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version 1.1.1.1, 2012/02/21 23:05:51	version 1.1.1.3, 2012/10/09 09:19:17
Line 1	Line 1
.TH PCREPERFORM 3	.TH PCREPERFORM 3 "09 January 2012" "PCRE 8.30"
.SH NAME	.SH NAME
PCRE - Perl-compatible regular expressions	PCRE - Perl-compatible regular expressions
.SH "PCRE PERFORMANCE"	.SH "PCRE PERFORMANCE"
Line 11 of them.	Line 11 of them.
.SH "COMPILED PATTERN MEMORY USAGE"	.SH "COMPILED PATTERN MEMORY USAGE"
.rs	.rs
.sp	.sp
Patterns are compiled by PCRE into a reasonably efficient byte code, so that	Patterns are compiled by PCRE into a reasonably efficient interpretive code, so
most simple patterns do not use much memory. However, there is one case where	that most simple patterns do not use much memory. However, there is one case
the memory usage of a compiled pattern can be unexpectedly large. If a	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	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	a limited maximum, the whole subpattern is repeated in the compiled code. For
example, the pattern	example, the pattern
Line 34 example, the very simple pattern	Line 34 example, the very simple pattern
.sp	.sp
((ab){1,1000}c){1,3}	((ab){1,1000}c){1,3}
.sp	.sp
uses 51K bytes when compiled. When PCRE is compiled with its default internal	uses 51K bytes when compiled using the 8-bit library. When PCRE is compiled
pointer size of two bytes, the size limit on a compiled pattern is 64K, and	with its default internal pointer size of two bytes, the size limit on a
this is reached with the above pattern if the outer repetition is increased	compiled pattern is 64K data units, and this is reached with the above pattern
from 3 to 4. PCRE can be compiled to use larger internal pointers and thus	if the outer repetition is increased from 3 to 4. PCRE can be compiled to use
handle larger compiled patterns, but it is better to try to rewrite your	larger internal pointers and thus handle larger compiled patterns, but it is
pattern to use less memory if you can.	better to try to rewrite your pattern to use less memory if you can.
.P	.P
One way of reducing the memory usage for such patterns is to make use of PCRE's	One way of reducing the memory usage for such patterns is to make use of PCRE's
.\" HTML <a href="pcrepattern.html#subpatternsassubroutines">	.\" HTML <a href="pcrepattern.html#subpatternsassubroutines">
Line 68 that PCRE cannot otherwise handle.	Line 68 that PCRE cannot otherwise handle.
.SH "STACK USAGE AT RUN TIME"	.SH "STACK USAGE AT RUN TIME"
.rs	.rs
.sp	.sp
When \fBpcre_exec()\fP is used for matching, certain kinds of pattern can cause	When \fBpcre_exec()\fP or \fBpcre16_exec()\fP is used for matching, certain
it to use large amounts of the process stack. In some environments the default	kinds of pattern can cause it to use large amounts of the process stack. In
process stack is quite small, and if it runs out the result is often SIGSEGV.	some environments the default process stack is quite small, and if it runs out
This issue is probably the most frequently raised problem with PCRE. Rewriting	the result is often SIGSEGV. This issue is probably the most frequently raised
your pattern can often help. The	problem with PCRE. Rewriting your pattern can often help. The
.\" HREF	.\" HREF
\fBpcrestack\fP	\fBpcrestack\fP
.\"	.\"
Line 101 character classes such as [:alpha:] do not use Unicode	Line 101 character classes such as [:alpha:] do not use Unicode
backwards compatibility, and partly for performance reasons. However, you can	backwards compatibility, and partly for performance reasons. However, you can
set PCRE_UCP if you want Unicode character properties to be used. This can	set PCRE_UCP if you want Unicode character properties to be used. This can
double the matching time for items such as \ed, when matched with	double the matching time for items such as \ed, when matched with
\fBpcre_exec()\fP; the performance loss is less with \fBpcre_dfa_exec()\fP, and	a traditional matching function; the performance loss is less with
in both cases there is not much difference for \eb.	a DFA matching function, and in both cases there is not much difference for
	\eb.
.P	.P
When a pattern begins with .* not in parentheses, or in parentheses that are	When a pattern begins with .* not in parentheses, or in parentheses that are
not the subject of a backreference, and the PCRE_DOTALL option is set, the	not the subject of a backreference, and the PCRE_DOTALL option is set, the
Line 172 Cambridge CB2 3QH, England.	Line 173 Cambridge CB2 3QH, England.
.rs	.rs
.sp	.sp
.nf	.nf
Last updated: 16 May 2010	Last updated: 09 January 2012
Copyright (c) 1997-2010 University of Cambridge.	Copyright (c) 1997-2012 University of Cambridge.
.fi	.fi

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	Added in v.1.1.1.3