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 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 (c) 1997-2015 University of Cambridge.