multiline
       mode  circumflex matches before "xyz" in the string "abc\r\nxyz" rather
       than after CR, even though CR on its own is a valid newline.  (It  also
       matches at the very start of the string, of course.)

       Note  that  the sequences \A, \Z, and \z can be used to match the start
       and end of the subject in both modes, and if all branches of a  pattern
       start  with \A it is always anchored, whether or not PCRE2_MULTILINE is
       set.


FULL STOP (PERIOD, DOT) AND \N

       Outside a character class, a dot in the pattern matches any one charac-
       ter  in  the subject string except (by default) a character that signi-
       fies the end of a line.

       When a line ending is defined as a single character, dot never  matches
       that  character; when the two-character sequence CRLF is used, dot does
       not match CR if it is immediately followed  by  LF,  but  otherwise  it
       matches  all characters (including isolated CRs and LFs). When any Uni-
       code line endings are being recognized, dot does not match CR or LF  or
       any of the other line ending characters.

       The  behaviour  of  dot  with regard to newlines can be changed. If the
       PCRE2_DOTALL option is set, a dot matches any  one  character,  without
       exception.   If  the two-character sequence CRLF is present in the sub-
       ject string, it takes two dots to match it.

       The handling of dot is entirely independent of the handling of  circum-
       flex  and  dollar,  the  only relationship being that they both involve
       newlines. Dot has no special meaning in a character class.

       The escape sequence \N behaves like  a  dot,  except  that  it  is  not
       affected  by  the  PCRE2_DOTALL  option. In other words, it matches any
       character except one that signifies the end of a line. Perl  also  uses
       \N to match characters by name; PCRE2 does not support this.


MATCHING A SINGLE CODE UNIT

       Outside  a character class, the escape sequence \C matches any one code
       unit, whether or not a UTF mode is set. In the 8-bit library, one  code
       unit  is  one  byte;  in the 16-bit library it is a 16-bit unit; in the
       32-bit library it is a 32-bit unit. Unlike a  dot,  \C  always  matches
       line-ending  characters.  The  feature  is provided in Perl in order to
       match individual bytes in UTF-8 mode, but it is unclear how it can use-
       fully be used.

       Because  \C  breaks  up characters into individual code units, matching
       one unit with \C in UTF-8 or UTF-16 mode means that  the  rest  of  the
       string  may  start  with  a malformed UTF character. This has undefined
       results, because PCRE2 assumes that it is matching character by charac-
       ter  in  a  valid UTF string (by default it checks the subject string's
       validity at the  start  of  processing  unless  the  PCRE2_NO_UTF_CHECK
       option is used).

       An   application   can   lock   out  the  use  of  \C  by  setting  the
       PCRE2_NEVER_BACKSLASH_C option when compiling a  pattern.  It  is  also
       possible to build PCRE2 with the use of \C permanently disabled.

       PCRE2  does  not allow \C to appear in lookbehind assertions (described
       below) in UTF-8 or UTF-16 modes, because this would make it  impossible
       to  calculate  the  length  of  the lookbehind. Neither the alternative
       matching function pcre2_dfa_match() nor the JIT optimizer support \C in
       these UTF modes.  The former gives a match-time error; the latter fails
       to optimize and so the match is always run using the interpreter.

       In the 32-bit library,  however,  \C  is  always  supported  (when  not
       explicitly  locked  out)  because it always matches a single code unit,
       whether or not UTF-32 is specified.

       In general, the \C escape sequence is best avoided. However, one way of
       using  it  that avoids the problem of malformed UTF-8 or UTF-16 charac-
       ters is to use a lookahead to check the length of the  next  character,
       as  in  this  pattern,  which could be used with a UTF-8 string (ignore
       white space and line breaks):

         (?| (?=[\x00-\x7f])(\C) |
             (?=[\x80-\x{7ff}])(\C)(\C) |
             (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
             (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))

       In this example, a group that starts  with  (?|  resets  the  capturing
       parentheses numbers in each alternative (see "Duplicate Subpattern Num-
       bers" below). The assertions at the start of each branch check the next
       UTF-8  character  for  values  whose encoding uses 1, 2, 3, or 4 bytes,
       respectively. The character's individual bytes are then captured by the
       appropriate number of \C groups.


SQUARE BRACKETS AND CHARACTER CLASSES

       An opening square bracket introduces a character class, terminated by a
       closing square bracket. A closing square bracket on its own is not spe-
       cial  by  default.  If a closing square bracket is required as a member
       of the class, it should be the first data character in the class (after
       an  initial  circumflex,  if present) or escaped with a backslash. This
       means that, by default, an empty class cannot be defined.  However,  if
       the  PCRE2_ALLOW_EMPTY_CLASS option is set, a closing square bracket at
       the start does end the (empty) class.

       A character class matches a single character in the subject. A  matched
       character must be in the set of characters defined by the class, unless
       the first character in the class definition is a circumflex,  in  which
       case the subject character must not be in the set defined by the class.
       If a circumflex is actually required as a member of the  class,  ensure
       it is not the first character, or escape it with a backslash.

       For  example, the character class [aeiou] matches any lower case vowel,
       while [^aeiou] matches any character that is not a  lower  case  vowel.
       Note that a circumflex is just a convenient notation for specifying the
       characters that are in the class by enumerating those that are  not.  A
       class  that starts with a circumflex is not an assertion; it still con-
       sumes a character from the subject string, and therefore  it  fails  if
       the current pointer is at the end of the string.

       When  caseless  matching  is set, any letters in a class represent both
       their upper case and lower case versions, so for  example,  a  caseless
       [aeiou]  matches  "A"  as well as "a", and a caseless [^aeiou] does not
       match "A", whereas a caseful version would.

       Characters that might indicate line breaks are  never  treated  in  any
       special  way  when  matching  character  classes,  whatever line-ending
       sequence is in use,  and  whatever  setting  of  the  PCRE2_DOTALL  and
       PCRE2_MULTILINE  options  is  used. A class such as [^a] always matches
       one of these characters.

       The minus (hyphen) character can be used to specify a range of  charac-
       ters  in  a  character  class.  For  example,  [d-m] matches any letter
       between d and m, inclusive. If a  minus  character  is  required  in  a
       class,  it  must  be  escaped  with a backslash or appear in a position
       where it cannot be interpreted as indicating a range, typically as  the
       first or last character in the class, or immediately after a range. For
       example, [b-d-z] matches letters in the range b to d, a hyphen  charac-
       ter, or z.

       It is not possible to have the literal character "]" as the end charac-
       ter of a range. A pattern such as [W-]46] is interpreted as a class  of
       two  characters ("W" and "-") followed by a literal string "46]", so it
       would match "W46]" or "-46]". However, if the "]"  is  escaped  with  a
       backslash  it is interpreted as the end of range, so [W-\]46] is inter-
       preted as a class containing a range followed by two other  characters.
       The  octal or hexadecimal representation of "]" can also be used to end
       a range.

       An error is generated if a POSIX character  class  (see  below)  or  an
       escape  sequence other than one that defines a single character appears
       at a point where a range ending character  is  expected.  For  example,
       [z-\xff] is valid, but [A-\d] and [A-[:digit:]] are not.

       Ranges normally include all code points between the start and end char-
       acters, inclusive. They can also be  used  for  code  points  specified
       numerically, for example [\000-\037]. Ranges can include any characters
       that are valid for the current mode.

       There is a special case in EBCDIC environments  for  ranges  whose  end
       points are both specified as literal letters in the same case. For com-
       patibility with Perl, EBCDIC code points within the range that are  not
       letters  are  omitted. For example, [h-k] matches only four characters,
       even though the codes for h and k are 0x88 and 0x92, a range of 11 code
       points.  However,  if  the range is specified numerically, for example,
       [\x88-\x92] or [h-\x92], all code points are included.

       If a range that includes letters is used when caseless matching is set,
       it matches the letters in either case. For example, [W-c] is equivalent
       to [][\\^_`wxyzabc], matched caselessly, and  in  a  non-UTF  mode,  if
       character  tables  for  a French locale are in use, [\xc8-\xcb] matches
       accented E characters in both cases.

       The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v,  \V,
       \w, and \W may appear in a character class, and add the characters that
       they match to the class. For example, [\dABCDEF] matches any  hexadeci-
       mal  digit.  In UTF modes, the PCRE2_UCP option affects the meanings of
       \d, \s, \w and their upper case partners, just as  it  does  when  they
       appear  outside a character class, as described in the section entitled
       "Generic character types" above. The escape sequence \b has a different
       meaning  inside  a character class; it matches the backspace character.
       The sequences \B, \N, \R, and \X are not  special  inside  a  character
       class.  Like  any  other  unrecognized  escape sequences, they cause an
       error.

       A circumflex can conveniently be used with  the  upper  case  character
       types  to specify a more restricted set of characters than the matching
       lower case type.  For example, the class [^\W_] matches any  letter  or
       digit, but not underscore, whereas [\w] includes underscore. A positive
       character class should be read as "something OR something OR ..." and a
       negative class as "NOT something AND NOT something AND NOT ...".

       The  only  metacharacters  that are recognized in character classes are
       backslash, hyphen (only where it can be  interpreted  as  specifying  a
       range),  circumflex  (only  at the start), opening square bracket (only
       when it can be interpreted as introducing a POSIX class name, or for  a
       special  compatibility  feature  -  see the next two sections), and the
       terminating  closing  square  bracket.  However,  escaping  other  non-
       alphanumeric characters does no harm.


POSIX CHARACTER CLASSES

       Perl supports the POSIX notation for character classes. This uses names
       enclosed by [: and :] within the enclosing square brackets. PCRE2  also
       supports this notation. For example,

         [01[:alpha:]%]

       matches "0", "1", any alphabetic character, or "%". The supported class
       names are:

         alnum    letters and digits
         alpha    letters
         ascii    character codes 0 - 127
         blank    space or tab only
         cntrl    control characters
         digit    decimal digits (same as \d)
         graph    printing characters, excluding space
         lower    lower case letters
         print    printing characters, including space
         punct    printing characters, excluding letters and digits and space
         space    white space (the same as \s from PCRE2 8.34)
         upper    upper case letters
         word     "word" characters (same as \w)
         xdigit   hexadecimal digits

       The default "space" characters are HT (9), LF (10), VT (11),  FF  (12),
       CR  (13),  and space (32). If locale-specific matching is taking place,
       the list of space characters may be different; there may  be  fewer  or
       more of them. "Space" and \s match the same set of characters.

       The  name  "word"  is  a Perl extension, and "blank" is a GNU extension
       from Perl 5.8. Another Perl extension is negation, which  is  indicated
       by a ^ character after the colon. For example,

         [12[:^digit:]]

       matches "1", "2", or any non-digit. PCRE2 (and Perl) also recognize the
       POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
       these are not supported, and an error is given if they are encountered.

       By default, characters with values greater than 127 do not match any of
       the POSIX character classes, although this may be different for charac-
       ters  in  the range 128-255 when locale-specific matching is happening.
       However, if the PCRE2_UCP option is passed to pcre2_compile(), some  of
       the  classes are changed so that Unicode character properties are used.
       This  is  achieved  by  replacing  certain  POSIX  classes  with  other
       sequences, as follows:

         [:alnum:]  becomes  \p{Xan}
         [:alpha:]  becomes  \p{L}
         [:blank:]  becomes  \h
         [:cntrl:]  becomes  \p{Cc}
         [:digit:]  becomes  \p{Nd}
         [:lower:]  becomes  \p{Ll}
         [:space:]  becomes  \p{Xps}
         [:upper:]  becomes  \p{Lu}
         [:word:]   becomes  \p{Xwd}

       Negated  versions, such as [:^alpha:] use \P instead of \p. Three other
       POSIX classes are handled specially in UCP mode:

       [:graph:] This matches characters that have glyphs that mark  the  page
                 when printed. In Unicode property terms, it matches all char-
                 acters with the L, M, N, P, S, or Cf properties, except for:

                   U+061C           Arabic Letter Mark
                   U+180E           Mongolian Vowel Separator
                   U+2066 - U+2069  Various "isolate"s


       [:print:] This matches the same  characters  as  [:graph:]  plus  space
                 characters  that  are  not controls, that is, characters with
                 the Zs property.

       [:punct:] This matches all characters that have the Unicode P (punctua-
                 tion)  property,  plus those characters with code points less
                 than 256 that have the S (Symbol) property.

       The other POSIX classes are unchanged, and match only  characters  with
       code points less than 256.


COMPATIBILITY FEATURE FOR WORD BOUNDARIES

       In  the POSIX.2 compliant library that was included in 4.4BSD Unix, the
       ugly syntax [[:<:]] and [[:>:]] is used for matching  "start  of  word"
       and "end of word". PCRE2 treats these items as follows:

         [[:<:]]  is converted to  \b(?=\w)
         [[:>:]]  is converted to  \b(?<=\w)

       Only these exact character sequences are recognized. A sequence such as
       [a[:<:]b] provokes error for an unrecognized  POSIX  class  name.  This
       support  is not compatible with Perl. It is provided to help migrations
       from other environments, and is best not used in any new patterns. Note
       that  \b matches at the start and the end of a word (see "Simple asser-
       tions" above), and in a Perl-style pattern the preceding  or  following
       character  normally  shows  which  is  wanted, without the need for the
       assertions that are used above in order to give exactly the  POSIX  be-
       haviour.


VERTICAL BAR

       Vertical  bar characters are used to separate alternative patterns. For
       example, the pattern

         gilbert|sullivan

       matches either "gilbert" or "sullivan". Any number of alternatives  may
       appear,  and  an  empty  alternative  is  permitted (matching the empty
       string). The matching process tries each alternative in turn, from left
       to  right, and the first one that succeeds is used. If the alternatives
       are within a subpattern (defined below), "succeeds" means matching  the
       rest of the main pattern as well as the alternative in the subpattern.


INTERNAL OPTION SETTING

       The  settings of the PCRE2_CASELESS, PCRE2_MULTILINE, PCRE2_DOTALL, and
       PCRE2_EXTENDED options (which are Perl-compatible) can be changed  from
       within  the  pattern  by  a  sequence  of  Perl option letters enclosed
       between "(?" and ")".  The option letters are

         i  for PCRE2_CASELESS
         m  for PCRE2_MULTILINE
         s  for PCRE2_DOTALL
         x  for PCRE2_EXTENDED

       For example, (?im) sets caseless, multiline matching. It is also possi-
       ble to unset these options by preceding the letter with a hyphen, and a
       combined setting and unsetting such as (?im-sx), which sets PCRE2_CASE-
       LESS    and    PCRE2_MULTILINE   while   unsetting   PCRE2_DOTALL   and
       PCRE2_EXTENDED, is also permitted. If a letter appears both before  and
       after  the  hyphen, the option is unset. An empty options setting "(?)"
       is allowed. Needless to say, it has no effect.

       The PCRE2-specific options PCRE2_DUPNAMES  and  PCRE2_UNGREEDY  can  be
       changed  in  the  same  way as the Perl-compatible options by using the
       characters J and U respectively.

       When one of these option changes occurs at  top  level  (that  is,  not
       inside  subpattern parentheses), the change applies to the remainder of
       the pattern that follows. If the change is placed right at the start of
       a  pattern,  PCRE2  extracts  it  into  the global options (and it will
       therefore show up in data extracted by the  pcre2_pattern_info()  func-
       tion).

       An  option  change  within a subpattern (see below for a description of
       subpatterns) affects only that part of the subpattern that follows  it,
       so

         (a(?i)b)c

       matches  abc  and  aBc and no other strings (assuming PCRE2_CASELESS is
       not used).  By this means, options can be made to have  different  set-
       tings in different parts of the pattern. Any changes made in one alter-
       native do carry on into subsequent branches within the same subpattern.
       For example,

         (a(?i)b|c)

       matches  "ab",  "aB",  "c",  and "C", even though when matching "C" the
       first branch is abandoned before the option setting.  This  is  because
       the  effects  of option settings happen at compile time. There would be
       some very weird behaviour otherwise.

       As a convenient shorthand, if any option settings are required  at  the
       start  of a non-capturing subpattern (see the next section), the option
       letters may appear between the "?" and the ":". Thus the two patterns

         (?i:saturday|sunday)
         (?:(?i)saturday|sunday)

       match exactly the same set of strings.

       Note: There are other PCRE2-specific options that can  be  set  by  the
       application when the compiling function is called. The pattern can con-
       tain special leading sequences such as (*CRLF)  to  override  what  the
       application  has  set  or what has been defaulted. Details are given in
       the section entitled "Newline sequences"  above.  There  are  also  the
       (*UTF)  and  (*UCP)  leading  sequences that can be used to set UTF and
       Unicode property modes; they are equivalent to  setting  the  PCRE2_UTF
       and  PCRE2_UCP  options, respectively. However, the application can set
       the PCRE2_NEVER_UTF and PCRE2_NEVER_UCP options, which lock out the use
       of the (*UTF) and (*UCP) sequences.


SUBPATTERNS

       Subpatterns are delimited by parentheses (round brackets), which can be
       nested.  Turning part of a pattern into a subpattern does two things:

       1. It localizes a set of alternatives. For example, the pattern

         cat(aract|erpillar|)

       matches "cataract", "caterpillar", or "cat". Without  the  parentheses,
       it would match "cataract", "erpillar" or an empty string.

       2.  It  sets  up  the  subpattern as a capturing subpattern. This means
       that, when the whole pattern matches, the portion of the subject string
       that  matched  the  subpattern is passed back to the caller, separately
       from the portion that matched the whole pattern. (This applies only  to
       the  traditional  matching function; the DFA matching function does not
       support capturing.)

       Opening parentheses are counted from left to right (starting from 1) to
       obtain  numbers  for  the  capturing  subpatterns.  For example, if the
       string "the red king" is matched against the pattern

         the ((red|white) (king|queen))

       the captured substrings are "red king", "red", and "king", and are num-
       bered 1, 2, and 3, respectively.

       The  fact  that  plain  parentheses  fulfil two functions is not always
       helpful.  There are often times when a grouping subpattern is  required
       without  a capturing requirement. If an opening parenthesis is followed
       by a question mark and a colon, the subpattern does not do any  captur-
       ing,  and  is  not  counted when computing the number of any subsequent
       capturing subpatterns. For example, if the string "the white queen"  is
       matched against the pattern

         the ((?:red|white) (king|queen))

       the captured substrings are "white queen" and "queen", and are numbered
       1 and 2. The maximum number of capturing subpatterns is 65535.

