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3rd/pcre2/doc/pcre2limits.3

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+.TH PCRE2LIMITS 3 "16 August 2023" "PCRE2 10.45"
+.SH NAME
+PCRE2 - Perl-compatible regular expressions (revised API)
+.SH "SIZE AND OTHER LIMITATIONS"
+.rs
+.sp
+There are some size limitations in PCRE2 but it is hoped that they will never
+in practice be relevant.
+.P
+The maximum size of a compiled pattern is approximately 64 thousand 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 \fBREADME\fP file in the source distribution and the
+.\" HREF
+\fBpcre2build\fP
+.\"
+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.
+.P
+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 \fBpcre2_compile()\fP can specify a smaller limit.
+.P
+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.
+.P
+All values in repeating quantifiers must be less than 65536.
+.P
+There are two different limits that apply to branches of lookbehind assertions.
+If every branch in such an assertion matches a fixed number of characters,
+the maximum length of any branch is 65535 characters. If any branch matches a
+variable number of characters, then the maximum matching length for every
+branch is limited. The default limit is set at compile time, defaulting to 255,
+but can be changed by the calling program.
+.P
+There is no limit to the number of parenthesized groups, but there can be no
+more than 65535 capture groups, and there is 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 default limit can be specified
+when PCRE2 is built; if not, the default is set to 250. An application can
+change this limit by calling pcre2_set_parens_nest_limit() to set the limit in
+a compile context.
+.P
+The maximum length of name for a named capture group is 32 code units, and the
+maximum number of such groups is 10000.
+.P
+The maximum length of a name in a (*MARK), (*PRUNE), (*SKIP), or (*THEN) verb
+is 255 code units for the 8-bit library and 65535 code units for the 16-bit and
+32-bit libraries.
+.P
+The maximum length of a string argument to a callout is the largest number a
+32-bit unsigned integer can hold.
+.P
+The maximum amount of heap memory used for matching is controlled by the heap
+limit, which can be set in a pattern or in a match context. The default is a
+very large number, effectively unlimited.
+.
+.
+.SH AUTHOR
+.rs
+.sp
+.nf
+Philip Hazel
+Retired from University Computing Service
+Cambridge, England.
+.fi
+.
+.
+.SH REVISION
+.rs
+.sp
+.nf
+Last updated: 16 August 2023
+Copyright (c) 1997-2023 University of Cambridge.
+.fi