doc/html/pcre2matching.html - platform/external/pcre - Gitiles

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 <head>
 <title>pcre2matching specification</title>
 </head>
 <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
 <h1>pcre2matching man page</h1>
 <p>
 Return to the <a href="index.html">PCRE2 index page</a>.
 </p>
 <p>
 This page is part of the PCRE2 HTML documentation. It was generated
 automatically from the original man page. If there is any nonsense in it,
 please consult the man page, in case the conversion went wrong.
 <br>
 <ul>
 <li><a name="TOC1" href="#SEC1">PCRE2 MATCHING ALGORITHMS</a>
 <li><a name="TOC2" href="#SEC2">REGULAR EXPRESSIONS AS TREES</a>
 <li><a name="TOC3" href="#SEC3">THE STANDARD MATCHING ALGORITHM</a>
 <li><a name="TOC4" href="#SEC4">THE ALTERNATIVE MATCHING ALGORITHM</a>
 <li><a name="TOC5" href="#SEC5">ADVANTAGES OF THE ALTERNATIVE ALGORITHM</a>
 <li><a name="TOC6" href="#SEC6">DISADVANTAGES OF THE ALTERNATIVE ALGORITHM</a>
 <li><a name="TOC7" href="#SEC7">AUTHOR</a>
 <li><a name="TOC8" href="#SEC8">REVISION</a>
 </ul>
 <br><a name="SEC1" href="#TOC1">PCRE2 MATCHING ALGORITHMS</a><br>
 <P>
 This document describes the two different algorithms that are available in
 PCRE2 for matching a compiled regular expression against a given subject
 string. The "standard" algorithm is the one provided by the <b>pcre2_match()</b>
 function. This works in the same as as Perl's matching function, and provide a
 Perl-compatible matching operation. The just-in-time (JIT) optimization that is
 described in the
 <a href="pcre2jit.html"><b>pcre2jit</b></a>
 documentation is compatible with this function.
 </P>
 <P>
 An alternative algorithm is provided by the <b>pcre2_dfa_match()</b> function;
 it operates in a different way, and is not Perl-compatible. This alternative
 has advantages 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
 pattern, the two algorithms give the same answer. A difference arises, however,
 when there are multiple possibilities. For example, if the pattern
 <pre>
   ^&#60;.*&#62;
 </pre>
 is matched against the string
 <pre>
   &#60;something&#62; &#60;something else&#62; &#60;something further&#62;
 </pre>
 there are three possible answers. The standard algorithm finds only one of
 them, whereas the alternative algorithm finds all three.
 </P>
 <br><a name="SEC2" href="#TOC1">REGULAR EXPRESSIONS AS TREES</a><br>
 <P>
 The set of strings that are matched by a regular expression can be represented
 as a tree structure. An unlimited repetition in the pattern makes the tree of
 infinite size, but it is still a tree. Matching the pattern to a given subject
 string (from a given starting point) can be thought of as a search of the tree.
 There are two ways to search a tree: depth-first and breadth-first, and these
 correspond to the two matching algorithms provided by PCRE2.
 </P>
 <br><a name="SEC3" href="#TOC1">THE STANDARD MATCHING ALGORITHM</a><br>
 <P>
 In the terminology of Jeffrey Friedl's book "Mastering Regular Expressions",
 the standard algorithm is an "NFA algorithm". It conducts a depth-first search
 of the pattern tree. That is, it proceeds along a single path through the tree,
 checking that the subject matches what is required. When there is a mismatch,
 the algorithm tries any alternatives at the current point, and if they all
 fail, it backs up to the previous branch point in the tree, and tries the next
 alternative branch at that level. This often involves backing up (moving to the
 left) in the subject string as well. The order in which repetition branches are
 tried is controlled by the greedy or ungreedy nature of the quantifier.
 </P>
 <P>
 If a leaf node is reached, a matching string has been found, and at that point
 the algorithm stops. Thus, if there is more than one possible match, this
 algorithm returns the first one that it finds. Whether this is the shortest,
 the longest, or some intermediate length depends on the way the alternations
 and the greedy or ungreedy repetition quantifiers are specified in the
 pattern.
 </P>
 <P>
 Because it ends up with a single path through the tree, it is relatively
 straightforward for this algorithm to keep track of the substrings that are
 matched by portions of the pattern in parentheses. This provides support for
 capturing parentheses and backreferences.