       As a convenient shorthand, if any option settings are required  at  the
       start  of  a  non-capturing  subpattern,  the option letters may appear
       between the "?" and the ":". Thus the two patterns

         (?i:saturday|sunday)
         (?:(?i)saturday|sunday)

       match exactly the same set of strings. Because alternative branches are
       tried  from  left  to right, and options are not reset until the end of
       the subpattern is reached, an option setting in one branch does  affect
       subsequent  branches,  so  the above patterns match "SUNDAY" as well as
       "Saturday".


DUPLICATE SUBPATTERN NUMBERS

       Perl 5.10 introduced a feature whereby each alternative in a subpattern
       uses  the same numbers for its capturing parentheses. Such a subpattern
       starts with (?| and is itself a non-capturing subpattern. For  example,
       consider this pattern:

         (?|(Sat)ur|(Sun))day

       Because  the two alternatives are inside a (?| group, both sets of cap-
       turing parentheses are numbered one. Thus, when  the  pattern  matches,
       you  can  look  at captured substring number one, whichever alternative
       matched. This construct is useful when you want to  capture  part,  but
       not all, of one of a number of alternatives. Inside a (?| group, paren-
       theses are numbered as usual, but the number is reset at the  start  of
       each  branch.  The numbers of any capturing parentheses that follow the
       subpattern start after the highest number used in any branch. The  fol-
       lowing example is taken from the Perl documentation. The numbers under-
       neath show in which buffer the captured content will be stored.

         # before  ---------------branch-reset----------- after
         / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
         # 1            2         2  3        2     3     4

       A back reference to a numbered subpattern uses the  most  recent  value
       that  is  set  for that number by any subpattern. The following pattern
       matches "abcabc" or "defdef":

         /(?|(abc)|(def))\1/

       In contrast, a subroutine call to a numbered subpattern  always  refers
       to  the  first  one in the pattern with the given number. The following
       pattern matches "abcabc" or "defabc":

         /(?|(abc)|(def))(?1)/

       A relative reference such as (?-1) is no different: it is just a conve-
       nient way of computing an absolute group number.

       If  a condition test for a subpattern's having matched refers to a non-
       unique number, the test is true if any of the subpatterns of that  num-
       ber have matched.

       An  alternative approach to using this "branch reset" feature is to use
       duplicate named subpatterns, as described in the next section.


NAMED SUBPATTERNS

       Identifying capturing parentheses by number is simple, but  it  can  be
       very  hard  to keep track of the numbers in complicated regular expres-
       sions. Furthermore, if an  expression  is  modified,  the  numbers  may
       change. To help with this difficulty, PCRE2 supports the naming of sub-
       patterns. This feature was not added to Perl until release 5.10. Python
       had  the feature earlier, and PCRE1 introduced it at release 4.0, using
       the Python syntax. PCRE2 supports both the Perl and the Python  syntax.
       Perl  allows  identically numbered subpatterns to have different names,
       but PCRE2 does not.

       In PCRE2, a subpattern can be named in one of three ways:  (?<name>...)
       or  (?'name'...)  as in Perl, or (?P<name>...) as in Python. References
       to capturing parentheses from other parts of the pattern, such as  back
       references,  recursion,  and conditions, can be made by name as well as
       by number.

       Names consist of up to 32 alphanumeric characters and underscores,  but
       must  start  with  a  non-digit.  Named capturing parentheses are still
       allocated numbers as well as names, exactly as if the  names  were  not
       present. The PCRE2 API provides function calls for extracting the name-
       to-number translation table from a compiled  pattern.  There  are  also
       convenience functions for extracting a captured substring by name.

       By  default, a name must be unique within a pattern, but it is possible
       to relax this constraint by setting the PCRE2_DUPNAMES option  at  com-
       pile  time.  (Duplicate names are also always permitted for subpatterns
       with the same number, set up as described  in  the  previous  section.)
       Duplicate  names  can be useful for patterns where only one instance of
       the named parentheses can match.  Suppose you want to match the name of
       a  weekday,  either as a 3-letter abbreviation or as the full name, and
       in both cases you  want  to  extract  the  abbreviation.  This  pattern
       (ignoring the line breaks) does the job:

         (?<DN>Mon|Fri|Sun)(?:day)?|
         (?<DN>Tue)(?:sday)?|
         (?<DN>Wed)(?:nesday)?|
         (?<DN>Thu)(?:rsday)?|
         (?<DN>Sat)(?:urday)?

       There  are  five capturing substrings, but only one is ever set after a
       match.  (An alternative way of solving this problem is to use a "branch
       reset" subpattern, as described in the previous section.)

       The  convenience  functions for extracting the data by name returns the
       substring for the first (and in this example, the only)  subpattern  of
       that  name  that  matched.  This saves searching to find which numbered
       subpattern it was.

       If you make a back reference to  a  non-unique  named  subpattern  from
       elsewhere  in the pattern, the subpatterns to which the name refers are
       checked in the order in which they appear in the overall  pattern.  The
       first one that is set is used for the reference. For example, this pat-
       tern matches both "foofoo" and "barbar" but not "foobar" or "barfoo":

         (?:(?<n>foo)|(?<n>bar))\k<n>


       If you make a subroutine call to a non-unique named subpattern, the one
       that  corresponds  to  the first occurrence of the name is used. In the
       absence of duplicate numbers (see the previous section) this is the one
       with the lowest number.

       If you use a named reference in a condition test (see the section about
       conditions below), either to check whether a subpattern has matched, or
       to  check for recursion, all subpatterns with the same name are tested.
       If the condition is true for any one of them, the overall condition  is
       true.  This  is  the  same  behaviour as testing by number. For further
       details of the interfaces  for  handling  named  subpatterns,  see  the
       pcre2api documentation.

       Warning: You cannot use different names to distinguish between two sub-
       patterns with the same number because PCRE2 uses only the numbers  when
       matching. For this reason, an error is given at compile time if differ-
       ent names are given to subpatterns with the same number.  However,  you
       can always give the same name to subpatterns with the same number, even
       when PCRE2_DUPNAMES is not set.


REPETITION

       Repetition is specified by quantifiers, which can  follow  any  of  the
       following items:

         a literal data character
         the dot metacharacter
         the \C escape sequence
         the \X escape sequence
         the \R escape sequence
         an escape such as \d or \pL that matches a single character
         a character class
         a back reference
         a parenthesized subpattern (including most assertions)
         a subroutine call to a subpattern (recursive or otherwise)

       The  general repetition quantifier specifies a minimum and maximum num-
       ber of permitted matches, by giving the two numbers in  curly  brackets
       (braces),  separated  by  a comma. The numbers must be less than 65536,
       and the first must be less than or equal to the second. For example:

         z{2,4}

       matches "zz", "zzz", or "zzzz". A closing brace on its  own  is  not  a
       special  character.  If  the second number is omitted, but the comma is
       present, there is no upper limit; if the second number  and  the  comma
       are  both omitted, the quantifier specifies an exact number of required
       matches. Thus

         [aeiou]{3,}

       matches at least 3 successive vowels, but may match many more, whereas

         \d{8}

       matches exactly 8 digits. An opening curly bracket that  appears  in  a
       position  where a quantifier is not allowed, or one that does not match
       the syntax of a quantifier, is taken as a literal character. For  exam-
       ple, {,6} is not a quantifier, but a literal string of four characters.

       In UTF modes, quantifiers apply to characters rather than to individual
       code units. Thus, for example, \x{100}{2} matches two characters,  each
       of which is represented by a two-byte sequence in a UTF-8 string. Simi-
       larly, \X{3} matches three Unicode extended grapheme clusters, each  of
       which  may  be  several  code  units long (and they may be of different
       lengths).

       The quantifier {0} is permitted, causing the expression to behave as if
       the previous item and the quantifier were not present. This may be use-
       ful for subpatterns that are referenced as subroutines  from  elsewhere
       in the pattern (but see also the section entitled "Defining subpatterns
       for use by reference only" below). Items other  than  subpatterns  that
       have a {0} quantifier are omitted from the compiled pattern.

       For  convenience, the three most common quantifiers have single-charac-
       ter abbreviations:

         *    is equivalent to {0,}
         +    is equivalent to {1,}
         ?    is equivalent to {0,1}

       It is possible to construct infinite loops by  following  a  subpattern
       that can match no characters with a quantifier that has no upper limit,
       for example:

         (a?)*

       Earlier versions of Perl and PCRE1 used to give  an  error  at  compile
       time for such patterns. However, because there are cases where this can
       be useful, such patterns are now accepted, but if any repetition of the
       subpattern  does in fact match no characters, the loop is forcibly bro-
       ken.

       By default, the quantifiers are "greedy", that is, they match  as  much
       as  possible  (up  to  the  maximum number of permitted times), without
       causing the rest of the pattern to fail. The classic example  of  where
       this gives problems is in trying to match comments in C programs. These
       appear between /* and */ and within the comment,  individual  *  and  /
       characters  may  appear. An attempt to match C comments by applying the
       pattern

         /\*.*\*/

       to the string

         /* first comment */  not comment  /* second comment */

       fails, because it matches the entire string owing to the greediness  of
       the .*  item.

       If a quantifier is followed by a question mark, it ceases to be greedy,
       and instead matches the minimum number of times possible, so  the  pat-
       tern

         /\*.*?\*/

       does  the  right  thing with the C comments. The meaning of the various
       quantifiers is not otherwise changed,  just  the  preferred  number  of
       matches.   Do  not  confuse this use of question mark with its use as a
       quantifier in its own right. Because it has two uses, it can  sometimes
       appear doubled, as in

         \d??\d

       which matches one digit by preference, but can match two if that is the
       only way the rest of the pattern matches.

       If the PCRE2_UNGREEDY option is set (an option that is not available in
       Perl),  the  quantifiers are not greedy by default, but individual ones
       can be made greedy by following them with a  question  mark.  In  other
       words, it inverts the default behaviour.

       When  a  parenthesized  subpattern  is quantified with a minimum repeat
       count that is greater than 1 or with a limited maximum, more memory  is
       required  for  the  compiled  pattern, in proportion to the size of the
       minimum or maximum.

       If a pattern starts with  .*  or  .{0,}  and  the  PCRE2_DOTALL  option
       (equivalent  to  Perl's /s) is set, thus allowing the dot to match new-
       lines, the pattern is implicitly  anchored,  because  whatever  follows
       will  be  tried against every character position in the subject string,
       so there is no point in retrying the  overall  match  at  any  position
       after the first. PCRE2 normally treats such a pattern as though it were
       preceded by \A.

       In cases where it is known that the subject  string  contains  no  new-
       lines,  it  is worth setting PCRE2_DOTALL in order to obtain this opti-
       mization, or alternatively, using ^ to indicate anchoring explicitly.

       However, there are some cases where the optimization  cannot  be  used.
       When .*  is inside capturing parentheses that are the subject of a back
       reference elsewhere in the pattern, a match at the start may fail where
       a later one succeeds. Consider, for example:

         (.*)abc\1

       If  the subject is "xyz123abc123" the match point is the fourth charac-
       ter. For this reason, such a pattern is not implicitly anchored.

       Another case where implicit anchoring is not applied is when the  lead-
       ing  .* is inside an atomic group. Once again, a match at the start may
       fail where a later one succeeds. Consider this pattern:

         (?>.*?a)b

       It matches "ab" in the subject "aab". The use of the backtracking  con-
       trol  verbs  (*PRUNE)  and  (*SKIP) also disable this optimization, and
       there is an option, PCRE2_NO_DOTSTAR_ANCHOR, to do so explicitly.

       When a capturing subpattern is repeated, the value captured is the sub-
       string that matched the final iteration. For example, after

         (tweedle[dume]{3}\s*)+

       has matched "tweedledum tweedledee" the value of the captured substring
       is "tweedledee". However, if there are  nested  capturing  subpatterns,
       the  corresponding captured values may have been set in previous itera-
       tions. For example, after

         (a|(b))+

       matches "aba" the value of the second captured substring is "b".


ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS

       With both maximizing ("greedy") and minimizing ("ungreedy"  or  "lazy")
       repetition,  failure  of what follows normally causes the repeated item
       to be re-evaluated to see if a different number of repeats  allows  the
       rest  of  the pattern to match. Sometimes it is useful to prevent this,
       either to change the nature of the match, or to cause it  fail  earlier
       than  it otherwise might, when the author of the pattern knows there is
       no point in carrying on.

       Consider, for example, the pattern \d+foo when applied to  the  subject
       line

         123456bar

       After matching all 6 digits and then failing to match "foo", the normal
       action of the matcher is to try again with only 5 digits  matching  the
       \d+  item,  and  then  with  4,  and  so on, before ultimately failing.
       "Atomic grouping" (a term taken from Jeffrey  Friedl's  book)  provides
       the  means for specifying that once a subpattern has matched, it is not
       to be re-evaluated in this way.

       If we use atomic grouping for the previous example, the  matcher  gives
       up  immediately  on failing to match "foo" the first time. The notation
       is a kind of special parenthesis, starting with (?> as in this example:

         (?>\d+)foo

       This kind of parenthesis "locks up" the  part of the  pattern  it  con-
       tains  once  it  has matched, and a failure further into the pattern is
       prevented from backtracking into it. Backtracking past it  to  previous
       items, however, works as normal.

       An  alternative  description  is that a subpattern of this type matches
       exactly the string of characters that an identical  standalone  pattern
       would match, if anchored at the current point in the subject string.

       Atomic grouping subpatterns are not capturing subpatterns. Simple cases
       such as the above example can be thought of as a maximizing repeat that
       must  swallow  everything  it can. So, while both \d+ and \d+? are pre-
       pared to adjust the number of digits they match in order  to  make  the
       rest of the pattern match, (?>\d+) can only match an entire sequence of
       digits.

       Atomic groups in general can of course contain arbitrarily  complicated
       subpatterns,  and  can  be  nested. However, when the subpattern for an
       atomic group is just a single repeated item, as in the example above, a
       simpler  notation,  called  a "possessive quantifier" can be used. This
       consists of an additional + character  following  a  quantifier.  Using
       this notation, the previous example can be rewritten as

         \d++foo

       Note that a possessive quantifier can be used with an entire group, for
       example:

         (abc|xyz){2,3}+

       Possessive  quantifiers  are  always  greedy;  the   setting   of   the
       PCRE2_UNGREEDY  option  is  ignored. They are a convenient notation for
       the simpler forms of atomic group. However, there is no  difference  in
       the meaning of a possessive quantifier and the equivalent atomic group,
       though there may be a performance  difference;  possessive  quantifiers
       should be slightly faster.

       The  possessive  quantifier syntax is an extension to the Perl 5.8 syn-
       tax.  Jeffrey Friedl originated the idea (and the name)  in  the  first
       edition of his book. Mike McCloskey liked it, so implemented it when he
       built Sun's Java package, and PCRE1 copied it from there. It ultimately
       found its way into Perl at release 5.10.

       PCRE2  has  an  optimization  that automatically "possessifies" certain
       simple pattern constructs. For example, the sequence A+B is treated  as
       A++B  because  there is no point in backtracking into a sequence of A's
       when B must follow.  This feature can be disabled by the PCRE2_NO_AUTO-
       POSSESS option, or starting the pattern with (*NO_AUTO_POSSESS).

       When  a  pattern  contains an unlimited repeat inside a subpattern that
       can itself be repeated an unlimited number of  times,  the  use  of  an
       atomic  group  is  the  only way to avoid some failing matches taking a
       very long time indeed. The pattern

         (\D+|<\d+>)*[!?]

       matches an unlimited number of substrings that either consist  of  non-
       digits,  or  digits  enclosed in <>, followed by either ! or ?. When it
       matches, it runs quickly. However, if it is applied to

         aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

       it takes a long time before reporting  failure.  This  is  because  the
       string  can be divided between the internal \D+ repeat and the external
       * repeat in a large number of ways, and all  have  to  be  tried.  (The
       example  uses  [!?]  rather than a single character at the end, because
       both PCRE2 and Perl have an optimization that allows for  fast  failure
       when  a single character is used. They remember the last single charac-
       ter that is required for a match, and fail early if it is  not  present
       in  the  string.)  If  the pattern is changed so that it uses an atomic
       group, like this:

         ((?>\D+)|<\d+>)*[!?]

       sequences of non-digits cannot be broken, and failure happens quickly.


BACK REFERENCES

       Outside a character class, a backslash followed by a digit greater than
       0 (and possibly further digits) is a back reference to a capturing sub-
       pattern earlier (that is, to its left) in the pattern,  provided  there
       have been that many previous capturing left parentheses.

       However,  if the decimal number following the backslash is less than 8,
       it is always taken as a back reference, and causes  an  error  only  if
       there  are  not that many capturing left parentheses in the entire pat-
       tern. In other words, the parentheses that are referenced need  not  be
       to  the  left of the reference for numbers less than 8. A "forward back
       reference" of this type can make sense when a  repetition  is  involved
       and  the  subpattern to the right has participated in an earlier itera-
       tion.

       It is not possible to have a numerical "forward back  reference"  to  a
       subpattern  whose  number  is  8  or  more  using this syntax because a
       sequence such as \50 is interpreted as a character  defined  in  octal.
       See the subsection entitled "Non-printing characters" above for further
       details of the handling of digits following a backslash.  There  is  no
       such  problem  when named parentheses are used. A back reference to any
       subpattern is possible using named parentheses (see below).

       Another way of avoiding the ambiguity inherent in  the  use  of  digits
       following  a  backslash  is  to use the \g escape sequence. This escape
       must be followed by an unsigned number or a negative number, optionally
       enclosed in braces. These examples are all identical:

         (ring), \1
         (ring), \g1
         (ring), \g{1}

       An  unsigned number specifies an absolute reference without the ambigu-
       ity that is present in the older syntax. It is also useful when literal
       digits follow the reference. A negative number is a relative reference.
       Consider this example:

         (abc(def)ghi)\g{-1}

       The sequence \g{-1} is a reference to the most recently started captur-
       ing subpattern before \g, that is, is it equivalent to \2 in this exam-
       ple.  Similarly, \g{-2} would be equivalent to \1. The use of  relative
       references  can  be helpful in long patterns, and also in patterns that
       are created by  joining  together  fragments  that  contain  references
       within themselves.

