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version 1.1.1.2, 2012/02/21 23:50:25	version 1.1.1.5, 2014/06/15 19:46:05
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.TH PCREMATCHING 3	.TH PCREMATCHING 3 "12 November 2013" "PCRE 8.34"
.SH NAME	.SH NAME
PCRE - Perl-compatible regular expressions	PCRE - Perl-compatible regular expressions
.SH "PCRE MATCHING ALGORITHMS"	.SH "PCRE MATCHING ALGORITHMS"
Line 6 PCRE - Perl-compatible regular expressions	Line 6 PCRE - Perl-compatible regular expressions
.sp	.sp
This document describes the two different algorithms that are available in PCRE	This document describes the two different algorithms that are available in PCRE
for matching a compiled regular expression against a given subject string. The	for matching a compiled regular expression against a given subject string. The
"standard" algorithm is the one provided by the \fBpcre_exec()\fP and	"standard" algorithm is the one provided by the \fBpcre_exec()\fP,
\fBpcre16_exec()\fP functions. These work in the same was as Perl's matching	\fBpcre16_exec()\fP and \fBpcre32_exec()\fP functions. These work in the same
function, and provide a Perl-compatible matching operation. The just-in-time	as as Perl's matching function, and provide a Perl-compatible matching operation.
(JIT) optimization that is described in the	The just-in-time (JIT) optimization that is described in the
.\" HREF	.\" HREF
\fBpcrejit\fP	\fBpcrejit\fP
.\"	.\"
documentation is compatible with these functions.	documentation is compatible with these functions.
.P	.P
An alternative algorithm is provided by the \fBpcre_dfa_exec()\fP and	An alternative algorithm is provided by the \fBpcre_dfa_exec()\fP,
\fBpcre16_dfa_exec()\fP functions; they operate in a different way, and are not	\fBpcre16_dfa_exec()\fP and \fBpcre32_dfa_exec()\fP functions; they operate in
Perl-compatible. This alternative has advantages and disadvantages compared	a different way, and are not Perl-compatible. This alternative has advantages
with the standard algorithm, and these are described below.	and disadvantages compared with the standard algorithm, and these are described
	below.
.P	.P
When there is only one possible way in which a given subject string can match a	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,	pattern, the two algorithms give the same answer. A difference arises, however,
Line 105 the three strings "caterpillar", "cater", and "cat" th	Line 106 the three strings "caterpillar", "cater", and "cat" th
character of the subject. The algorithm does not automatically move on to find	character of the subject. The algorithm does not automatically move on to find
matches that start at later positions.	matches that start at later positions.
.P	.P
	PCRE's "auto-possessification" optimization usually applies to character
	repeats at the end of a pattern (as well as internally). For example, the
	pattern "a\ed+" is compiled as if it were "a\ed++" 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\ed+?") or set the PCRE_NO_AUTO_POSSESS option when compiling.
	.P
There are a number of features of PCRE regular expressions that are not	There are a number of features of PCRE regular expressions that are not
supported by the alternative matching algorithm. They are as follows:	supported by the alternative matching algorithm. They are as follows:
.P	.P
Line 140 and not on others), is not supported. It causes an err	Line 149 and not on others), is not supported. It causes an err
always 1, and the value of the \fIcapture_last\fP field is always -1.	always 1, and the value of the \fIcapture_last\fP field is always -1.
.P	.P
7. The \eC escape sequence, which (in the standard algorithm) always matches a	7. The \eC escape sequence, which (in the standard algorithm) always matches a
single data unit, even in UTF-8 or UTF-16 modes, is not supported in these	single data unit, even in UTF-8, UTF-16 or UTF-32 modes, is not supported in
modes, because the alternative algorithm moves through the subject string one	these modes, because the alternative algorithm moves through the subject string
character (not data unit) at a time, for all active paths through the tree.	one character (not data unit) at a time, for all active paths through the tree.
.P	.P
8. Except for (FAIL), the backtracking control verbs such as (PRUNE) are not	8. Except for (FAIL), the backtracking control verbs such as (PRUNE) are not
supported. (*FAIL) is supported, and behaves like a failing negative assertion.	supported. (*FAIL) is supported, and behaves like a failing negative assertion.
Line 200 Cambridge CB2 3QH, England.	Line 209 Cambridge CB2 3QH, England.
.rs	.rs
.sp	.sp
.nf	.nf
Last updated: 08 January 2012	Last updated: 12 November 2013
Copyright (c) 1997-2012 University of Cambridge.	Copyright (c) 1997-2012 University of Cambridge.
.fi	.fi

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