 </P>
 <br><a name="SEC4" href="#TOC1">THE ALTERNATIVE MATCHING ALGORITHM</a><br>
 <P>
 This algorithm conducts a breadth-first search of the tree. Starting from the
 first matching point in the subject, it scans the subject string from left to
 right, once, character by character, and as it does this, it remembers all the
 paths through the tree that represent valid matches. In Friedl's terminology,
 this is a kind of "DFA algorithm", though it is not implemented as a
 traditional finite state machine (it keeps multiple states active
 simultaneously).
 </P>
 <P>
 Although the general principle of this matching algorithm is that it scans the
 subject string only once, without backtracking, there is one exception: when a
 lookaround assertion is encountered, the characters following or preceding the
 current point have to be independently inspected.
 </P>
 <P>
 The scan continues until either the end of the subject is reached, or there are
 no more unterminated paths. At this point, terminated paths represent the
 different matching possibilities (if there are none, the match has failed).
 Thus, if there is more than one possible match, this algorithm finds all of
 them, and in particular, it finds the longest. The matches are returned in
 the output vector in decreasing order of length. There is an option to stop the
 algorithm after the first match (which is necessarily the shortest) is found.
 </P>
 <P>
 Note that the size of vector needed to contain all the results depends on the
 number of simultaneous matches, not on the number of parentheses in the
 pattern. Using <b>pcre2_match_data_create_from_pattern()</b> to create the match
 data block is therefore not advisable when doing DFA matching.
 </P>
 <P>
 Note also that all the matches that are found start at the same point in the
 subject. If the pattern
 <pre>
   cat(er(pillar)?)?
 </pre>
 is matched against the string "the caterpillar catchment", the result is the
 three strings "caterpillar", "cater", and "cat" that start at the fifth
 character of the subject. The algorithm does not automatically move on to find
 matches that start at later positions.
 </P>
 <P>
 PCRE2's "auto-possessification" optimization usually applies to character
 repeats at the end of a pattern (as well as internally). For example, the
 pattern "a\d+" is compiled as if it were "a\d++" 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\d+?") or set the PCRE2_NO_AUTO_POSSESS option when compiling.
 </P>
 <P>
 There are a number of features of PCRE2 regular expressions that are not
 supported or behave differently in the alternative matching function. Those
 that are not supported cause an error if encountered.
 </P>
 <P>
 1. Because the algorithm finds all possible matches, the greedy or ungreedy
 nature of repetition quantifiers is not relevant (though it may affect
 auto-possessification, as just described). During matching, greedy and ungreedy
 quantifiers are treated in exactly the same way. However, possessive
 quantifiers can make a difference when what follows could also match what is
 quantified, for example in a pattern like this:
 <pre>
   ^a++\w!
 </pre>
 This pattern matches "aaab!" but not "aaa!", which would be matched by a
 non-possessive quantifier. Similarly, if an atomic group is present, it is
 matched as if it were a standalone pattern at the current point, and the
 longest match is then "locked in" for the rest of the overall pattern.
 </P>
 <P>
 2. When dealing with multiple paths through the tree simultaneously, it is not
 straightforward to keep track of captured substrings for the different matching
 possibilities, and PCRE2's implementation of this algorithm does not attempt to
 do this. This means that no captured substrings are available.
 </P>
 <P>
 3. Because no substrings are captured, backreferences within the pattern are
 not supported.
 </P>
 <P>
 4. For the same reason, conditional expressions that use a backreference as the
 condition or test for a specific group recursion are not supported.
 </P>
 <P>
 5. Again for the same reason, script runs are not supported.
 </P>
 <P>
 6. Because many paths through the tree may be active, the \K escape sequence,
 which resets the start of the match when encountered (but may be on some paths
 and not on others), is not supported.
 </P>
 <P>
 7. Callouts are supported, but the value of the <i>capture_top</i> field is
 always 1, and the value of the <i>capture_last</i> field is always 0.
 </P>
 <P>
 8. The \C escape sequence, which (in the standard algorithm) always matches a
 single code unit, even in a UTF mode, is not supported in these modes, because
 the alternative algorithm moves through the subject string one character (not
 code unit) at a time, for all active paths through the tree.