       A  back  reference matches whatever actually matched the capturing sub-
       pattern in the current subject string, rather  than  anything  matching
       the subpattern itself (see "Subpatterns as subroutines" below for a way
       of doing that). So the pattern

         (sens|respons)e and \1ibility

       matches "sense and sensibility" and "response and responsibility",  but
       not  "sense and responsibility". If caseful matching is in force at the
       time of the back reference, the case of letters is relevant. For  exam-
       ple,

         ((?i)rah)\s+\1

       matches  "rah  rah"  and  "RAH RAH", but not "RAH rah", even though the
       original capturing subpattern is matched caselessly.

       There are several different ways of writing back  references  to  named
       subpatterns.  The  .NET syntax \k{name} and the Perl syntax \k<name> or
       \k'name' are supported, as is the Python syntax (?P=name). Perl  5.10's
       unified back reference syntax, in which \g can be used for both numeric
       and named references, is also supported. We  could  rewrite  the  above
       example in any of the following ways:

         (?<p1>(?i)rah)\s+\k<p1>
         (?'p1'(?i)rah)\s+\k{p1}
         (?P<p1>(?i)rah)\s+(?P=p1)
         (?<p1>(?i)rah)\s+\g{p1}

       A  subpattern  that  is  referenced  by  name may appear in the pattern
       before or after the reference.

       There may be more than one back reference to the same subpattern. If  a
       subpattern  has  not actually been used in a particular match, any back
       references to it always fail by default. For example, the pattern

         (a|(bc))\2

       always fails if it starts to match "a" rather than  "bc".  However,  if
       the  PCRE2_MATCH_UNSET_BACKREF  option  is  set at compile time, a back
       reference to an unset value matches an empty string.

       Because there may be many capturing parentheses in a pattern, all  dig-
       its  following a backslash are taken as part of a potential back refer-
       ence number.  If the pattern continues with  a  digit  character,  some
       delimiter  must  be  used  to  terminate  the  back  reference.  If the
       PCRE2_EXTENDED option is set, this can be white space.  Otherwise,  the
       \g{ syntax or an empty comment (see "Comments" below) can be used.

   Recursive back references

       A  back reference that occurs inside the parentheses to which it refers
       fails when the subpattern is first used, so, for example,  (a\1)  never
       matches.   However,  such references can be useful inside repeated sub-
       patterns. For example, the pattern

         (a|b\1)+

       matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-
       ation  of  the  subpattern,  the  back  reference matches the character
       string corresponding to the previous iteration. In order  for  this  to
       work,  the  pattern must be such that the first iteration does not need
       to match the back reference. This can be done using alternation, as  in
       the example above, or by a quantifier with a minimum of zero.

       Back  references of this type cause the group that they reference to be
       treated as an atomic group.  Once the whole group has been  matched,  a
       subsequent  matching  failure cannot cause backtracking into the middle
       of the group.


ASSERTIONS

       An assertion is a test on the characters  following  or  preceding  the
       current matching point that does not consume any characters. The simple
       assertions coded as \b, \B, \A, \G, \Z,  \z,  ^  and  $  are  described
       above.

       More  complicated  assertions  are  coded as subpatterns. There are two
       kinds: those that look ahead of the current  position  in  the  subject
       string,  and  those  that  look  behind  it. An assertion subpattern is
       matched in the normal way, except that it does not  cause  the  current
       matching position to be changed.

       Assertion  subpatterns are not capturing subpatterns. If such an asser-
       tion contains capturing subpatterns within it, these  are  counted  for
       the  purposes  of numbering the capturing subpatterns in the whole pat-
       tern. However, substring capturing is carried  out  only  for  positive
       assertions. (Perl sometimes, but not always, does do capturing in nega-
       tive assertions.)

       For  compatibility  with  Perl,  most  assertion  subpatterns  may   be
       repeated;  though  it  makes  no sense to assert the same thing several
       times, the side effect of capturing  parentheses  may  occasionally  be
       useful.  However,  an  assertion  that forms the condition for a condi-
       tional subpattern may not be quantified. In practice, for other  asser-
       tions, there only three cases:

       (1)  If  the  quantifier  is  {0}, the assertion is never obeyed during
       matching.  However, it may  contain  internal  capturing  parenthesized
       groups that are called from elsewhere via the subroutine mechanism.

       (2)  If quantifier is {0,n} where n is greater than zero, it is treated
       as if it were {0,1}. At run time, the rest  of  the  pattern  match  is
       tried with and without the assertion, the order depending on the greed-
       iness of the quantifier.

       (3) If the minimum repetition is greater than zero, the  quantifier  is
       ignored.   The  assertion  is  obeyed just once when encountered during
       matching.

   Lookahead assertions

       Lookahead assertions start with (?= for positive assertions and (?! for
       negative assertions. For example,

         \w+(?=;)

       matches  a word followed by a semicolon, but does not include the semi-
       colon in the match, and

         foo(?!bar)

       matches any occurrence of "foo" that is not  followed  by  "bar".  Note
       that the apparently similar pattern

         (?!foo)bar

       does  not  find  an  occurrence  of "bar" that is preceded by something
       other than "foo"; it finds any occurrence of "bar" whatsoever,  because
       the assertion (?!foo) is always true when the next three characters are
       "bar". A lookbehind assertion is needed to achieve the other effect.

       If you want to force a matching failure at some point in a pattern, the
       most  convenient  way  to  do  it  is with (?!) because an empty string
       always matches, so an assertion that requires there not to be an  empty
       string must always fail.  The backtracking control verb (*FAIL) or (*F)
       is a synonym for (?!).

   Lookbehind assertions

       Lookbehind assertions start with (?<= for positive assertions and  (?<!
       for negative assertions. For example,

         (?<!foo)bar

       does  find  an  occurrence  of "bar" that is not preceded by "foo". The
       contents of a lookbehind assertion are restricted  such  that  all  the
       strings it matches must have a fixed length. However, if there are sev-
       eral top-level alternatives, they do not all  have  to  have  the  same
       fixed length. Thus

         (?<=bullock|donkey)

       is permitted, but

         (?<!dogs?|cats?)

       causes  an  error at compile time. Branches that match different length
       strings are permitted only at the top level of a lookbehind  assertion.
       This is an extension compared with Perl, which requires all branches to
       match the same length of string. An assertion such as

         (?<=ab(c|de))

       is not permitted, because its single top-level  branch  can  match  two
       different  lengths,  but  it is acceptable to PCRE2 if rewritten to use
       two top-level branches:

         (?<=abc|abde)

       In some cases, the escape sequence \K (see above) can be  used  instead
       of a lookbehind assertion to get round the fixed-length restriction.

       The  implementation  of lookbehind assertions is, for each alternative,
       to temporarily move the current position back by the fixed  length  and
       then try to match. If there are insufficient characters before the cur-
       rent position, the assertion fails.

       In a UTF mode, PCRE2 does not allow the \C escape (which matches a sin-
       gle  code  unit even in a UTF mode) to appear in lookbehind assertions,
       because it makes it impossible to calculate the length of  the  lookbe-
       hind.  The \X and \R escapes, which can match different numbers of code
       units, are also not permitted.

       "Subroutine" calls (see below) such as (?2) or (?&X) are  permitted  in
       lookbehinds,  as  long as the subpattern matches a fixed-length string.
       Recursion, however, is not supported.

       Possessive quantifiers can  be  used  in  conjunction  with  lookbehind
       assertions to specify efficient matching of fixed-length strings at the
       end of subject strings. Consider a simple pattern such as

         abcd$

       when applied to a long string that does  not  match.  Because  matching
       proceeds  from  left to right, PCRE2 will look for each "a" in the sub-
       ject and then see if what follows matches the rest of the  pattern.  If
       the pattern is specified as

         ^.*abcd$

       the  initial .* matches the entire string at first, but when this fails
       (because there is no following "a"), it backtracks to match all but the
       last  character,  then all but the last two characters, and so on. Once
       again the search for "a" covers the entire string, from right to  left,
       so we are no better off. However, if the pattern is written as

         ^.*+(?<=abcd)

       there can be no backtracking for the .*+ item because of the possessive
       quantifier; it can match only the entire string. The subsequent lookbe-
       hind  assertion  does  a single test on the last four characters. If it
       fails, the match fails immediately. For  long  strings,  this  approach
       makes a significant difference to the processing time.

   Using multiple assertions

       Several assertions (of any sort) may occur in succession. For example,

         (?<=\d{3})(?<!999)foo

       matches  "foo" preceded by three digits that are not "999". Notice that
       each of the assertions is applied independently at the  same  point  in
       the  subject  string.  First  there  is a check that the previous three
       characters are all digits, and then there is  a  check  that  the  same
       three characters are not "999".  This pattern does not match "foo" pre-
       ceded by six characters, the first of which are  digits  and  the  last
       three  of  which  are not "999". For example, it doesn't match "123abc-
       foo". A pattern to do that is

         (?<=\d{3}...)(?<!999)foo

       This time the first assertion looks at the  preceding  six  characters,
       checking that the first three are digits, and then the second assertion
       checks that the preceding three characters are not "999".

       Assertions can be nested in any combination. For example,

         (?<=(?<!foo)bar)baz

       matches an occurrence of "baz" that is preceded by "bar" which in  turn
       is not preceded by "foo", while

         (?<=\d{3}(?!999)...)foo

       is  another pattern that matches "foo" preceded by three digits and any
       three characters that are not "999".


CONDITIONAL SUBPATTERNS

       It is possible to cause the matching process to obey a subpattern  con-
       ditionally  or to choose between two alternative subpatterns, depending
       on the result of an assertion, or whether a specific capturing  subpat-
       tern  has  already  been matched. The two possible forms of conditional
       subpattern are:

         (?(condition)yes-pattern)
         (?(condition)yes-pattern|no-pattern)

       If the condition is satisfied, the yes-pattern is used;  otherwise  the
       no-pattern  (if  present)  is used. If there are more than two alterna-
       tives in the subpattern, a compile-time error occurs. Each of  the  two
       alternatives may itself contain nested subpatterns of any form, includ-
       ing  conditional  subpatterns;  the  restriction  to  two  alternatives
       applies only at the level of the condition. This pattern fragment is an
       example where the alternatives are complex:

         (?(1) (A|B|C) | (D | (?(2)E|F) | E) )


       There are five kinds of condition: references  to  subpatterns,  refer-
       ences  to  recursion,  two pseudo-conditions called DEFINE and VERSION,
       and assertions.

   Checking for a used subpattern by number

       If the text between the parentheses consists of a sequence  of  digits,
       the condition is true if a capturing subpattern of that number has pre-
       viously matched. If there is more than one  capturing  subpattern  with
       the  same  number  (see  the earlier section about duplicate subpattern
       numbers), the condition is true if any of them have matched. An  alter-
       native  notation is to precede the digits with a plus or minus sign. In
       this case, the subpattern number is relative rather than absolute.  The
       most  recently opened parentheses can be referenced by (?(-1), the next
       most recent by (?(-2), and so on. Inside loops it can also  make  sense
       to refer to subsequent groups. The next parentheses to be opened can be
       referenced as (?(+1), and so on. (The value zero in any of these  forms
       is not used; it provokes a compile-time error.)

       Consider  the  following  pattern, which contains non-significant white
       space to make it more readable (assume the PCRE2_EXTENDED  option)  and
       to divide it into three parts for ease of discussion:

         ( \( )?    [^()]+    (?(1) \) )

       The  first  part  matches  an optional opening parenthesis, and if that
       character is present, sets it as the first captured substring. The sec-
       ond  part  matches one or more characters that are not parentheses. The
       third part is a conditional subpattern that tests whether  or  not  the
       first  set  of  parentheses  matched.  If they did, that is, if subject
       started with an opening parenthesis, the condition is true, and so  the
       yes-pattern  is  executed and a closing parenthesis is required. Other-
       wise, since no-pattern is not present, the subpattern matches  nothing.
       In  other  words,  this  pattern matches a sequence of non-parentheses,
       optionally enclosed in parentheses.

       If you were embedding this pattern in a larger one,  you  could  use  a
       relative reference:

         ...other stuff... ( \( )?    [^()]+    (?(-1) \) ) ...

       This  makes  the  fragment independent of the parentheses in the larger
       pattern.

   Checking for a used subpattern by name

       Perl uses the syntax (?(<name>)...) or (?('name')...)  to  test  for  a
       used  subpattern  by  name.  For compatibility with earlier versions of
       PCRE1, which had this facility before Perl, the syntax (?(name)...)  is
       also recognized.

       Rewriting the above example to use a named subpattern gives this:

         (?<OPEN> \( )?    [^()]+    (?(<OPEN>) \) )

       If  the  name used in a condition of this kind is a duplicate, the test
       is applied to all subpatterns of the same name, and is true if any  one
       of them has matched.

   Checking for pattern recursion

       If the condition is the string (R), and there is no subpattern with the
       name R, the condition is true if a recursive call to the whole  pattern
       or any subpattern has been made. If digits or a name preceded by amper-
       sand follow the letter R, for example:

         (?(R3)...) or (?(R&name)...)

       the condition is true if the most recent recursion is into a subpattern
       whose number or name is given. This condition does not check the entire
       recursion stack. If the name used in a condition  of  this  kind  is  a
       duplicate, the test is applied to all subpatterns of the same name, and
       is true if any one of them is the most recent recursion.

       At "top level", all these recursion test  conditions  are  false.   The
       syntax for recursive patterns is described below.

   Defining subpatterns for use by reference only

       If  the  condition  is  the string (DEFINE), and there is no subpattern
       with the name DEFINE, the condition is  always  false.  In  this  case,
       there  may  be  only  one  alternative  in the subpattern. It is always
       skipped if control reaches this point  in  the  pattern;  the  idea  of
       DEFINE  is that it can be used to define subroutines that can be refer-
       enced from elsewhere. (The use of subroutines is described below.)  For
       example,  a  pattern  to match an IPv4 address such as "192.168.23.245"
       could be written like this (ignore white space and line breaks):

         (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
         \b (?&byte) (\.(?&byte)){3} \b

       The first part of the pattern is a DEFINE group inside which a  another
       group  named "byte" is defined. This matches an individual component of
       an IPv4 address (a number less than 256). When  matching  takes  place,
       this  part  of  the pattern is skipped because DEFINE acts like a false
       condition. The rest of the pattern uses references to the  named  group
       to  match the four dot-separated components of an IPv4 address, insist-
       ing on a word boundary at each end.

   Checking the PCRE2 version

       Programs that link with a PCRE2 library can check the version by  call-
       ing  pcre2_config()  with  appropriate arguments. Users of applications
       that do not have access to the underlying code cannot do this.  A  spe-
       cial  "condition" called VERSION exists to allow such users to discover
       which version of PCRE2 they are dealing with by using this condition to
       match  a string such as "yesno". VERSION must be followed either by "="
       or ">=" and a version number.  For example:

         (?(VERSION>=10.4)yes|no)

       This pattern matches "yes" if the PCRE2 version is greater or equal  to
       10.4,  or "no" otherwise. The fractional part of the version number may
       not contain more than two digits.

   Assertion conditions

       If the condition is not in any of the above  formats,  it  must  be  an
       assertion.   This may be a positive or negative lookahead or lookbehind
       assertion. Consider  this  pattern,  again  containing  non-significant
       white space, and with the two alternatives on the second line:

         (?(?=[^a-z]*[a-z])
         \d{2}-[a-z]{3}-\d{2}  |  \d{2}-\d{2}-\d{2} )

       The  condition  is  a  positive  lookahead  assertion  that  matches an
       optional sequence of non-letters followed by a letter. In other  words,
       it  tests  for the presence of at least one letter in the subject. If a
       letter is found, the subject is matched against the first  alternative;
       otherwise  it  is  matched  against  the  second.  This pattern matches
       strings in one of the two forms dd-aaa-dd or dd-dd-dd,  where  aaa  are
       letters and dd are digits.


COMMENTS

       There are two ways of including comments in patterns that are processed
       by PCRE2. In both cases, the start of the comment  must  not  be  in  a
       character  class,  nor  in  the middle of any other sequence of related
       characters such as (?: or a subpattern name or number.  The  characters
       that make up a comment play no part in the pattern matching.

       The  sequence (?# marks the start of a comment that continues up to the
       next closing parenthesis. Nested parentheses are not permitted. If  the
       PCRE2_EXTENDED  option is set, an unescaped # character also introduces
       a comment, which in this case continues to immediately after  the  next
       newline  character  or character sequence in the pattern. Which charac-
       ters are interpreted as newlines is controlled by an option  passed  to
       the  compiling  function  or  by a special sequence at the start of the
       pattern, as described in the  section  entitled  "Newline  conventions"
       above.  Note  that the end of this type of comment is a literal newline
       sequence in the pattern; escape sequences that happen  to  represent  a
       newline   do  not  count.  For  example,  consider  this  pattern  when
       PCRE2_EXTENDED is set, and the default  newline  convention  (a  single
       linefeed character) is in force:

         abc #comment \n still comment

       On  encountering  the # character, pcre2_compile() skips along, looking
       for a newline in the pattern. The sequence \n is still literal at  this
       stage,  so  it does not terminate the comment. Only an actual character
       with the code value 0x0a (the default newline) does so.


RECURSIVE PATTERNS

       Consider the problem of matching a string in parentheses, allowing  for
       unlimited  nested  parentheses.  Without the use of recursion, the best
       that can be done is to use a pattern that  matches  up  to  some  fixed
       depth  of  nesting.  It  is not possible to handle an arbitrary nesting
       depth.

       For some time, Perl has provided a facility that allows regular expres-
       sions  to recurse (amongst other things). It does this by interpolating
       Perl code in the expression at run time, and the code can refer to  the
       expression itself. A Perl pattern using code interpolation to solve the
       parentheses problem can be created like this:

         $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;

       The (?p{...}) item interpolates Perl code at run time, and in this case
       refers recursively to the pattern in which it appears.

       Obviously,  PCRE2  cannot  support  the  interpolation  of  Perl  code.
       Instead, it supports special syntax for recursion of  the  entire  pat-
       tern, and also for individual subpattern recursion. After its introduc-
       tion in PCRE1 and Python,  this  kind  of  recursion  was  subsequently
       introduced into Perl at release 5.10.

