# pcre2limits man page

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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 building 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 subpatterns 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 © 1997-2015 University of Cambridge.

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