 </P>
 <P>
 9. Except for (*FAIL), the backtracking control verbs such as (*PRUNE) are not
 supported. (*FAIL) is supported, and behaves like a failing negative assertion.
 </P>
 <P>
 10. The PCRE2_MATCH_INVALID_UTF option for <b>pcre2_compile()</b> is not
 supported by <b>pcre2_dfa_match()</b>.
 </P>
 <br><a name="SEC5" href="#TOC1">ADVANTAGES OF THE ALTERNATIVE ALGORITHM</a><br>
 <P>
 The main advantage of the alternative algorithm is that all possible matches
 (at a single point in the subject) are automatically found, and in particular,
 the longest match is found. To find more than one match at the same point using
 the standard algorithm, you have to do kludgy things with callouts.
 </P>
 <P>
 Partial matching is possible with this algorithm, though it has some
 limitations. The
 <a href="pcre2partial.html"><b>pcre2partial</b></a>
 documentation gives details of partial matching and discusses multi-segment
 matching.
 </P>
 <br><a name="SEC6" href="#TOC1">DISADVANTAGES OF THE ALTERNATIVE ALGORITHM</a><br>
 <P>
 The alternative algorithm suffers from a number of disadvantages:
 </P>
 <P>
 1. It is substantially slower than the standard algorithm. This is partly
 because it has to search for all possible matches, but is also because it is
 less susceptible to optimization.
 </P>
 <P>
 2. Capturing parentheses, backreferences, script runs, and matching within
 invalid UTF string are not supported.
 </P>
 <P>
 3. Although atomic groups are supported, their use does not provide the
 performance advantage that it does for the standard algorithm.
 </P>
 <P>
 4. JIT optimization is not supported.
 </P>
 <br><a name="SEC7" href="#TOC1">AUTHOR</a><br>
 <P>
 Philip Hazel
 <br>
 Retired from University Computing Service
 <br>
 Cambridge, England.
 <br>
 </P>
 <br><a name="SEC8" href="#TOC1">REVISION</a><br>
 <P>
 Last updated: 28 August 2021
 <br>
 Copyright &copy; 1997-2021 University of Cambridge.
 <br>
 <p>
 Return to the <a href="index.html">PCRE2 index page</a>.
 </p>
	<html>
	<head>
	<title>pcre2matching specification</title>
	</head>
	<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
	<h1>pcre2matching man page</h1>
	<p>
	Return to the <a href="index.html">PCRE2 index page</a>.
	</p>
	<p>
	This page is part of the PCRE2 HTML documentation. It was generated
	automatically from the original man page. If there is any nonsense in it,
	please consult the man page, in case the conversion went wrong.
	<br>
	<ul>
	<li><a name="TOC1" href="#SEC1">PCRE2 MATCHING ALGORITHMS</a>
	<li><a name="TOC2" href="#SEC2">REGULAR EXPRESSIONS AS TREES</a>
	<li><a name="TOC3" href="#SEC3">THE STANDARD MATCHING ALGORITHM</a>
	<li><a name="TOC4" href="#SEC4">THE ALTERNATIVE MATCHING ALGORITHM</a>
	<li><a name="TOC5" href="#SEC5">ADVANTAGES OF THE ALTERNATIVE ALGORITHM</a>
	<li><a name="TOC6" href="#SEC6">DISADVANTAGES OF THE ALTERNATIVE ALGORITHM</a>
	<li><a name="TOC7" href="#SEC7">AUTHOR</a>
	<li><a name="TOC8" href="#SEC8">REVISION</a>
	</ul>
	<br><a name="SEC1" href="#TOC1">PCRE2 MATCHING ALGORITHMS</a><br>
	<P>
	This document describes the two different algorithms that are available in
	PCRE2 for matching a compiled regular expression against a given subject
	string. The "standard" algorithm is the one provided by the <b>pcre2_match()</b>
	function. This works in the same as as Perl's matching function, and provide a
	Perl-compatible matching operation. The just-in-time (JIT) optimization that is
	described in the
	<a href="pcre2jit.html"><b>pcre2jit</b></a>
	documentation is compatible with this function.