       A  special  item  that consists of (? followed by a number greater than
       zero and a closing parenthesis is a recursive subroutine  call  of  the
       subpattern  of  the  given  number, provided that it occurs inside that
       subpattern. (If not, it is a non-recursive subroutine  call,  which  is
       described  in  the  next  section.)  The special item (?R) or (?0) is a
       recursive call of the entire regular expression.

       This PCRE2 pattern solves the nested parentheses  problem  (assume  the
       PCRE2_EXTENDED option is set so that white space is ignored):

         \( ( [^()]++ | (?R) )* \)

       First  it matches an opening parenthesis. Then it matches any number of
       substrings which can either be a  sequence  of  non-parentheses,  or  a
       recursive  match  of the pattern itself (that is, a correctly parenthe-
       sized substring).  Finally there is a closing parenthesis. Note the use
       of a possessive quantifier to avoid backtracking into sequences of non-
       parentheses.

       If this were part of a larger pattern, you would not  want  to  recurse
       the entire pattern, so instead you could use this:

         ( \( ( [^()]++ | (?1) )* \) )

       We  have  put the pattern into parentheses, and caused the recursion to
       refer to them instead of the whole pattern.

       In a larger pattern,  keeping  track  of  parenthesis  numbers  can  be
       tricky.  This is made easier by the use of relative references. Instead
       of (?1) in the pattern above you can write (?-2) to refer to the second
       most  recently  opened  parentheses  preceding  the recursion. In other
       words, a negative number counts capturing  parentheses  leftwards  from
       the point at which it is encountered.

       Be aware however, that if duplicate subpattern numbers are in use, rel-
       ative references refer to the earliest subpattern with the  appropriate
       number. Consider, for example:

         (?|(a)|(b)) (c) (?-2)

       The  first  two  capturing  groups (a) and (b) are both numbered 1, and
       group (c) is number 2. When the reference  (?-2)  is  encountered,  the
       second most recently opened parentheses has the number 1, but it is the
       first such group (the (a) group) to which the  recursion  refers.  This
       would  be  the  same  if  an absolute reference (?1) was used. In other
       words, relative references are just a shorthand for computing  a  group
       number.

       It  is  also  possible  to refer to subsequently opened parentheses, by
       writing references such as (?+2). However, these  cannot  be  recursive
       because  the  reference  is  not inside the parentheses that are refer-
       enced. They are always non-recursive subroutine calls, as described  in
       the next section.

       An  alternative  approach  is to use named parentheses. The Perl syntax
       for this is (?&name); PCRE1's earlier syntax  (?P>name)  is  also  sup-
       ported. We could rewrite the above example as follows:

         (?<pn> \( ( [^()]++ | (?&pn) )* \) )

       If  there  is more than one subpattern with the same name, the earliest
       one is used.

       The example pattern that we have been looking at contains nested unlim-
       ited  repeats,  and  so the use of a possessive quantifier for matching
       strings of non-parentheses is important when applying  the  pattern  to
       strings that do not match. For example, when this pattern is applied to

         (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()

       it  yields  "no  match" quickly. However, if a possessive quantifier is
       not used, the match runs for a very long time indeed because there  are
       so  many  different  ways the + and * repeats can carve up the subject,
       and all have to be tested before failure can be reported.

       At the end of a match, the values of capturing  parentheses  are  those
       from  the outermost level. If you want to obtain intermediate values, a
       callout function can be used (see below and the pcre2callout documenta-
       tion). If the pattern above is matched against

         (ab(cd)ef)

       the  value  for  the  inner capturing parentheses (numbered 2) is "ef",
       which is the last value taken on at the top level. If a capturing  sub-
       pattern  is  not  matched at the top level, its final captured value is
       unset, even if it was (temporarily) set at a deeper  level  during  the
       matching process.

       If there are more than 15 capturing parentheses in a pattern, PCRE2 has
       to obtain extra memory from the heap to store data during a  recursion.
       If   no   memory   can   be   obtained,   the   match  fails  with  the
       PCRE2_ERROR_NOMEMORY error.

       Do not confuse the (?R) item with the condition (R),  which  tests  for
       recursion.   Consider  this pattern, which matches text in angle brack-
       ets, allowing for arbitrary nesting. Only digits are allowed in  nested
       brackets  (that is, when recursing), whereas any characters are permit-
       ted at the outer level.

         < (?: (?(R) \d++  | [^<>]*+) | (?R)) * >

       In this pattern, (?(R) is the start of a conditional  subpattern,  with
       two  different  alternatives for the recursive and non-recursive cases.
       The (?R) item is the actual recursive call.

   Differences in recursion processing between PCRE2 and Perl

       Recursion processing in PCRE2 differs from Perl in two important  ways.
       In PCRE2 (like Python, but unlike Perl), a recursive subpattern call is
       always treated as an atomic group. That is, once it has matched some of
       the subject string, it is never re-entered, even if it contains untried
       alternatives and there is a subsequent matching failure.  This  can  be
       illustrated  by the following pattern, which purports to match a palin-
       dromic string that contains an odd number of characters  (for  example,
       "a", "aba", "abcba", "abcdcba"):

         ^(.|(.)(?1)\2)$

       The idea is that it either matches a single character, or two identical
       characters surrounding a sub-palindrome. In Perl, this  pattern  works;
       in  PCRE2  it  does not if the pattern is longer than three characters.
       Consider the subject string "abcba":

       At the top level, the first character is matched, but as it is  not  at
       the end of the string, the first alternative fails; the second alterna-
       tive is taken and the recursion kicks in. The recursive call to subpat-
       tern  1  successfully  matches the next character ("b"). (Note that the
       beginning and end of line tests are not part of the recursion).

       Back at the top level, the next character ("c") is compared  with  what
       subpattern  2 matched, which was "a". This fails. Because the recursion
       is treated as an atomic group, there are now  no  backtracking  points,
       and  so  the  entire  match fails. (Perl is able, at this point, to re-
       enter the recursion and try the second alternative.)  However,  if  the
       pattern is written with the alternatives in the other order, things are
       different:

         ^((.)(?1)\2|.)$

       This time, the recursing alternative is tried first, and  continues  to
       recurse  until  it runs out of characters, at which point the recursion
       fails. But this time we do have  another  alternative  to  try  at  the
       higher  level.  That  is  the  big difference: in the previous case the
       remaining alternative is at a deeper recursion level, which PCRE2  can-
       not use.

       To  change  the pattern so that it matches all palindromic strings, not
       just those with an odd number of characters, it is tempting  to  change
       the pattern to this:

         ^((.)(?1)\2|.?)$

       Again,  this  works in Perl, but not in PCRE2, and for the same reason.
       When a deeper recursion has matched a single character,  it  cannot  be
       entered  again  in  order  to match an empty string. The solution is to
       separate the two cases, and write out the odd and even cases as  alter-
       natives at the higher level:

         ^(?:((.)(?1)\2|)|((.)(?3)\4|.))

       If  you  want  to match typical palindromic phrases, the pattern has to
       ignore all non-word characters, which can be done like this:

         ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$

       If run with the PCRE2_CASELESS option,  this  pattern  matches  phrases
       such  as  "A  man, a plan, a canal: Panama!" and it works in both PCRE2
       and Perl. Note the use of the possessive quantifier *+ to  avoid  back-
       tracking  into  sequences  of  non-word characters. Without this, PCRE2
       takes a great deal longer (ten times or more) to match typical phrases,
       and Perl takes so long that you think it has gone into a loop.

       WARNING:  The  palindrome-matching patterns above work only if the sub-
       ject string does not start with a palindrome that is shorter  than  the
       entire  string.  For example, although "abcba" is correctly matched, if
       the subject is "ababa", PCRE2 finds the palindrome "aba" at the  start,
       then  fails at top level because the end of the string does not follow.
       Once again, it cannot jump back into the recursion to try other  alter-
       natives, so the entire match fails.

       The  second  way in which PCRE2 and Perl differ in their recursion pro-
       cessing is in the handling of captured values. In Perl, when a  subpat-
       tern  is  called recursively or as a subpattern (see the next section),
       it has no access to any values that were captured  outside  the  recur-
       sion,  whereas  in  PCRE2 these values can be referenced. Consider this
       pattern:

         ^(.)(\1|a(?2))

       In PCRE2, this pattern matches "bab". The first  capturing  parentheses
       match  "b",  then in the second group, when the back reference \1 fails
       to match "b", the second alternative matches "a" and then recurses.  In
       the  recursion,  \1 does now match "b" and so the whole match succeeds.
       In Perl, the pattern fails to match because inside the  recursive  call
       \1 cannot access the externally set value.


SUBPATTERNS AS SUBROUTINES

       If  the  syntax for a recursive subpattern call (either by number or by
       name) is used outside the parentheses to which it refers,  it  operates
       like  a subroutine in a programming language. The called subpattern may
       be defined before or after the reference. A numbered reference  can  be
       absolute or relative, as in these examples:

         (...(absolute)...)...(?2)...
         (...(relative)...)...(?-1)...
         (...(?+1)...(relative)...

       An earlier example pointed out that the pattern

         (sens|respons)e and \1ibility

       matches  "sense and sensibility" and "response and responsibility", but
       not "sense and responsibility". If instead the pattern

         (sens|respons)e and (?1)ibility

       is used, it does match "sense and responsibility" as well as the  other
       two  strings.  Another  example  is  given  in the discussion of DEFINE
       above.

       All subroutine calls, whether recursive or not, are always  treated  as
       atomic  groups. That is, once a subroutine has matched some of the sub-
       ject string, it is never re-entered, even if it contains untried alter-
       natives  and  there  is  a  subsequent  matching failure. Any capturing
       parentheses that are set during the subroutine  call  revert  to  their
       previous values afterwards.

       Processing  options  such as case-independence are fixed when a subpat-
       tern is defined, so if it is used as a subroutine, such options  cannot
       be changed for different calls. For example, consider this pattern:

         (abc)(?i:(?-1))

       It  matches  "abcabc". It does not match "abcABC" because the change of
       processing option does not affect the called subpattern.


ONIGURUMA SUBROUTINE SYNTAX

       For compatibility with Oniguruma, the non-Perl syntax \g followed by  a
       name or a number enclosed either in angle brackets or single quotes, is
       an alternative syntax for referencing a  subpattern  as  a  subroutine,
       possibly  recursively. Here are two of the examples used above, rewrit-
       ten using this syntax:

         (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )
         (sens|respons)e and \g'1'ibility

       PCRE2 supports an extension to Oniguruma: if a number is preceded by  a
       plus or a minus sign it is taken as a relative reference. For example:

         (abc)(?i:\g<-1>)

       Note  that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not
       synonymous. The former is a back reference; the latter is a  subroutine
       call.


CALLOUTS

       Perl has a feature whereby using the sequence (?{...}) causes arbitrary
       Perl code to be obeyed in the middle of matching a regular  expression.
       This makes it possible, amongst other things, to extract different sub-
       strings that match the same pair of parentheses when there is a repeti-
       tion.

       PCRE2  provides  a  similar feature, but of course it cannot obey arbi-
       trary Perl code. The feature is called "callout". The caller  of  PCRE2
       provides  an  external  function  by putting its entry point in a match
       context using the function pcre2_set_callout(), and then  passing  that
       context  to  pcre2_match() or pcre2_dfa_match(). If no match context is
       passed, or if the callout entry point is set to NULL, callouts are dis-
       abled.

       Within  a  regular expression, (?C<arg>) indicates a point at which the
       external function is to be called. There  are  two  kinds  of  callout:
       those  with a numerical argument and those with a string argument. (?C)
       on its own with no argument is treated as (?C0). A  numerical  argument
       allows  the  application  to  distinguish  between  different callouts.
       String arguments were added for release 10.20 to make it  possible  for
       script  languages that use PCRE2 to embed short scripts within patterns
       in a similar way to Perl.

       During matching, when PCRE2 reaches a callout point, the external func-
       tion  is  called.  It is provided with the number or string argument of
       the callout, the position in the pattern, and one item of data that  is
       also set in the match block. The callout function may cause matching to
       proceed, to backtrack, or to fail.

       By default, PCRE2 implements a  number  of  optimizations  at  matching
       time,  and  one  side-effect is that sometimes callouts are skipped. If
       you need all possible callouts to happen, you need to set options  that
       disable  the relevant optimizations. More details, including a complete
       description of the programming interface to the callout  function,  are
       given in the pcre2callout documentation.

   Callouts with numerical arguments

       If  you  just  want  to  have  a means of identifying different callout
       points, put a number less than 256 after the  letter  C.  For  example,
       this pattern has two callout points:

         (?C1)abc(?C2)def

       If  the PCRE2_AUTO_CALLOUT flag is passed to pcre2_compile(), numerical
       callouts are automatically installed before each item in  the  pattern.
       They  are all numbered 255. If there is a conditional group in the pat-
       tern whose condition is an assertion, an additional callout is inserted
       just  before the condition. An explicit callout may also be set at this
       position, as in this example:

         (?(?C9)(?=a)abc|def)

       Note that this applies only to assertion conditions, not to other types
       of condition.

   Callouts with string arguments

       A  delimited  string may be used instead of a number as a callout argu-
       ment. The starting delimiter must be one of ` ' " ^ % #  $  {  and  the
       ending delimiter is the same as the start, except for {, where the end-
       ing delimiter is }. If  the  ending  delimiter  is  needed  within  the
       string, it must be doubled. For example:

         (?C'ab ''c'' d')xyz(?C{any text})pqr

       The  doubling  is  removed  before  the string is passed to the callout
       function.


BACKTRACKING CONTROL

       Perl 5.10 introduced a number of "Special Backtracking Control  Verbs",
       which  are  still  described in the Perl documentation as "experimental
       and subject to change or removal in a future version of Perl". It  goes
       on  to  say:  "Their  usage in production code should be noted to avoid
       problems during upgrades." The same remarks apply to the PCRE2 features
       described in this section.

       The  new verbs make use of what was previously invalid syntax: an open-
       ing parenthesis followed by an asterisk. They are generally of the form
       (*VERB) or (*VERB:NAME). Some verbs take either form, possibly behaving
       differently depending on whether or not a name is present.

       By default, for compatibility with Perl, a  name  is  any  sequence  of
       characters that does not include a closing parenthesis. The name is not
       processed in any way, and it is  not  possible  to  include  a  closing
       parenthesis in the name.  However, if the PCRE2_ALT_VERBNAMES option is
       set, normal backslash processing is applied to verb names and  only  an
       unescaped  closing parenthesis terminates the name. A closing parenthe-
       sis can be included in a name either as \) or between \Q and \E. If the
       PCRE2_EXTENDED  option  is  set,  unescaped whitespace in verb names is
       skipped and #-comments are recognized, exactly as in the  rest  of  the
       pattern.

       The  maximum  length of a name is 255 in the 8-bit library and 65535 in
       the 16-bit and 32-bit libraries. If the name is empty, that is, if  the
       closing  parenthesis immediately follows the colon, the effect is as if
       the colon were not there. Any number of these verbs may occur in a pat-
       tern.

       Since  these  verbs  are  specifically related to backtracking, most of
       them can be used only when the pattern is to be matched using the  tra-
       ditional matching function, because these use a backtracking algorithm.
       With the exception of (*FAIL), which behaves like  a  failing  negative
       assertion, the backtracking control verbs cause an error if encountered
       by the DFA matching function.

       The behaviour of these verbs in repeated  groups,  assertions,  and  in
       subpatterns called as subroutines (whether or not recursively) is docu-
       mented below.

   Optimizations that affect backtracking verbs

       PCRE2 contains some optimizations that are used to speed up matching by
       running some checks at the start of each match attempt. For example, it
       may know the minimum length of matching subject, or that  a  particular
       character must be present. When one of these optimizations bypasses the
       running of a match,  any  included  backtracking  verbs  will  not,  of
       course, be processed. You can suppress the start-of-match optimizations
       by setting the PCRE2_NO_START_OPTIMIZE option when  calling  pcre2_com-
       pile(),  or by starting the pattern with (*NO_START_OPT). There is more
       discussion of this option in the section entitled "Compiling a pattern"
       in the pcre2api documentation.

       Experiments  with  Perl  suggest that it too has similar optimizations,
       sometimes leading to anomalous results.

   Verbs that act immediately

       The following verbs act as soon as they are encountered. They  may  not
       be followed by a name.

          (*ACCEPT)

       This  verb causes the match to end successfully, skipping the remainder
       of the pattern. However, when it is inside a subpattern that is  called
       as  a  subroutine, only that subpattern is ended successfully. Matching
       then continues at the outer level. If (*ACCEPT) in triggered in a posi-
       tive  assertion,  the  assertion succeeds; in a negative assertion, the
       assertion fails.

       If (*ACCEPT) is inside capturing parentheses, the data so far  is  cap-
       tured. For example:

         A((?:A|B(*ACCEPT)|C)D)

       This  matches  "AB", "AAD", or "ACD"; when it matches "AB", "B" is cap-
       tured by the outer parentheses.

         (*FAIL) or (*F)

       This verb causes a matching failure, forcing backtracking to occur.  It
       is  equivalent to (?!) but easier to read. The Perl documentation notes
       that it is probably useful only when combined  with  (?{})  or  (??{}).
       Those  are, of course, Perl features that are not present in PCRE2. The
       nearest equivalent is the callout feature, as for example in this  pat-
       tern:

         a+(?C)(*FAIL)

       A  match  with the string "aaaa" always fails, but the callout is taken
       before each backtrack happens (in this example, 10 times).

   Recording which path was taken

       There is one verb whose main purpose  is  to  track  how  a  match  was
       arrived  at,  though  it  also  has a secondary use in conjunction with
       advancing the match starting point (see (*SKIP) below).

         (*MARK:NAME) or (*:NAME)

       A name is always  required  with  this  verb.  There  may  be  as  many
       instances  of  (*MARK) as you like in a pattern, and their names do not
       have to be unique.

       When a match succeeds, the name of the  last-encountered  (*MARK:NAME),
       (*PRUNE:NAME),  or  (*THEN:NAME) on the matching path is passed back to
       the caller as described in  the  section  entitled  "Other  information
       about  the  match" in the pcre2api documentation. Here is an example of
       pcre2test output, where the "mark" modifier requests the retrieval  and
       outputting of (*MARK) data:

           re> /X(*MARK:A)Y|X(*MARK:B)Z/mark
         data> XY
          0: XY
         MK: A
         XZ
          0: XZ
         MK: B

       The (*MARK) name is tagged with "MK:" in this output, and in this exam-
       ple it indicates which of the two alternatives matched. This is a  more
       efficient  way of obtaining this information than putting each alterna-
       tive in its own capturing parentheses.

       If a verb with a name is encountered in a positive  assertion  that  is
       true,  the  name  is recorded and passed back if it is the last-encoun-
       tered. This does not happen for negative assertions or failing positive
       assertions.

       After  a  partial match or a failed match, the last encountered name in
       the entire match process is returned. For example:

           re> /X(*MARK:A)Y|X(*MARK:B)Z/mark
         data> XP
         No match, mark = B

       Note that in this unanchored example the  mark  is  retained  from  the
       match attempt that started at the letter "X" in the subject. Subsequent
       match attempts starting at "P" and then with an empty string do not get
       as far as the (*MARK) item, but nevertheless do not reset it.

       If  you  are  interested  in  (*MARK)  values after failed matches, you
       should probably set the PCRE2_NO_START_OPTIMIZE option (see  above)  to
       ensure that the match is always attempted.

   Verbs that act after backtracking

       The following verbs do nothing when they are encountered. Matching con-
       tinues with what follows, but if there is no subsequent match,  causing
       a  backtrack  to  the  verb, a failure is forced. That is, backtracking
       cannot pass to the left of the verb. However, when one of  these  verbs
       appears inside an atomic group (which includes any group that is called
       as a subroutine) or in an assertion that is true, its  effect  is  con-
       fined  to that group, because once the group has been matched, there is
       never any backtracking into it. In this situation, backtracking has  to
       jump to the left of the entire atomic group or assertion.

       These  verbs  differ  in exactly what kind of failure occurs when back-
       tracking reaches them. The behaviour described below  is  what  happens
       when  the  verb is not in a subroutine or an assertion. Subsequent sec-
       tions cover these special cases.

         (*COMMIT)

       This verb, which may not be followed by a name, causes the whole  match
       to fail outright if there is a later matching failure that causes back-
       tracking to reach it. Even if the pattern  is  unanchored,  no  further
       attempts to find a match by advancing the starting point take place. If
       (*COMMIT) is the only backtracking verb that is  encountered,  once  it
       has  been  passed  pcre2_match() is committed to finding a match at the
       current starting point, or not at all. For example:

         a+(*COMMIT)b

       This matches "xxaab" but not "aacaab". It can be thought of as  a  kind
       of dynamic anchor, or "I've started, so I must finish." The name of the
       most recently passed (*MARK) in the path is passed back when  (*COMMIT)
       forces a match failure.

       If  there  is more than one backtracking verb in a pattern, a different
       one that follows (*COMMIT) may be triggered first,  so  merely  passing
       (*COMMIT) during a match does not always guarantee that a match must be
       at this starting point.

       Note that (*COMMIT) at the start of a pattern is not  the  same  as  an
       anchor,  unless PCRE2's start-of-match optimizations are turned off, as
       shown in this output from pcre2test:

           re> /(*COMMIT)abc/
         data> xyzabc
          0: abc
         data>
         re> /(*COMMIT)abc/no_start_optimize
         data> xyzabc
         No match

       For the first pattern, PCRE2 knows that any match must start with  "a",
       so  the optimization skips along the subject to "a" before applying the
       pattern to the first set of data. The match attempt then succeeds.  The
       second  pattern disables the optimization that skips along to the first
       character. The pattern is now applied  starting  at  "x",  and  so  the
       (*COMMIT)  causes  the  match to fail without trying any other starting
       points.

         (*PRUNE) or (*PRUNE:NAME)

       This verb causes the match to fail at the current starting position  in
       the subject if there is a later matching failure that causes backtrack-
       ing to reach it. If the pattern is unanchored, the  normal  "bumpalong"
       advance  to  the next starting character then happens. Backtracking can
       occur as usual to the left of (*PRUNE), before it is reached,  or  when
       matching  to  the  right  of  (*PRUNE), but if there is no match to the
       right, backtracking cannot cross (*PRUNE). In simple cases, the use  of
       (*PRUNE)  is just an alternative to an atomic group or possessive quan-
       tifier, but there are some uses of (*PRUNE) that cannot be expressed in
       any  other  way. In an anchored pattern (*PRUNE) has the same effect as
       (*COMMIT).

       The   behaviour   of   (*PRUNE:NAME)   is   the   not   the   same   as
       (*MARK:NAME)(*PRUNE).   It  is  like  (*MARK:NAME)  in that the name is
       remembered for  passing  back  to  the  caller.  However,  (*SKIP:NAME)
       searches  only  for  names  set  with  (*MARK),  ignoring  those set by
       (*PRUNE) or (*THEN).

         (*SKIP)

       This verb, when given without a name, is like (*PRUNE), except that  if
       the  pattern  is unanchored, the "bumpalong" advance is not to the next
       character, but to the position in the subject where (*SKIP) was encoun-
       tered.  (*SKIP)  signifies that whatever text was matched leading up to
       it cannot be part of a successful match. Consider:

         a+(*SKIP)b

       If the subject is "aaaac...",  after  the  first  match  attempt  fails
       (starting  at  the  first  character in the string), the starting point
       skips on to start the next attempt at "c". Note that a possessive quan-
       tifer  does not have the same effect as this example; although it would
       suppress backtracking  during  the  first  match  attempt,  the  second
       attempt  would  start at the second character instead of skipping on to
       "c".

         (*SKIP:NAME)

       When (*SKIP) has an associated name, its behaviour is modified. When it
       is triggered, the previous path through the pattern is searched for the
       most recent (*MARK) that has the  same  name.  If  one  is  found,  the
       "bumpalong" advance is to the subject position that corresponds to that
       (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with
       a matching name is found, the (*SKIP) is ignored.

       Note  that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It
       ignores names that are set by (*PRUNE:NAME) or (*THEN:NAME).

         (*THEN) or (*THEN:NAME)

       This verb causes a skip to the next innermost  alternative  when  back-
       tracking  reaches  it.  That  is,  it  cancels any further backtracking
       within the current alternative. Its name  comes  from  the  observation
       that it can be used for a pattern-based if-then-else block:

         ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...

       If  the COND1 pattern matches, FOO is tried (and possibly further items
       after the end of the group if FOO succeeds); on  failure,  the  matcher
       skips  to  the second alternative and tries COND2, without backtracking
       into COND1. If that succeeds and BAR fails, COND3 is tried.  If  subse-
       quently  BAZ fails, there are no more alternatives, so there is a back-
       track to whatever came before the  entire  group.  If  (*THEN)  is  not
       inside an alternation, it acts like (*PRUNE).

       The    behaviour   of   (*THEN:NAME)   is   the   not   the   same   as
       (*MARK:NAME)(*THEN).  It is like  (*MARK:NAME)  in  that  the  name  is
       remembered  for  passing  back  to  the  caller.  However, (*SKIP:NAME)
       searches only for  names  set  with  (*MARK),  ignoring  those  set  by
       (*PRUNE) and (*THEN).

       A  subpattern that does not contain a | character is just a part of the
       enclosing alternative; it is not a nested  alternation  with  only  one
       alternative.  The effect of (*THEN) extends beyond such a subpattern to
       the enclosing alternative. Consider this pattern, where A, B, etc.  are
       complex  pattern fragments that do not contain any | characters at this
       level:

         A (B(*THEN)C) | D

       If A and B are matched, but there is a failure in C, matching does  not
       backtrack into A; instead it moves to the next alternative, that is, D.
       However, if the subpattern containing (*THEN) is given an  alternative,
       it behaves differently:

         A (B(*THEN)C | (*FAIL)) | D

       The  effect of (*THEN) is now confined to the inner subpattern. After a
       failure in C, matching moves to (*FAIL), which causes the whole subpat-
       tern  to  fail  because  there are no more alternatives to try. In this
       case, matching does now backtrack into A.

       Note that a conditional subpattern is  not  considered  as  having  two
       alternatives,  because  only  one  is  ever used. In other words, the |
       character in a conditional subpattern has a different meaning. Ignoring
       white space, consider:

         ^.*? (?(?=a) a | b(*THEN)c )

       If  the  subject  is  "ba", this pattern does not match. Because .*? is
       ungreedy, it initially matches zero  characters.  The  condition  (?=a)
       then  fails,  the  character  "b"  is  matched, but "c" is not. At this
       point, matching does not backtrack to .*? as might perhaps be  expected
       from  the  presence  of  the | character. The conditional subpattern is
       part of the single alternative that comprises the whole pattern, and so
       the  match  fails.  (If  there was a backtrack into .*?, allowing it to
       match "b", the match would succeed.)

       The verbs just described provide four different "strengths" of  control
       when subsequent matching fails. (*THEN) is the weakest, carrying on the
       match at the next alternative. (*PRUNE) comes next, failing  the  match
       at  the  current starting position, but allowing an advance to the next
       character (for an unanchored pattern). (*SKIP) is similar, except  that
       the advance may be more than one character. (*COMMIT) is the strongest,
       causing the entire match to fail.

   More than one backtracking verb

       If more than one backtracking verb is present in  a  pattern,  the  one
       that  is  backtracked  onto first acts. For example, consider this pat-
       tern, where A, B, etc. are complex pattern fragments:

         (A(*COMMIT)B(*THEN)C|ABD)

       If A matches but B fails, the backtrack to (*COMMIT) causes the  entire
       match to fail. However, if A and B match, but C fails, the backtrack to
       (*THEN) causes the next alternative (ABD) to be tried.  This  behaviour
       is  consistent,  but is not always the same as Perl's. It means that if
       two or more backtracking verbs appear in succession, all the  the  last
       of them has no effect. Consider this example:

         ...(*COMMIT)(*PRUNE)...

       If there is a matching failure to the right, backtracking onto (*PRUNE)
       causes it to be triggered, and its action is taken. There can never  be
       a backtrack onto (*COMMIT).

   Backtracking verbs in repeated groups

       PCRE2  differs  from  Perl  in  its  handling  of backtracking verbs in
       repeated groups. For example, consider:

         /(a(*COMMIT)b)+ac/

       If the subject is "abac", Perl matches, but  PCRE2  fails  because  the
       (*COMMIT) in the second repeat of the group acts.

   Backtracking verbs in assertions

       (*FAIL)  in  an assertion has its normal effect: it forces an immediate
       backtrack.

       (*ACCEPT) in a positive assertion causes the assertion to succeed with-
       out  any  further processing. In a negative assertion, (*ACCEPT) causes
       the assertion to fail without any further processing.

       The other backtracking verbs are not treated specially if  they  appear
       in  a  positive  assertion.  In  particular,  (*THEN) skips to the next
       alternative in the innermost enclosing  group  that  has  alternations,
       whether or not this is within the assertion.

       Negative  assertions  are,  however, different, in order to ensure that
       changing a positive assertion into a  negative  assertion  changes  its
       result. Backtracking into (*COMMIT), (*SKIP), or (*PRUNE) causes a neg-
       ative assertion to be true, without considering any further alternative
       branches in the assertion.  Backtracking into (*THEN) causes it to skip
       to the next enclosing alternative within the assertion (the normal  be-
       haviour),  but  if  the  assertion  does  not have such an alternative,
       (*THEN) behaves like (*PRUNE).

   Backtracking verbs in subroutines

       These behaviours occur whether or not the subpattern is  called  recur-
       sively.  Perl's treatment of subroutines is different in some cases.

       (*FAIL)  in  a subpattern called as a subroutine has its normal effect:
       it forces an immediate backtrack.

       (*ACCEPT) in a subpattern called as a subroutine causes the  subroutine
       match  to succeed without any further processing. Matching then contin-
       ues after the subroutine call.

       (*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine
       cause the subroutine match to fail.

       (*THEN)  skips to the next alternative in the innermost enclosing group
       within the subpattern that has alternatives. If there is no such  group
       within the subpattern, (*THEN) causes the subroutine match to fail.


SEE ALSO

       pcre2api(3),    pcre2callout(3),    pcre2matching(3),   pcre2syntax(3),
       pcre2(3).


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge, England.


REVISION

       Last updated: 20 June 2016
       Copyright (c) 1997-2016 University of Cambridge.
------------------------------------------------------------------------------


PCRE2PERFORM(3)            Library Functions Manual            PCRE2PERFORM(3)



NAME
       PCRE2 - Perl-compatible regular expressions (revised API)

PCRE2 PERFORMANCE

       Two  aspects  of performance are discussed below: memory usage and pro-
       cessing time. The way you express your pattern as a regular  expression
       can affect both of them.


COMPILED PATTERN MEMORY USAGE

       Patterns are compiled by PCRE2 into a reasonably efficient interpretive
       code, so that most simple patterns do not  use  much  memory.  However,
       there  is  one case where the memory usage of a compiled pattern can be
       unexpectedly large. If a parenthesized subpattern has a quantifier with
       a minimum greater than 1 and/or a limited maximum, the whole subpattern
       is repeated in the compiled code. For example, the pattern

         (abc|def){2,4}

       is compiled as if it were

         (abc|def)(abc|def)((abc|def)(abc|def)?)?

       (Technical aside: It is done this way so that backtrack  points  within
       each of the repetitions can be independently maintained.)

       For  regular expressions whose quantifiers use only small numbers, this
       is not usually a problem. However, if the numbers are large,  and  par-
       ticularly  if  such repetitions are nested, the memory usage can become
       an embarrassment. For example, the very simple pattern

         ((ab){1,1000}c){1,3}

       uses 51K bytes when compiled using the 8-bit  library.  When  PCRE2  is
       compiled  with its default internal pointer size of two bytes, the size
       limit on a compiled pattern is 64K code units in the 8-bit  and  16-bit
       libraries, and this is reached with the above pattern if the outer rep-
       etition is increased from 3 to 4. PCRE2 can be compiled to  use  larger
       internal  pointers  and thus handle larger compiled patterns, but it is
       better to try to rewrite your pattern to use less memory if you can.

       One way of reducing the memory usage for such patterns is to  make  use
       of PCRE2's "subroutine" facility. Re-writing the above pattern as

         ((ab)(?2){0,999}c)(?1){0,2}

       reduces the memory requirements to 18K, and indeed it remains under 20K
       even with the outer repetition increased to 100. However, this  pattern
       is  not  exactly equivalent, because the "subroutine" calls are treated
       as atomic groups into which there can be no backtracking if there is  a
       subsequent  matching  failure.  Therefore, PCRE2 cannot do this kind of
       rewriting automatically.  Furthermore, there is a  noticeable  loss  of
       speed  when executing the modified pattern. Nevertheless, if the atomic
       grouping is not a problem and the loss of  speed  is  acceptable,  this
       kind  of rewriting will allow you to process patterns that PCRE2 cannot
       otherwise handle.


STACK USAGE AT RUN TIME

       When pcre2_match() is used for matching, certain kinds of  pattern  can
       cause  it  to  use large amounts of the process stack. In some environ-
       ments the default process stack is quite small, and if it runs out  the
       result  is  often  SIGSEGV.  Rewriting your pattern can often help. The
       pcre2stack documentation discusses this issue in detail.


PROCESSING TIME

       Certain items in regular expression patterns are processed  more  effi-
       ciently than others. It is more efficient to use a character class like
       [aeiou]  than  a  set  of   single-character   alternatives   such   as
       (a|e|i|o|u).  In  general,  the simplest construction that provides the
       required behaviour is usually the most efficient. Jeffrey Friedl's book
       contains  a  lot  of useful general discussion about optimizing regular
       expressions for efficient performance. This  document  contains  a  few
       observations about PCRE2.

       Using  Unicode  character  properties  (the  \p, \P, and \X escapes) is
       slow, because PCRE2 has to use a multi-stage table lookup  whenever  it
       needs  a  character's  property. If you can find an alternative pattern
       that does not use character properties, it will probably be faster.

       By default, the escape sequences \b, \d, \s,  and  \w,  and  the  POSIX
       character  classes  such  as  [:alpha:]  do not use Unicode properties,
       partly for backwards compatibility, and partly for performance reasons.
       However,  you  can  set  the PCRE2_UCP option or start the pattern with
       (*UCP) if you want Unicode character properties to be  used.  This  can
       double  the  matching  time  for  items  such  as \d, when matched with
       pcre2_match(); the performance loss is less with a DFA  matching  func-
       tion, and in both cases there is not much difference for \b.

       When  a pattern begins with .* not in atomic parentheses, nor in paren-
       theses that are the subject of a backreference,  and  the  PCRE2_DOTALL
       option  is  set,  the pattern is implicitly anchored by PCRE2, since it
       can match only at the start of a subject string.  If  the  pattern  has
       multiple top-level branches, they must all be anchorable. The optimiza-
       tion can be disabled by  the  PCRE2_NO_DOTSTAR_ANCHOR  option,  and  is
       automatically disabled if the pattern contains (*PRUNE) or (*SKIP).

       If  PCRE2_DOTALL  is  not  set,  PCRE2  cannot  make this optimization,
       because the dot metacharacter does not then match a newline, and if the
       subject  string contains newlines, the pattern may match from the char-
       acter immediately following one of them instead of from the very start.
       For example, the pattern

         .*second

       matches  the subject "first\nand second" (where \n stands for a newline
       character), with the match starting at the seventh character. In  order
       to  do  this, PCRE2 has to retry the match starting after every newline
       in the subject.

       If you are using such a pattern with subject strings that do  not  con-
       tain   newlines,   the   best   performance   is  obtained  by  setting
       PCRE2_DOTALL, or starting the pattern with  ^.*  or  ^.*?  to  indicate
       explicit anchoring. That saves PCRE2 from having to scan along the sub-
       ject looking for a newline to restart at.

       Beware of patterns that contain nested indefinite  repeats.  These  can
       take  a  long time to run when applied to a string that does not match.
       Consider the pattern fragment

         ^(a+)*

       This can match "aaaa" in 16 different ways, and this  number  increases
       very  rapidly  as the string gets longer. (The * repeat can match 0, 1,
       2, 3, or 4 times, and for each of those cases other than 0 or 4, the  +
       repeats  can  match  different numbers of times.) When the remainder of
       the pattern is such that the entire match is going to fail,  PCRE2  has
       in  principle  to  try  every  possible variation, and this can take an
       extremely long time, even for relatively short strings.