	</P>
	<P>
	An alternative algorithm is provided by the <b>pcre2_dfa_match()</b> function;
	it operates in a different way, and is not Perl-compatible. This alternative
	has advantages 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
	pattern, the two algorithms give the same answer. A difference arises, however,
	when there are multiple possibilities. For example, if the pattern
	<pre>
	^<.*>
	</pre>
	is matched against the string
	<pre>
	<something> <something else> <something further>
	</pre>
	there are three possible answers. The standard algorithm finds only one of
	them, whereas the alternative algorithm finds all three.
	</P>
	<br><a name="SEC2" href="#TOC1">REGULAR EXPRESSIONS AS TREES</a><br>
	<P>
	The set of strings that are matched by a regular expression can be represented
	as a tree structure. An unlimited repetition in the pattern makes the tree of
	infinite size, but it is still a tree. Matching the pattern to a given subject
	string (from a given starting point) can be thought of as a search of the tree.
	There are two ways to search a tree: depth-first and breadth-first, and these
	correspond to the two matching algorithms provided by PCRE2.
	</P>
	<br><a name="SEC3" href="#TOC1">THE STANDARD MATCHING ALGORITHM</a><br>
	<P>
	In the terminology of Jeffrey Friedl's book "Mastering Regular Expressions",
	the standard algorithm is an "NFA algorithm". It conducts a depth-first search
	of the pattern tree. That is, it proceeds along a single path through the tree,
	checking that the subject matches what is required. When there is a mismatch,
	the algorithm tries any alternatives at the current point, and if they all
	fail, it backs up to the previous branch point in the tree, and tries the next
	alternative branch at that level. This often involves backing up (moving to the
	left) in the subject string as well. The order in which repetition branches are
	tried is controlled by the greedy or ungreedy nature of the quantifier.
	</P>
	<P>
	If a leaf node is reached, a matching string has been found, and at that point
	the algorithm stops. Thus, if there is more than one possible match, this
	algorithm returns the first one that it finds. Whether this is the shortest,
	the longest, or some intermediate length depends on the way the alternations
	and the greedy or ungreedy repetition quantifiers are specified in the
	pattern.
	</P>
	<P>
	Because it ends up with a single path through the tree, it is relatively
	straightforward for this algorithm to keep track of the substrings that are
	matched by portions of the pattern in parentheses. This provides support for
	capturing parentheses and backreferences.
	</P>
	<br><a name="SEC4" href="#TOC1">THE ALTERNATIVE MATCHING ALGORITHM</a><br>
	<P>
	This algorithm conducts a breadth-first search of the tree. Starting from the
	first matching point in the subject, it scans the subject string from left to
	right, once, character by character, and as it does this, it remembers all the
	paths through the tree that represent valid matches. In Friedl's terminology,
	this is a kind of "DFA algorithm", though it is not implemented as a
	traditional finite state machine (it keeps multiple states active
	simultaneously).
	</P>
	<P>
	Although the general principle of this matching algorithm is that it scans the
	subject string only once, without backtracking, there is one exception: when a
	lookaround assertion is encountered, the characters following or preceding the
	current point have to be independently inspected.
	</P>
	<P>
	The scan continues until either the end of the subject is reached, or there are
	no more unterminated paths. At this point, terminated paths represent the
	different matching possibilities (if there are none, the match has failed).
	Thus, if there is more than one possible match, this algorithm finds all of
	them, and in particular, it finds the longest. The matches are returned in
	the output vector in decreasing order of length. There is an option to stop the
	algorithm after the first match (which is necessarily the shortest) is found.
	</P>
	<P>
	Note that the size of vector needed to contain all the results depends on the
	number of simultaneous matches, not on the number of parentheses in the
	pattern. Using <b>pcre2_match_data_create_from_pattern()</b> to create the match
	data block is therefore not advisable when doing DFA matching.
	</P>
	<P>
	Note also that all the matches that are found start at the same point in the
	subject. If the pattern
	<pre>
	cat(er(pillar)?)?
	</pre>
	is matched against the string "the caterpillar catchment", the result is the
	three strings "caterpillar", "cater", and "cat" that start at the fifth
	character of the subject. The algorithm does not automatically move on to find
	matches that start at later positions.
	</P>
	<P>
	PCRE2's "auto-possessification" optimization usually applies to character
	repeats at the end of a pattern (as well as internally). For example, the
	pattern "a\d+" is compiled as if it were "a\d++" 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\d+?") or set the PCRE2_NO_AUTO_POSSESS option when compiling.