       An optimization catches some of the more simple cases such as

         (a+)*b

       where a literal character follows. Before  embarking  on  the  standard
       matching  procedure, PCRE2 checks that there is a "b" later in the sub-
       ject string, and if there is not, it fails the match immediately.  How-
       ever,  when  there  is no following literal this optimization cannot be
       used. You can see the difference by comparing the behaviour of

         (a+)*\d

       with the pattern above. The former gives  a  failure  almost  instantly
       when  applied  to  a  whole  line of "a" characters, whereas the latter
       takes an appreciable time with strings longer than about 20 characters.

       In many cases, the solution to this kind of performance issue is to use
       an atomic group or a possessive quantifier.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge, England.


REVISION

       Last updated: 02 January 2015
       Copyright (c) 1997-2015 University of Cambridge.
------------------------------------------------------------------------------


PCRE2POSIX(3)              Library Functions Manual              PCRE2POSIX(3)



NAME
       PCRE2 - Perl-compatible regular expressions (revised API)

SYNOPSIS

       #include <pcre2posix.h>

       int regcomp(regex_t *preg, const char *pattern,
            int cflags);

       int regexec(const regex_t *preg, const char *string,
            size_t nmatch, regmatch_t pmatch[], int eflags);

       size_t regerror(int errcode, const regex_t *preg,
            char *errbuf, size_t errbuf_size);

       void regfree(regex_t *preg);


DESCRIPTION

       This  set of functions provides a POSIX-style API for the PCRE2 regular
       expression 8-bit library. See the pcre2api documentation for a descrip-
       tion  of PCRE2's native API, which contains much additional functional-
       ity. There are no POSIX-style wrappers for PCRE2's  16-bit  and  32-bit
       libraries.

       The functions described here are just wrapper functions that ultimately
       call the  PCRE2  native  API.  Their  prototypes  are  defined  in  the
       pcre2posix.h  header  file,  and  on Unix systems the library itself is
       called libpcre2-posix.a, so can be accessed by adding -lpcre2-posix  to
       the  command  for  linking  an  application that uses them. Because the
       POSIX functions call the native ones,  it  is  also  necessary  to  add
       -lpcre2-8.

       Those  POSIX  option bits that can reasonably be mapped to PCRE2 native
       options have been implemented. In addition, the option REG_EXTENDED  is
       defined  with  the  value  zero. This has no effect, but since programs
       that are written to the POSIX interface often use  it,  this  makes  it
       easier  to  slot in PCRE2 as a replacement library. Other POSIX options
       are not even defined.

       There are also some options that are not defined by POSIX.  These  have
       been  added  at  the  request  of users who want to make use of certain
       PCRE2-specific features via the POSIX calling interface.

       When PCRE2 is called via these functions, it is only the  API  that  is
       POSIX-like  in  style.  The syntax and semantics of the regular expres-
       sions themselves are still those of Perl, subject  to  the  setting  of
       various  PCRE2 options, as described below. "POSIX-like in style" means
       that the API approximates to the POSIX  definition;  it  is  not  fully
       POSIX-compatible,  and  in  multi-unit  encoding domains it is probably
       even less compatible.

       The header for these functions is supplied as pcre2posix.h to avoid any
       potential  clash  with  other  POSIX  libraries.  It can, of course, be
       renamed or aliased as regex.h, which is the "correct" name. It provides
       two  structure  types,  regex_t  for  compiled internal forms, and reg-
       match_t for returning captured substrings. It also  defines  some  con-
       stants  whose  names  start  with  "REG_";  these  are used for setting
       options and identifying error codes.


COMPILING A PATTERN

       The function regcomp() is called to compile a pattern into an  internal
       form.  The  pattern  is  a C string terminated by a binary zero, and is
       passed in the argument pattern. The preg argument is  a  pointer  to  a
       regex_t  structure that is used as a base for storing information about
       the compiled regular expression.

       The argument cflags is either zero, or contains one or more of the bits
       defined by the following macros:

         REG_DOTALL

       The  PCRE2_DOTALL  option  is set when the regular expression is passed
       for compilation to the native function. Note  that  REG_DOTALL  is  not
       part of the POSIX standard.

         REG_ICASE

       The  PCRE2_CASELESS option is set when the regular expression is passed
       for compilation to the native function.

         REG_NEWLINE

       The PCRE2_MULTILINE option is set when the regular expression is passed
       for  compilation  to the native function. Note that this does not mimic
       the defined POSIX behaviour for REG_NEWLINE  (see  the  following  sec-
       tion).

         REG_NOSUB

       When  a  pattern that is compiled with this flag is passed to regexec()
       for matching, the nmatch and pmatch arguments are ignored, and no  cap-
       tured strings are returned. Versions of the PCRE library prior to 10.22
       used to set the  PCRE2_NO_AUTO_CAPTURE  compile  option,  but  this  no
       longer happens because it disables the use of back references.

         REG_UCP

       The  PCRE2_UCP  option is set when the regular expression is passed for
       compilation to the native function. This causes PCRE2  to  use  Unicode
       properties  when  matchine  \d,  \w,  etc., instead of just recognizing
       ASCII values. Note that REG_UCP is not part of the POSIX standard.

         REG_UNGREEDY

       The PCRE2_UNGREEDY option is set when the regular expression is  passed
       for  compilation  to the native function. Note that REG_UNGREEDY is not
       part of the POSIX standard.

         REG_UTF

       The PCRE2_UTF option is set when the regular expression is  passed  for
       compilation  to the native function. This causes the pattern itself and
       all data strings used for matching it to be treated as  UTF-8  strings.
       Note that REG_UTF is not part of the POSIX standard.

       In  the  absence  of  these  flags, no options are passed to the native
       function.  This means the the regex  is  compiled  with  PCRE2  default
       semantics.  In particular, the way it handles newline characters in the
       subject string is the Perl way, not the POSIX way.  Note  that  setting
       PCRE2_MULTILINE has only some of the effects specified for REG_NEWLINE.
       It does not affect the way newlines are matched by the dot  metacharac-
       ter (they are not) or by a negative class such as [^a] (they are).

       The  yield of regcomp() is zero on success, and non-zero otherwise. The
       preg structure is filled in on success, and one member of the structure
       is  public: re_nsub contains the number of capturing subpatterns in the
       regular expression. Various error codes are defined in the header file.

       NOTE: If the yield of regcomp() is non-zero, you must  not  attempt  to
       use the contents of the preg structure. If, for example, you pass it to
       regexec(), the result is undefined and your program is likely to crash.


MATCHING NEWLINE CHARACTERS

       This area is not simple, because POSIX and Perl take different views of
       things.   It  is not possible to get PCRE2 to obey POSIX semantics, but
       then PCRE2 was never intended to be a POSIX engine. The following table
       lists  the  different  possibilities for matching newline characters in
       Perl and PCRE2:

                                 Default   Change with

         . matches newline          no     PCRE2_DOTALL
         newline matches [^a]       yes    not changeable
         $ matches \n at end        yes    PCRE2_DOLLAR_ENDONLY
         $ matches \n in middle     no     PCRE2_MULTILINE
         ^ matches \n in middle     no     PCRE2_MULTILINE

       This is the equivalent table for a POSIX-compatible pattern matcher:

                                 Default   Change with

         . matches newline          yes    REG_NEWLINE
         newline matches [^a]       yes    REG_NEWLINE
         $ matches \n at end        no     REG_NEWLINE
         $ matches \n in middle     no     REG_NEWLINE
         ^ matches \n in middle     no     REG_NEWLINE

       This behaviour is not what happens when PCRE2 is called via  its  POSIX
       API.  By  default, PCRE2's behaviour is the same as Perl's, except that
       there is no equivalent for PCRE2_DOLLAR_ENDONLY in Perl. In both  PCRE2
       and Perl, there is no way to stop newline from matching [^a].

       Default  POSIX newline handling can be obtained by setting PCRE2_DOTALL
       and PCRE2_DOLLAR_ENDONLY when  calling  pcre2_compile()  directly,  but
       there  is  no  way  to make PCRE2 behave exactly as for the REG_NEWLINE
       action. When using the POSIX API, passing REG_NEWLINE to  PCRE2's  reg-
       comp() function causes PCRE2_MULTILINE to be passed to pcre2_compile(),
       and REG_DOTALL passes PCRE2_DOTALL. There is no way to pass  PCRE2_DOL-
       LAR_ENDONLY.


MATCHING A PATTERN

       The  function  regexec()  is  called  to  match a compiled pattern preg
       against a given string, which is by default terminated by a  zero  byte
       (but  see  REG_STARTEND below), subject to the options in eflags. These
       can be:

         REG_NOTBOL

       The PCRE2_NOTBOL option is set when calling the underlying PCRE2 match-
       ing function.

         REG_NOTEMPTY

       The  PCRE2_NOTEMPTY  option  is  set  when calling the underlying PCRE2
       matching function. Note that REG_NOTEMPTY is  not  part  of  the  POSIX
       standard.  However, setting this option can give more POSIX-like behav-
       iour in some situations.

         REG_NOTEOL

       The PCRE2_NOTEOL option is set when calling the underlying PCRE2 match-
       ing function.

         REG_STARTEND

       The  string  is  considered to start at string + pmatch[0].rm_so and to
       have a terminating NUL located at string + pmatch[0].rm_eo (there  need
       not  actually  be  a  NUL at that location), regardless of the value of
       nmatch. This is a BSD extension, compatible with but not  specified  by
       IEEE  Standard  1003.2  (POSIX.2),  and  should be used with caution in
       software intended to be portable to other systems. Note that a non-zero
       rm_so does not imply REG_NOTBOL; REG_STARTEND affects only the location
       of the string, not how it is matched. Setting REG_STARTEND and  passing
       pmatch  as  NULL  are  mutually  exclusive;  the  error  REG_INVARG  is
       returned.

       If the pattern was compiled with the REG_NOSUB flag, no data about  any
       matched  strings  is  returned.  The  nmatch  and  pmatch  arguments of
       regexec() are ignored (except possibly as input for REG_STARTEND).

       The value of nmatch may be zero, and  the  value  pmatch  may  be  NULL
       (unless  REG_STARTEND  is  set);  in both these cases no data about any
       matched strings is returned.

       Otherwise, the portion of the string that was  matched,  and  also  any
       captured substrings, are returned via the pmatch argument, which points
       to an array of nmatch structures of  type  regmatch_t,  containing  the
       members  rm_so  and  rm_eo.  These contain the byte offset to the first
       character of each substring and the offset to the first character after
       the  end of each substring, respectively. The 0th element of the vector
       relates to the entire portion of string that  was  matched;  subsequent
       elements relate to the capturing subpatterns of the regular expression.
       Unused entries in the array have both structure members set to -1.

       A successful match yields  a  zero  return;  various  error  codes  are
       defined  in  the  header  file,  of which REG_NOMATCH is the "expected"
       failure code.


ERROR MESSAGES

       The regerror() function maps a non-zero errorcode from either regcomp()
       or  regexec()  to  a  printable message. If preg is not NULL, the error
       should have arisen from the use of that structure. A message terminated
       by  a binary zero is placed in errbuf. If the buffer is too short, only
       the first errbuf_size - 1 characters of the error message are used. The
       yield  of  the  function is the size of buffer needed to hold the whole
       message, including the terminating zero. This  value  is  greater  than
       errbuf_size if the message was truncated.


MEMORY USAGE

       Compiling  a regular expression causes memory to be allocated and asso-
       ciated with the preg structure. The function regfree() frees  all  such
       memory,  after  which  preg may no longer be used as a compiled expres-
       sion.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge, England.


REVISION

       Last updated: 31 January 2016
       Copyright (c) 1997-2016 University of Cambridge.
------------------------------------------------------------------------------


PCRE2SAMPLE(3)             Library Functions Manual             PCRE2SAMPLE(3)



NAME
       PCRE2 - Perl-compatible regular expressions (revised API)

PCRE2 SAMPLE PROGRAM

       A  simple, complete demonstration program to get you started with using
       PCRE2 is supplied in the file pcre2demo.c in the src directory  in  the
       PCRE2 distribution. A listing of this program is given in the pcre2demo
       documentation. If you do not have a copy of the PCRE2 distribution, you
       can save this listing to re-create the contents of pcre2demo.c.

       The  demonstration  program compiles the regular expression that is its
       first argument, and matches it against the subject string in its second
       argument.  No  PCRE2  options are set, and default character tables are
       used. If matching succeeds, the program outputs the portion of the sub-
       ject  that  matched,  together  with  the contents of any captured sub-
       strings.

       If the -g option is given on the command line, the program then goes on
       to check for further matches of the same regular expression in the same
       subject string. The logic is a little bit tricky because of the  possi-
       bility  of  matching an empty string. Comments in the code explain what
       is going on.

       The code in pcre2demo.c is an 8-bit program that uses the  PCRE2  8-bit
       library.  It  handles  strings  and characters that are stored in 8-bit
       code units.  By default, one character corresponds to  one  code  unit,
       but  if  the  pattern starts with "(*UTF)", both it and the subject are
       treated as UTF-8 strings, where characters  may  occupy  multiple  code
       units.

       If  PCRE2  is installed in the standard include and library directories
       for your operating system, you should be able to compile the demonstra-
       tion program using a command like this:

         cc -o pcre2demo pcre2demo.c -lpcre2-8

       If PCRE2 is installed elsewhere, you may need to add additional options
       to the command line. For example, on a Unix-like system that has  PCRE2
       installed  in  /usr/local,  you  can  compile the demonstration program
       using a command like this:

         cc -o pcre2demo -I/usr/local/include pcre2demo.c \
            -L/usr/local/lib -lpcre2-8

       Once you have built the demonstration program, you can run simple tests
       like this:

         ./pcre2demo 'cat|dog' 'the cat sat on the mat'
         ./pcre2demo -g 'cat|dog' 'the dog sat on the cat'

       Note  that  there  is  a  much  more comprehensive test program, called
       pcre2test, which supports many  more  facilities  for  testing  regular
       expressions using all three PCRE2 libraries (8-bit, 16-bit, and 32-bit,
       though not all three need be installed). The pcre2demo program is  pro-
       vided as a relatively simple coding example.

       If you try to run pcre2demo when PCRE2 is not installed in the standard
       library directory, you may get an error like  this  on  some  operating
       systems (e.g. Solaris):

         ld.so.1: pcre2demo: fatal: libpcre2-8.so.0: open failed: No such file
       or directory

       This is caused by the way shared library support works  on  those  sys-
       tems. You need to add

         -R/usr/local/lib

       (for example) to the compile command to get round this problem.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge, England.


REVISION

       Last updated: 02 February 2016
       Copyright (c) 1997-2016 University of Cambridge.
------------------------------------------------------------------------------
PCRE2SERIALIZE(3)          Library Functions Manual          PCRE2SERIALIZE(3)



NAME
       PCRE2 - Perl-compatible regular expressions (revised API)

SAVING AND RE-USING PRECOMPILED PCRE2 PATTERNS

       int32_t pcre2_serialize_decode(pcre2_code **codes,
         int32_t number_of_codes, const uint32_t *bytes,
         pcre2_general_context *gcontext);

       int32_t pcre2_serialize_encode(pcre2_code **codes,
         int32_t number_of_codes, uint32_t **serialized_bytes,
         PCRE2_SIZE *serialized_size, pcre2_general_context *gcontext);

       void pcre2_serialize_free(uint8_t *bytes);

       int32_t pcre2_serialize_get_number_of_codes(const uint8_t *bytes);

       If  you  are running an application that uses a large number of regular
       expression patterns, it may be useful to store them  in  a  precompiled
       form  instead  of  having to compile them every time the application is
       run. However, if you are using the just-in-time  optimization  feature,
       it is not possible to save and reload the JIT data, because it is posi-
       tion-dependent. The host on which the patterns  are  reloaded  must  be
       running  the  same version of PCRE2, with the same code unit width, and
       must also have the same endianness, pointer width and PCRE2_SIZE  type.
       For  example, patterns compiled on a 32-bit system using PCRE2's 16-bit
       library cannot be reloaded on a 64-bit system, nor can they be reloaded
       using the 8-bit library.


SECURITY CONCERNS

       The facility for saving and restoring compiled patterns is intended for
       use within individual applications.  As  such,  the  data  supplied  to
       pcre2_serialize_decode()  is expected to be trusted data, not data from
       arbitrary external sources.  There  is  only  some  simple  consistency
       checking, not complete validation of what is being re-loaded.


SAVING COMPILED PATTERNS

       Before compiled patterns can be saved they must be serialized, that is,
       converted to a stream of bytes. A single byte stream  may  contain  any
       number  of  compiled patterns, but they must all use the same character
       tables. A single copy of the tables is included in the byte stream (its
       size is 1088 bytes). For more details of character tables, see the sec-
       tion on locale support in the pcre2api documentation.

       The function pcre2_serialize_encode() creates a serialized byte  stream
       from  a  list of compiled patterns. Its first two arguments specify the
       list, being a pointer to a vector of pointers to compiled patterns, and
       the length of the vector. The third and fourth arguments point to vari-
       ables which are set to point to the created byte stream and its length,
       respectively.  The  final  argument  is a pointer to a general context,
       which can be used to specify custom memory  mangagement  functions.  If
       this  argument  is NULL, malloc() is used to obtain memory for the byte
       stream. The yield of the function is the number of serialized patterns,
       or one of the following negative error codes:

         PCRE2_ERROR_BADDATA      the number of patterns is zero or less
         PCRE2_ERROR_BADMAGIC     mismatch of id bytes in one of the patterns
         PCRE2_ERROR_MEMORY       memory allocation failed
         PCRE2_ERROR_MIXEDTABLES  the patterns do not all use the same tables
         PCRE2_ERROR_NULL         the 1st, 3rd, or 4th argument is NULL

       PCRE2_ERROR_BADMAGIC  means  either that a pattern's code has been cor-
       rupted, or that a slot in the vector does not point to a compiled  pat-
       tern.

       Once a set of patterns has been serialized you can save the data in any
       appropriate manner. Here is sample code that compiles two patterns  and
       writes them to a file. It assumes that the variable fd refers to a file
       that is open for output. The error checking that should be present in a
       real application has been omitted for simplicity.

         int errorcode;
         uint8_t *bytes;
         PCRE2_SIZE erroroffset;
         PCRE2_SIZE bytescount;
         pcre2_code *list_of_codes[2];
         list_of_codes[0] = pcre2_compile("first pattern",
           PCRE2_ZERO_TERMINATED, 0, &errorcode, &erroroffset, NULL);
         list_of_codes[1] = pcre2_compile("second pattern",
           PCRE2_ZERO_TERMINATED, 0, &errorcode, &erroroffset, NULL);
         errorcode = pcre2_serialize_encode(list_of_codes, 2, &bytes,
           &bytescount, NULL);
         errorcode = fwrite(bytes, 1, bytescount, fd);

       Note  that  the  serialized data is binary data that may contain any of
       the 256 possible byte  values.  On  systems  that  make  a  distinction
       between binary and non-binary data, be sure that the file is opened for
       binary output.

       Serializing a set of patterns leaves the original  data  untouched,  so
       they  can  still  be used for matching. Their memory must eventually be
       freed in the usual way by calling pcre2_code_free(). When you have fin-
       ished with the byte stream, it too must be freed by calling pcre2_seri-
       alize_free().


RE-USING PRECOMPILED PATTERNS

       In order to re-use a set of saved patterns  you  must  first  make  the
       serialized  byte stream available in main memory (for example, by read-
       ing from a file). The management of this memory  block  is  up  to  the
       application.  You  can  use  the  pcre2_serialize_get_number_of_codes()
       function to find out how many compiled patterns are in  the  serialized
       data without actually decoding the patterns:

         uint8_t *bytes = <serialized data>;
         int32_t number_of_codes = pcre2_serialize_get_number_of_codes(bytes);

       The pcre2_serialize_decode() function reads a byte stream and recreates
       the compiled patterns in new memory blocks, setting pointers to them in
       a  vector.  The  first two arguments are a pointer to a suitable vector
       and its length, and the third argument points to  a  byte  stream.  The
       final  argument is a pointer to a general context, which can be used to
       specify custom memory mangagement functions for the  decoded  patterns.
       If this argument is NULL, malloc() and free() are used. After deserial-
       ization, the byte stream is no longer needed and can be discarded.

         int32_t number_of_codes;
         pcre2_code *list_of_codes[2];
         uint8_t *bytes = <serialized data>;
         int32_t number_of_codes =
           pcre2_serialize_decode(list_of_codes, 2, bytes, NULL);

       If the vector is not large enough for all  the  patterns  in  the  byte
       stream,  it  is  filled  with  those  that  fit,  and the remainder are
       ignored. The yield of the function is the number of  decoded  patterns,
       or one of the following negative error codes:

         PCRE2_ERROR_BADDATA    second argument is zero or less
         PCRE2_ERROR_BADMAGIC   mismatch of id bytes in the data
         PCRE2_ERROR_BADMODE    mismatch of code unit size or PCRE2 version
         PCRE2_ERROR_BADSERIALIZEDDATA  other sanity check failure
         PCRE2_ERROR_MEMORY     memory allocation failed
         PCRE2_ERROR_NULL       first or third argument is NULL

       PCRE2_ERROR_BADMAGIC  may mean that the data is corrupt, or that it was
       compiled on a system with different endianness.

       Decoded patterns can be used for matching in the usual way, and must be
       freed  by  calling pcre2_code_free(). However, be aware that there is a
       potential race issue if you  are  using  multiple  patterns  that  were
       decoded  from  a  single  byte stream in a multithreaded application. A
       single copy of the character tables is used by all the decoded patterns
       and a reference count is used to arrange for its memory to be automati-
       cally freed when the last pattern is freed, but there is no locking  on
       this  reference count. Therefore, if you want to call pcre2_code_free()
       for these patterns in different threads,  you  must  arrange  your  own
       locking,  and  ensure  that  pcre2_code_free()  cannot be called by two
       threads at the same time.

       If a pattern was processed by pcre2_jit_compile() before being  serial-
       ized,  the  JIT data is discarded and so is no longer available after a
       save/restore cycle. You can, however, process a restored  pattern  with
       pcre2_jit_compile() if you wish.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge, England.


REVISION

       Last updated: 24 May 2016
       Copyright (c) 1997-2016 University of Cambridge.
------------------------------------------------------------------------------


PCRE2STACK(3)              Library Functions Manual              PCRE2STACK(3)



NAME
       PCRE2 - Perl-compatible regular expressions (revised API)

PCRE2 DISCUSSION OF STACK USAGE

       When  you  call  pcre2_match(),  it  makes  use of an internal function
       called match(). This calls itself recursively at branch points  in  the
       pattern,  in  order  to  remember the state of the match so that it can
       back up and try a different alternative after a  failure.  As  matching
       proceeds  deeper  and deeper into the tree of possibilities, the recur-
       sion depth increases. The match() function is also called in other cir-
       cumstances,  for  example,  whenever  a  parenthesized  sub-pattern  is
       entered, and in certain cases of repetition.

       Not all calls of match() increase the recursion depth; for an item such
       as  a* it may be called several times at the same level, after matching
       different numbers of a's. Furthermore, in a number of cases  where  the
       result  of  the  recursive call would immediately be passed back as the
       result of the current call (a "tail recursion"), the function  is  just
       restarted instead.

       Each  time the internal match() function is called recursively, it uses
       memory from the process stack. For certain kinds of pattern  and  data,
       very  large  amounts of stack may be needed, despite the recognition of
       "tail recursion". Note that if  PCRE2  is  compiled  with  the  -fsani-
       tize=address  option  of  the  GCC compiler, the stack requirements are
       greatly increased.

       The above comments apply when pcre2_match() is run in its normal inter-
       pretive manner. If the compiled pattern was processed by pcre2_jit_com-
       pile(), and just-in-time compiling  was  successful,  and  the  options
       passed  to  pcre2_match()  were  not incompatible, the matching process
       uses the JIT-compiled code instead of the  match()  function.  In  this
       case, the memory requirements are handled entirely differently. See the
       pcre2jit documentation for details.

       The  pcre2_dfa_match()  function  operates  in  a  different   way   to
       pcre2_match(),  and uses recursion only when there is a regular expres-
       sion recursion or subroutine call in the  pattern.  This  includes  the
       processing  of assertion and "once-only" subpatterns, which are handled
       like subroutine calls.  Normally, these are never very  deep,  and  the
       limit  on  the  complexity  of  pcre2_dfa_match()  is controlled by the
       amount of workspace it is given.  However, it is possible to write pat-
       terns  with  runaway  infinite  recursions;  such  patterns  will cause
       pcre2_dfa_match() to run out of stack. At present, there is no  protec-
       tion against this.

       The  comments  that  follow do NOT apply to pcre2_dfa_match(); they are
       relevant only for pcre2_match() without the JIT optimization.

   Reducing pcre2_match()'s stack usage

       You can often reduce the amount of recursion, and therefore the  amount
       of  stack  used,  by  modifying the pattern that is being matched. Con-
       sider, for example, this pattern:

         ([^<]|<(?!inet))+

       It matches from wherever it starts until it encounters "<inet"  or  the
       end  of  the  data,  and is the kind of pattern that might be used when
       processing an XML file. Each iteration of the outer parentheses matches
       either  one  character that is not "<" or a "<" that is not followed by
       "inet". However, each time a  parenthesis  is  processed,  a  recursion
       occurs, so this formulation uses a stack frame for each matched charac-
       ter. For a long string, a lot of stack is required. Consider  now  this
       rewritten pattern, which matches exactly the same strings:

         ([^<]++|<(?!inet))+

       This  uses very much less stack, because runs of characters that do not
       contain "<" are "swallowed" in one item inside the parentheses.  Recur-
       sion  happens  only when a "<" character that is not followed by "inet"
       is encountered (and we assume this is relatively  rare).  A  possessive
       quantifier  is  used  to stop any backtracking into the runs of non-"<"
       characters, but that is not related to stack usage.

       This example shows that one way of avoiding stack problems when  match-
       ing long subject strings is to write repeated parenthesized subpatterns
       to match more than one character whenever possible.

   Compiling PCRE2 to use heap instead of stack for pcre2_match()

       In environments where stack memory is constrained, you  might  want  to
       compile PCRE2 to use heap memory instead of stack for remembering back-
       up points when pcre2_match() is running. This makes it run more slowly,
       however. Details of how to do this are given in the pcre2build documen-
       tation. When built in this way, instead of using the stack, PCRE2  gets
       memory  for  remembering  backup  points from the heap. By default, the
       memory is obtained by calling the system malloc() function, but you can
       arrange to supply your own memory management function. For details, see
       the section entitled "The match context" in the pcre2api documentation.
       Since the block sizes are always the same, it may be possible to imple-
       ment customized a memory handler that is more efficient than the  stan-
       dard function. The memory blocks obtained for this purpose are retained
       and re-used if possible while pcre2_match() is running.  They  are  all
       freed just before it exits.

   Limiting pcre2_match()'s stack usage

       You can set limits on the number of times the internal match() function
       is called, both in total and  recursively.  If  a  limit  is  exceeded,
       pcre2_match()  returns  an  error  code. Setting suitable limits should
       prevent it from running out of stack. The default values of the  limits
       are  very large, and unlikely ever to operate. They can be changed when
       PCRE2 is built, and they can also be set when pcre2_match() is  called.
       For  details  of these interfaces, see the pcre2build documentation and
       the section entitled "The match context" in the pcre2api documentation.

       As a very rough rule of thumb, you should reckon on about 500 bytes per
       recursion.  Thus,  if  you  want  to limit your stack usage to 8Mb, you
       should set the limit at 16000 recursions. A 64Mb stack,  on  the  other
       hand, can support around 128000 recursions.

       The  pcre2test  test program has a modifier called "find_limits" which,
       if applied to a subject line, causes it to  find  the  smallest  limits
       that  allow a a pattern to match. This is done by calling pcre2_match()
       repeatedly with different limits.

   Changing stack size in Unix-like systems

       In Unix-like environments, there is not often a problem with the  stack
       unless  very  long  strings  are  involved, though the default limit on
       stack size varies from system to system. Values from 8Mb  to  64Mb  are
       common. You can find your default limit by running the command:

         ulimit -s

       Unfortunately,  the  effect  of  running out of stack is often SIGSEGV,
       though sometimes a more explicit error message is given. You  can  nor-
       mally increase the limit on stack size by code such as this:

         struct rlimit rlim;
         getrlimit(RLIMIT_STACK, &rlim);
         rlim.rlim_cur = 100*1024*1024;
         setrlimit(RLIMIT_STACK, &rlim);

       This  reads  the current limits (soft and hard) using getrlimit(), then
       attempts to increase the soft limit to  100Mb  using  setrlimit().  You
       must do this before calling pcre2_match().

   Changing stack size in Mac OS X

       Using setrlimit(), as described above, should also work on Mac OS X. It
       is also possible to set a stack size when linking a program. There is a
       discussion   about   stack  sizes  in  Mac  OS  X  at  this  web  site:
       http://developer.apple.com/qa/qa2005/qa1419.html.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge, England.


REVISION

       Last updated: 21 November 2014
       Copyright (c) 1997-2014 University of Cambridge.
------------------------------------------------------------------------------


PCRE2SYNTAX(3)             Library Functions Manual             PCRE2SYNTAX(3)



NAME
       PCRE2 - Perl-compatible regular expressions (revised API)

PCRE2 REGULAR EXPRESSION SYNTAX SUMMARY

       The  full syntax and semantics of the regular expressions that are sup-
       ported by PCRE2 are described in the pcre2pattern  documentation.  This
       document contains a quick-reference summary of the syntax.


QUOTING

         \x         where x is non-alphanumeric is a literal x
         \Q...\E    treat enclosed characters as literal


ESCAPED CHARACTERS

       This table applies to ASCII and Unicode environments.

         \a         alarm, that is, the BEL character (hex 07)
         \cx        "control-x", where x is any ASCII printing character
         \e         escape (hex 1B)
         \f         form feed (hex 0C)
         \n         newline (hex 0A)
         \r         carriage return (hex 0D)
         \t         tab (hex 09)
         \0dd       character with octal code 0dd
         \ddd       character with octal code ddd, or backreference
         \o{ddd..}  character with octal code ddd..
         \U         "U" if PCRE2_ALT_BSUX is set (otherwise is an error)
         \uhhhh     character with hex code hhhh (if PCRE2_ALT_BSUX is set)
         \xhh       character with hex code hh
         \x{hhh..}  character with hex code hhh..

       Note that \0dd is always an octal code. The treatment of backslash fol-
       lowed by a non-zero digit is complicated; for details see  the  section
       "Non-printing  characters"  in  the  pcre2pattern  documentation, where
       details of escape processing in EBCDIC environments are also given.

       When \x is not followed by {, from zero to two hexadecimal  digits  are
       read, but if PCRE2_ALT_BSUX is set, \x must be followed by two hexadec-
       imal digits to be recognized as  a  hexadecimal  escape;  otherwise  it
       matches  a literal "x".  Likewise, if \u (in ALT_BSUX mode) is not fol-
       lowed by four hexadecimal digits, it matches a literal "u".


CHARACTER TYPES

         .          any character except newline;
                      in dotall mode, any character whatsoever
         \C         one code unit, even in UTF mode (best avoided)
         \d         a decimal digit
         \D         a character that is not a decimal digit
         \h         a horizontal white space character
         \H         a character that is not a horizontal white space character
         \N         a character that is not a newline
         \p{xx}     a character with the xx property
         \P{xx}     a character without the xx property
         \R         a newline sequence
         \s         a white space character
         \S         a character that is not a white space character
         \v         a vertical white space character
         \V         a character that is not a vertical white space character
         \w         a "word" character
         \W         a "non-word" character
         \X         a Unicode extended grapheme cluster

       \C is dangerous because it may leave the current matching point in  the
       middle of a UTF-8 or UTF-16 character. The application can lock out the
       use of \C by setting the PCRE2_NEVER_BACKSLASH_C  option.  It  is  also
       possible to build PCRE2 with the use of \C permanently disabled.

       By  default,  \d, \s, and \w match only ASCII characters, even in UTF-8
       mode or in the 16-bit and 32-bit libraries. However, if locale-specific
       matching  is  happening,  \s and \w may also match characters with code
       points in the range 128-255. If the PCRE2_UCP option is set, the behav-
       iour of these escape sequences is changed to use Unicode properties and
       they match many more characters.


GENERAL CATEGORY PROPERTIES FOR \p and \P

         C          Other
         Cc         Control
         Cf         Format
         Cn         Unassigned
         Co         Private use
         Cs         Surrogate

         L          Letter
         Ll         Lower case letter
         Lm         Modifier letter
         Lo         Other letter
         Lt         Title case letter
         Lu         Upper case letter
         L&         Ll, Lu, or Lt

         M          Mark
         Mc         Spacing mark
         Me         Enclosing mark
         Mn         Non-spacing mark

         N          Number
         Nd         Decimal number
         Nl         Letter number
         No         Other number

         P          Punctuation
         Pc         Connector punctuation
         Pd         Dash punctuation
         Pe         Close punctuation
         Pf         Final punctuation
         Pi         Initial punctuation
         Po         Other punctuation
         Ps         Open punctuation

         S          Symbol
         Sc         Currency symbol
         Sk         Modifier symbol
         Sm         Mathematical symbol
         So         Other symbol

         Z          Separator
         Zl         Line separator
         Zp         Paragraph separator
         Zs         Space separator


PCRE2 SPECIAL CATEGORY PROPERTIES FOR \p and \P

         Xan        Alphanumeric: union of properties L and N
         Xps        POSIX space: property Z or tab, NL, VT, FF, CR
         Xsp        Perl space: property Z or tab, NL, VT, FF, CR
         Xuc        Univerally-named character: one that can be
                      represented by a Universal Character Name
         Xwd        Perl word: property Xan or underscore

       Perl and POSIX space are now the same. Perl added VT to its space char-
       acter set at release 5.18.


SCRIPT NAMES FOR \p AND \P

       Ahom,   Anatolian_Hieroglyphs,  Arabic,  Armenian,  Avestan,  Balinese,
       Bamum, Bassa_Vah, Batak, Bengali, Bopomofo, Brahmi, Braille,  Buginese,
       Buhid,  Canadian_Aboriginal,  Carian, Caucasian_Albanian, Chakma, Cham,
       Cherokee,  Common,  Coptic,  Cuneiform,  Cypriot,  Cyrillic,   Deseret,
       Devanagari,  Duployan,  Egyptian_Hieroglyphs,  Elbasan, Ethiopic, Geor-
       gian, Glagolitic, Gothic,  Grantha,  Greek,  Gujarati,  Gurmukhi,  Han,
       Hangul, Hanunoo, Hatran, Hebrew, Hiragana, Imperial_Aramaic, Inherited,
       Inscriptional_Pahlavi, Inscriptional_Parthian, Javanese,  Kaithi,  Kan-
       nada,  Katakana,  Kayah_Li,  Kharoshthi, Khmer, Khojki, Khudawadi, Lao,
       Latin, Lepcha, Limbu, Linear_A, Linear_B, Lisu, Lycian,  Lydian,  Maha-
       jani,  Malayalam,  Mandaic,  Manichaean,  Meetei_Mayek,  Mende_Kikakui,
       Meroitic_Cursive, Meroitic_Hieroglyphs,  Miao,  Modi,  Mongolian,  Mro,
       Multani,   Myanmar,   Nabataean,  New_Tai_Lue,  Nko,  Ogham,  Ol_Chiki,
       Old_Hungarian, Old_Italic, Old_North_Arabian, Old_Permic,  Old_Persian,
       Old_South_Arabian, Old_Turkic, Oriya, Osmanya, Pahawh_Hmong, Palmyrene,
       Pau_Cin_Hau,  Phags_Pa,  Phoenician,  Psalter_Pahlavi,  Rejang,  Runic,
       Samaritan, Saurashtra, Sharada, Shavian, Siddham, SignWriting, Sinhala,
       Sora_Sompeng,  Sundanese,  Syloti_Nagri,  Syriac,  Tagalog,   Tagbanwa,
       Tai_Le,   Tai_Tham,  Tai_Viet,  Takri,  Tamil,  Telugu,  Thaana,  Thai,
       Tibetan, Tifinagh, Tirhuta, Ugaritic, Vai, Warang_Citi, Yi.


CHARACTER CLASSES

         [...]       positive character class
         [^...]      negative character class
         [x-y]       range (can be used for hex characters)
         [[:xxx:]]   positive POSIX named set
         [[:^xxx:]]  negative POSIX named set

         alnum       alphanumeric
         alpha       alphabetic
         ascii       0-127
         blank       space or tab
         cntrl       control character
         digit       decimal digit
         graph       printing, excluding space
         lower       lower case letter
         print       printing, including space
         punct       printing, excluding alphanumeric
         space       white space
         upper       upper case letter
         word        same as \w
         xdigit      hexadecimal digit

       In PCRE2, POSIX character set names recognize only ASCII characters  by
       default,  but  some of them use Unicode properties if PCRE2_UCP is set.
       You can use \Q...\E inside a character class.