	</P>
	<P>
	There are a number of features of PCRE2 regular expressions that are not
	supported or behave differently in the alternative matching function. Those
	that are not supported cause an error if encountered.
	</P>
	<P>
	1. Because the algorithm finds all possible matches, the greedy or ungreedy
	nature of repetition quantifiers is not relevant (though it may affect
	auto-possessification, as just described). During matching, greedy and ungreedy
	quantifiers are treated in exactly the same way. However, possessive
	quantifiers can make a difference when what follows could also match what is
	quantified, for example in a pattern like this:
	<pre>
	^a++\w!
	</pre>
	This pattern matches "aaab!" but not "aaa!", which would be matched by a
	non-possessive quantifier. Similarly, if an atomic group is present, it is
	matched as if it were a standalone pattern at the current point, and the
	longest match is then "locked in" for the rest of the overall pattern.
	</P>
	<P>
	2. When dealing with multiple paths through the tree simultaneously, it is not
	straightforward to keep track of captured substrings for the different matching
	possibilities, and PCRE2's implementation of this algorithm does not attempt to
	do this. This means that no captured substrings are available.
	</P>
	<P>
	3. Because no substrings are captured, backreferences within the pattern are
	not supported.
	</P>
	<P>
	4. For the same reason, conditional expressions that use a backreference as the
	condition or test for a specific group recursion are not supported.
	</P>
	<P>
	5. Again for the same reason, script runs are not supported.
	</P>
	<P>
	6. Because many paths through the tree may be active, the \K escape sequence,
	which resets the start of the match when encountered (but may be on some paths
	and not on others), is not supported.
	</P>
	<P>
	7. Callouts are supported, but the value of the <i>capture_top</i> field is
	always 1, and the value of the <i>capture_last</i> field is always 0.
	</P>
	<P>
	8. The \C escape sequence, which (in the standard algorithm) always matches a
	single code unit, even in a UTF mode, is not supported in these modes, because
	the alternative algorithm moves through the subject string one character (not
	code unit) at a time, for all active paths through the tree.
	</P>
	<P>
	9. Except for (FAIL), the backtracking control verbs such as (PRUNE) are not
	supported. (*FAIL) is supported, and behaves like a failing negative assertion.
	</P>
	<P>
	10. The PCRE2_MATCH_INVALID_UTF option for <b>pcre2_compile()</b> is not
	supported by <b>pcre2_dfa_match()</b>.
	</P>
	<br><a name="SEC5" href="#TOC1">ADVANTAGES OF THE ALTERNATIVE ALGORITHM</a><br>
	<P>
	The main advantage of the alternative algorithm is that all possible matches
	(at a single point in the subject) are automatically found, and in particular,
	the longest match is found. To find more than one match at the same point using
	the standard algorithm, you have to do kludgy things with callouts.
	</P>
	<P>
	Partial matching is possible with this algorithm, though it has some
	limitations. The
	<a href="pcre2partial.html"><b>pcre2partial</b></a>
	documentation gives details of partial matching and discusses multi-segment
	matching.
	</P>
	<br><a name="SEC6" href="#TOC1">DISADVANTAGES OF THE ALTERNATIVE ALGORITHM</a><br>
	<P>
	The alternative algorithm suffers from a number of disadvantages:
	</P>
	<P>
	1. It is substantially slower than the standard algorithm. This is partly
	because it has to search for all possible matches, but is also because it is
	less susceptible to optimization.
	</P>
	<P>
	2. Capturing parentheses, backreferences, script runs, and matching within
	invalid UTF string are not supported.
	</P>
	<P>
	3. Although atomic groups are supported, their use does not provide the
	performance advantage that it does for the standard algorithm.
	</P>
	<P>
	4. JIT optimization is not supported.
	</P>
	<br><a name="SEC7" href="#TOC1">AUTHOR</a><br>
	<P>
	Philip Hazel
	<br>
	Retired from University Computing Service
	<br>
	Cambridge, England.
	<br>
	</P>
	<br><a name="SEC8" href="#TOC1">REVISION</a><br>
	<P>
	Last updated: 28 August 2021
	<br>
	Copyright © 1997-2021 University of Cambridge.
	<br>
	<p>
	Return to the <a href="index.html">PCRE2 index page</a>.
	</p>