QUANTIFIERS

         ?           0 or 1, greedy
         ?+          0 or 1, possessive
         ??          0 or 1, lazy
         *           0 or more, greedy
         *+          0 or more, possessive
         *?          0 or more, lazy
         +           1 or more, greedy
         ++          1 or more, possessive
         +?          1 or more, lazy
         {n}         exactly n
         {n,m}       at least n, no more than m, greedy
         {n,m}+      at least n, no more than m, possessive
         {n,m}?      at least n, no more than m, lazy
         {n,}        n or more, greedy
         {n,}+       n or more, possessive
         {n,}?       n or more, lazy


ANCHORS AND SIMPLE ASSERTIONS

         \b          word boundary
         \B          not a word boundary
         ^           start of subject
                       also after an internal newline in multiline mode
                       (after any newline if PCRE2_ALT_CIRCUMFLEX is set)
         \A          start of subject
         $           end of subject
                       also before newline at end of subject
                       also before internal newline in multiline mode
         \Z          end of subject
                       also before newline at end of subject
         \z          end of subject
         \G          first matching position in subject


MATCH POINT RESET

         \K          reset start of match

       \K is honoured in positive assertions, but ignored in negative ones.


ALTERNATION

         expr|expr|expr...


CAPTURING

         (...)           capturing group
         (?<name>...)    named capturing group (Perl)
         (?'name'...)    named capturing group (Perl)
         (?P<name>...)   named capturing group (Python)
         (?:...)         non-capturing group
         (?|...)         non-capturing group; reset group numbers for
                          capturing groups in each alternative


ATOMIC GROUPS

         (?>...)         atomic, non-capturing group


COMMENT

         (?#....)        comment (not nestable)


OPTION SETTING

         (?i)            caseless
         (?J)            allow duplicate names
         (?m)            multiline
         (?s)            single line (dotall)
         (?U)            default ungreedy (lazy)
         (?x)            extended (ignore white space)
         (?-...)         unset option(s)

       The following are recognized only at the very start  of  a  pattern  or
       after  one  of the newline or \R options with similar syntax. More than
       one of them may appear.

         (*LIMIT_MATCH=d) set the match limit to d (decimal number)
         (*LIMIT_RECURSION=d) set the recursion limit to d (decimal number)
         (*NOTEMPTY)     set PCRE2_NOTEMPTY when matching
         (*NOTEMPTY_ATSTART) set PCRE2_NOTEMPTY_ATSTART when matching
         (*NO_AUTO_POSSESS) no auto-possessification (PCRE2_NO_AUTO_POSSESS)
         (*NO_DOTSTAR_ANCHOR) no .* anchoring (PCRE2_NO_DOTSTAR_ANCHOR)
         (*NO_JIT)       disable JIT optimization
         (*NO_START_OPT) no start-match optimization (PCRE2_NO_START_OPTIMIZE)
         (*UTF)          set appropriate UTF mode for the library in use
         (*UCP)          set PCRE2_UCP (use Unicode properties for \d etc)

       Note that LIMIT_MATCH and LIMIT_RECURSION can only reduce the value  of
       the  limits  set by the caller of pcre2_match(), not increase them. The
       application can lock out the use of (*UTF) and (*UCP)  by  setting  the
       PCRE2_NEVER_UTF  or  PCRE2_NEVER_UCP  options, respectively, at compile
       time.


NEWLINE CONVENTION

       These are recognized only at the very start of  the  pattern  or  after
       option settings with a similar syntax.

         (*CR)           carriage return only
         (*LF)           linefeed only
         (*CRLF)         carriage return followed by linefeed
         (*ANYCRLF)      all three of the above
         (*ANY)          any Unicode newline sequence


WHAT \R MATCHES

       These  are  recognized  only  at the very start of the pattern or after
       option setting with a similar syntax.

         (*BSR_ANYCRLF)  CR, LF, or CRLF
         (*BSR_UNICODE)  any Unicode newline sequence


LOOKAHEAD AND LOOKBEHIND ASSERTIONS

         (?=...)         positive look ahead
         (?!...)         negative look ahead
         (?<=...)        positive look behind
         (?<!...)        negative look behind

       Each top-level branch of a look behind must be of a fixed length.


BACKREFERENCES

         \n              reference by number (can be ambiguous)
         \gn             reference by number
         \g{n}           reference by number
         \g{-n}          relative reference by number
         \k<name>        reference by name (Perl)
         \k'name'        reference by name (Perl)
         \g{name}        reference by name (Perl)
         \k{name}        reference by name (.NET)
         (?P=name)       reference by name (Python)


SUBROUTINE REFERENCES (POSSIBLY RECURSIVE)

         (?R)            recurse whole pattern
         (?n)            call subpattern by absolute number
         (?+n)           call subpattern by relative number
         (?-n)           call subpattern by relative number
         (?&name)        call subpattern by name (Perl)
         (?P>name)       call subpattern by name (Python)
         \g<name>        call subpattern by name (Oniguruma)
         \g'name'        call subpattern by name (Oniguruma)
         \g<n>           call subpattern by absolute number (Oniguruma)
         \g'n'           call subpattern by absolute number (Oniguruma)
         \g<+n>          call subpattern by relative number (PCRE2 extension)
         \g'+n'          call subpattern by relative number (PCRE2 extension)
         \g<-n>          call subpattern by relative number (PCRE2 extension)
         \g'-n'          call subpattern by relative number (PCRE2 extension)


CONDITIONAL PATTERNS

         (?(condition)yes-pattern)
         (?(condition)yes-pattern|no-pattern)

         (?(n)               absolute reference condition
         (?(+n)              relative reference condition
         (?(-n)              relative reference condition
         (?(<name>)          named reference condition (Perl)
         (?('name')          named reference condition (Perl)
         (?(name)            named reference condition (PCRE2)
         (?(R)               overall recursion condition
         (?(Rn)              specific group recursion condition
         (?(R&name)          specific recursion condition
         (?(DEFINE)          define subpattern for reference
         (?(VERSION[>]=n.m)  test PCRE2 version
         (?(assert)          assertion condition


BACKTRACKING CONTROL

       The following act immediately they are reached:

         (*ACCEPT)       force successful match
         (*FAIL)         force backtrack; synonym (*F)
         (*MARK:NAME)    set name to be passed back; synonym (*:NAME)

       The following act only when a subsequent match failure causes  a  back-
       track to reach them. They all force a match failure, but they differ in
       what happens afterwards. Those that advance the start-of-match point do
       so only if the pattern is not anchored.

         (*COMMIT)       overall failure, no advance of starting point
         (*PRUNE)        advance to next starting character
         (*PRUNE:NAME)   equivalent to (*MARK:NAME)(*PRUNE)
         (*SKIP)         advance to current matching position
         (*SKIP:NAME)    advance to position corresponding to an earlier
                         (*MARK:NAME); if not found, the (*SKIP) is ignored
         (*THEN)         local failure, backtrack to next alternation
         (*THEN:NAME)    equivalent to (*MARK:NAME)(*THEN)


CALLOUTS

         (?C)            callout (assumed number 0)
         (?Cn)           callout with numerical data n
         (?C"text")      callout with string data

       The allowed string delimiters are ` ' " ^ % # $ (which are the same for
       the start and the end), and the starting delimiter { matched  with  the
       ending  delimiter  }. To encode the ending delimiter within the string,
       double it.


SEE ALSO

       pcre2pattern(3),   pcre2api(3),   pcre2callout(3),    pcre2matching(3),
       pcre2(3).


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge, England.


REVISION

       Last updated: 16 October 2015
       Copyright (c) 1997-2015 University of Cambridge.
------------------------------------------------------------------------------


PCRE2UNICODE(3)            Library Functions Manual            PCRE2UNICODE(3)



NAME
       PCRE - Perl-compatible regular expressions (revised API)

UNICODE AND UTF SUPPORT

       When PCRE2 is built with Unicode support (which is the default), it has
       knowledge of Unicode character properties and can process text  strings
       in  UTF-8, UTF-16, or UTF-32 format (depending on the code unit width).
       However, by default, PCRE2 assumes that one code unit is one character.
       To  process  a  pattern  as a UTF string, where a character may require
       more than one  code  unit,  you  must  call  pcre2_compile()  with  the
       PCRE2_UTF  option  flag,  or  the  pattern must start with the sequence
       (*UTF). When either of these is the case, both the pattern and any sub-
       ject  strings  that  are  matched against it are treated as UTF strings
       instead of strings of individual one-code-unit characters.

       If you do not need Unicode support you can build PCRE2 without  it,  in
       which case the library will be smaller.


UNICODE PROPERTY SUPPORT

       When  PCRE2 is built with Unicode support, the escape sequences \p{..},
       \P{..}, and \X can be used. The Unicode properties that can  be  tested
       are  limited to the general category properties such as Lu for an upper
       case letter or Nd for a decimal number, the Unicode script  names  such
       as Arabic or Han, and the derived properties Any and L&. Full lists are
       given in the pcre2pattern and pcre2syntax documentation. Only the short
       names  for  properties are supported. For example, \p{L} matches a let-
       ter. Its Perl synonym, \p{Letter}, is not supported.   Furthermore,  in
       Perl,  many properties may optionally be prefixed by "Is", for compati-
       bility with Perl 5.6. PCRE does not support this.


WIDE CHARACTERS AND UTF MODES

       Codepoints less than 256 can be specified in patterns by either  braced
       or unbraced hexadecimal escape sequences (for example, \x{b3} or \xb3).
       Larger values have to use braced sequences. Unbraced octal code  points
       up to \777 are also recognized; larger ones can be coded using \o{...}.

       In  UTF modes, repeat quantifiers apply to complete UTF characters, not
       to individual code units.

       In UTF modes, the dot metacharacter matches one UTF  character  instead
       of a single code unit.

       The escape sequence \C can be used to match a single code unit in a UTF
       mode, but its use can lead to some strange effects because it breaks up
       multi-unit  characters  (see  the description of \C in the pcre2pattern
       documentation).

       The use of \C is not supported by  the  alternative  matching  function
       pcre2_dfa_match() when in UTF-8 or UTF-16 mode, that is, when a charac-
       ter may consist of more than one code unit. The  use  of  \C  in  these
       modes  provokes a match-time error. Also, the JIT optimization does not
       support \C in these modes. If JIT optimization is requested for a UTF-8
       or  UTF-16  pattern  that contains \C, it will not succeed, and so when
       pcre2_match() is called, the matching will be carried out by the normal
       interpretive function.

       The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly test
       characters of any code value, but,  by  default,  the  characters  that
       PCRE2  recognizes as digits, spaces, or word characters remain the same
       set as in non-UTF mode, all  with  code  points  less  than  256.  This
       remains  true  even  when  PCRE2  is  built to include Unicode support,
       because to do otherwise would slow down matching in many common  cases.
       Note  that  this also applies to \b and \B, because they are defined in
       terms of \w and \W. If you want to test for  a  wider  sense  of,  say,
       "digit",  you  can  use explicit Unicode property tests such as \p{Nd}.
       Alternatively, if you set the PCRE2_UCP option, the way that the  char-
       acter  escapes  work  is changed so that Unicode properties are used to
       determine which characters match. There are more details in the section
       on generic character types in the pcre2pattern documentation.

       Similarly,  characters that match the POSIX named character classes are
       all low-valued characters, unless the PCRE2_UCP option is set.

       However, the special  horizontal  and  vertical  white  space  matching
       escapes (\h, \H, \v, and \V) do match all the appropriate Unicode char-
       acters, whether or not PCRE2_UCP is set.

       Case-insensitive matching in UTF mode makes use of Unicode  properties.
       A  few  Unicode characters such as Greek sigma have more than two code-
       points that are case-equivalent, and these are treated as such.


VALIDITY OF UTF STRINGS

       When the PCRE2_UTF option is set, the strings passed  as  patterns  and
       subjects are (by default) checked for validity on entry to the relevant
       functions.  If an invalid UTF string is passed, an negative error  code
       is  returned.  The  code  unit offset to the offending character can be
       extracted from the match data block by  calling  pcre2_get_startchar(),
       which is used for this purpose after a UTF error.

       UTF-16 and UTF-32 strings can indicate their endianness by special code
       knows as a byte-order mark (BOM). The PCRE2  functions  do  not  handle
       this, expecting strings to be in host byte order.

       A UTF string is checked before any other processing takes place. In the
       case of pcre2_match()  and  pcre2_dfa_match()  calls  with  a  non-zero
       starting  offset, the check is applied only to that part of the subject
       that could be inspected during matching, and there is a check that  the
       starting  offset points to the first code unit of a character or to the
       end of the subject. If there are no lookbehind assertions in  the  pat-
       tern,  the check starts at the starting offset. Otherwise, it starts at
       the length of the longest lookbehind before the starting offset, or  at
       the  start  of the subject if there are not that many characters before
       the starting offset. Note that the sequences \b and \B are  one-charac-
       ter lookbehinds.

       In  addition  to checking the format of the string, there is a check to
       ensure that all code points lie in the range U+0 to U+10FFFF, excluding
       the  surrogate  area. The so-called "non-character" code points are not
       excluded because Unicode corrigendum #9 makes it clear that they should
       not be.

       Characters  in  the "Surrogate Area" of Unicode are reserved for use by
       UTF-16, where they are used in pairs to encode code points with  values
       greater  than  0xFFFF. The code points that are encoded by UTF-16 pairs
       are available independently in the  UTF-8  and  UTF-32  encodings.  (In
       other  words,  the  whole  surrogate  thing is a fudge for UTF-16 which
       unfortunately messes up UTF-8 and UTF-32.)

       In some situations, you may already know that your strings  are  valid,
       and  therefore  want  to  skip these checks in order to improve perfor-
       mance, for example in the case of a long subject string that  is  being
       scanned  repeatedly.   If you set the PCRE2_NO_UTF_CHECK option at com-
       pile time or at match time, PCRE2 assumes that the pattern  or  subject
       it is given (respectively) contains only valid UTF code unit sequences.

       Passing  PCRE2_NO_UTF_CHECK  to pcre2_compile() just disables the check
       for the pattern; it does not also apply to subject strings. If you want
       to  disable the check for a subject string you must pass this option to
       pcre2_match() or pcre2_dfa_match().

       If you pass an invalid UTF string when PCRE2_NO_UTF_CHECK is  set,  the
       result is undefined and your program may crash or loop indefinitely.

   Errors in UTF-8 strings

       The following negative error codes are given for invalid UTF-8 strings:

         PCRE2_ERROR_UTF8_ERR1
         PCRE2_ERROR_UTF8_ERR2
         PCRE2_ERROR_UTF8_ERR3
         PCRE2_ERROR_UTF8_ERR4
         PCRE2_ERROR_UTF8_ERR5

       The  string  ends  with a truncated UTF-8 character; the code specifies
       how many bytes are missing (1 to 5). Although RFC 3629 restricts  UTF-8
       characters  to  be  no longer than 4 bytes, the encoding scheme (origi-
       nally defined by RFC 2279) allows for  up  to  6  bytes,  and  this  is
       checked first; hence the possibility of 4 or 5 missing bytes.

         PCRE2_ERROR_UTF8_ERR6
         PCRE2_ERROR_UTF8_ERR7
         PCRE2_ERROR_UTF8_ERR8
         PCRE2_ERROR_UTF8_ERR9
         PCRE2_ERROR_UTF8_ERR10

       The two most significant bits of the 2nd, 3rd, 4th, 5th, or 6th byte of
       the character do not have the binary value 0b10 (that  is,  either  the
       most significant bit is 0, or the next bit is 1).

         PCRE2_ERROR_UTF8_ERR11
         PCRE2_ERROR_UTF8_ERR12

       A  character that is valid by the RFC 2279 rules is either 5 or 6 bytes
       long; these code points are excluded by RFC 3629.

         PCRE2_ERROR_UTF8_ERR13

       A 4-byte character has a value greater than 0x10fff; these code  points
       are excluded by RFC 3629.

         PCRE2_ERROR_UTF8_ERR14

       A  3-byte  character  has  a  value in the range 0xd800 to 0xdfff; this
       range of code points are reserved by RFC 3629 for use with UTF-16,  and
       so are excluded from UTF-8.

         PCRE2_ERROR_UTF8_ERR15
         PCRE2_ERROR_UTF8_ERR16
         PCRE2_ERROR_UTF8_ERR17
         PCRE2_ERROR_UTF8_ERR18
         PCRE2_ERROR_UTF8_ERR19

       A  2-, 3-, 4-, 5-, or 6-byte character is "overlong", that is, it codes
       for a value that can be represented by fewer bytes, which  is  invalid.
       For  example,  the two bytes 0xc0, 0xae give the value 0x2e, whose cor-
       rect coding uses just one byte.

         PCRE2_ERROR_UTF8_ERR20

       The two most significant bits of the first byte of a character have the
       binary  value 0b10 (that is, the most significant bit is 1 and the sec-
       ond is 0). Such a byte can only validly occur as the second  or  subse-
       quent byte of a multi-byte character.

         PCRE2_ERROR_UTF8_ERR21

       The  first byte of a character has the value 0xfe or 0xff. These values
       can never occur in a valid UTF-8 string.

   Errors in UTF-16 strings

       The following  negative  error  codes  are  given  for  invalid  UTF-16
       strings:

         PCRE2_ERROR_UTF16_ERR1  Missing low surrogate at end of string
         PCRE2_ERROR_UTF16_ERR2  Invalid low surrogate follows high surrogate
         PCRE2_ERROR_UTF16_ERR3  Isolated low surrogate


   Errors in UTF-32 strings

       The  following  negative  error  codes  are  given  for  invalid UTF-32
       strings:

         PCRE2_ERROR_UTF32_ERR1  Surrogate character (0xd800 to 0xdfff)
         PCRE2_ERROR_UTF32_ERR2  Code point is greater than 0x10ffff


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge, England.


REVISION

       Last updated: 03 July 2016
       Copyright (c) 1997-2016 University of Cambridge.
------------------------------------------------------------------------------


