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				3	<title>pcrematching specification</title>
				4	</head>
				5	<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
				6	<h1>pcrematching man page</h1>
				7	<p>
				8	Return to the <a href="index.html">PCRE index page</a>.
				9	</p>
				10	<p>
				11	This page is part of the PCRE HTML documentation. It was generated automatically
				12	from the original man page. If there is any nonsense in it, please consult the
				13	man page, in case the conversion went wrong.
				14	<br>
				15	<ul>
				16	<li><a name="TOC1" href="#SEC1">PCRE MATCHING ALGORITHMS</a>
				17	<li><a name="TOC2" href="#SEC2">REGULAR EXPRESSIONS AS TREES</a>
				18	<li><a name="TOC3" href="#SEC3">THE STANDARD MATCHING ALGORITHM</a>
				19	<li><a name="TOC4" href="#SEC4">THE ALTERNATIVE MATCHING ALGORITHM</a>
				20	<li><a name="TOC5" href="#SEC5">ADVANTAGES OF THE ALTERNATIVE ALGORITHM</a>
				21	<li><a name="TOC6" href="#SEC6">DISADVANTAGES OF THE ALTERNATIVE ALGORITHM</a>
				22	<li><a name="TOC7" href="#SEC7">AUTHOR</a>
				23	<li><a name="TOC8" href="#SEC8">REVISION</a>
				24	</ul>
				25	<br><a name="SEC1" href="#TOC1">PCRE MATCHING ALGORITHMS</a><br>
				26	<P>
				27	This document describes the two different algorithms that are available in PCRE
				28	for matching a compiled regular expression against a given subject string. The
				29	"standard" algorithm is the one provided by the <b>pcre_exec()</b> function.
				30	This works in the same was as Perl's matching function, and provides a
				31	Perl-compatible matching operation.
				32	</P>
				33	<P>
				34	An alternative algorithm is provided by the <b>pcre_dfa_exec()</b> function;
				35	this operates in a different way, and is not Perl-compatible. It has advantages
				36	and disadvantages compared with the standard algorithm, and these are described
				37	below.
				38	</P>
				39	<P>
				40	When there is only one possible way in which a given subject string can match a
				41	pattern, the two algorithms give the same answer. A difference arises, however,
				42	when there are multiple possibilities. For example, if the pattern
				43	<pre>
				44	^<.*>
				45	</pre>
				46	is matched against the string
				47	<pre>
				48	<something> <something else> <something further>
				49	</pre>
				50	there are three possible answers. The standard algorithm finds only one of
				51	them, whereas the alternative algorithm finds all three.
				52	</P>
				53	<br><a name="SEC2" href="#TOC1">REGULAR EXPRESSIONS AS TREES</a><br>
				54	<P>
				55	The set of strings that are matched by a regular expression can be represented
				56	as a tree structure. An unlimited repetition in the pattern makes the tree of
				57	infinite size, but it is still a tree. Matching the pattern to a given subject
				58	string (from a given starting point) can be thought of as a search of the tree.
				59	There are two ways to search a tree: depth-first and breadth-first, and these
				60	correspond to the two matching algorithms provided by PCRE.
				61	</P>
				62	<br><a name="SEC3" href="#TOC1">THE STANDARD MATCHING ALGORITHM</a><br>
				63	<P>
				64	In the terminology of Jeffrey Friedl's book "Mastering Regular
				65	Expressions", the standard algorithm is an "NFA algorithm". It conducts a
				66	depth-first search of the pattern tree. That is, it proceeds along a single
				67	path through the tree, checking that the subject matches what is required. When
				68	there is a mismatch, the algorithm tries any alternatives at the current point,
				69	and if they all fail, it backs up to the previous branch point in the tree, and
				70	tries the next alternative branch at that level. This often involves backing up
				71	(moving to the left) in the subject string as well. The order in which
				72	repetition branches are tried is controlled by the greedy or ungreedy nature of
				73	the quantifier.
				74	</P>
				75	<P>
				76	If a leaf node is reached, a matching string has been found, and at that point
				77	the algorithm stops. Thus, if there is more than one possible match, this
				78	algorithm returns the first one that it finds. Whether this is the shortest,
				79	the longest, or some intermediate length depends on the way the greedy and
				80	ungreedy repetition quantifiers are specified in the pattern.
				81	</P>
				82	<P>
				83	Because it ends up with a single path through the tree, it is relatively
				84	straightforward for this algorithm to keep track of the substrings that are
				85	matched by portions of the pattern in parentheses. This provides support for
				86	capturing parentheses and back references.
				87	</P>
				88	<br><a name="SEC4" href="#TOC1">THE ALTERNATIVE MATCHING ALGORITHM</a><br>
				89	<P>
				90	This algorithm conducts a breadth-first search of the tree. Starting from the
				91	first matching point in the subject, it scans the subject string from left to
				92	right, once, character by character, and as it does this, it remembers all the
				93	paths through the tree that represent valid matches. In Friedl's terminology,
				94	this is a kind of "DFA algorithm", though it is not implemented as a
				95	traditional finite state machine (it keeps multiple states active
				96	simultaneously).
				97	</P>
				98	<P>
				99	Although the general principle of this matching algorithm is that it scans the
				100	subject string only once, without backtracking, there is one exception: when a
				101	lookaround assertion is encountered, the characters following or preceding the
				102	current point have to be independently inspected.
				103	</P>
				104	<P>
				105	The scan continues until either the end of the subject is reached, or there are
				106	no more unterminated paths. At this point, terminated paths represent the
				107	different matching possibilities (if there are none, the match has failed).
				108	Thus, if there is more than one possible match, this algorithm finds all of
				109	them, and in particular, it finds the longest. The matches are returned in
				110	decreasing order of length. There is an option to stop the algorithm after the
				111	first match (which is necessarily the shortest) is found.
				112	</P>
				113	<P>
				114	Note that all the matches that are found start at the same point in the
				115	subject. If the pattern
				116	<pre>
				117	cat(er(pillar)?)?
				118	</pre>
				119	is matched against the string "the caterpillar catchment", the result will be
				120	the three strings "caterpillar", "cater", and "cat" that start at the fifth
				121	character of the subject. The algorithm does not automatically move on to find
				122	matches that start at later positions.
				123	</P>
				124	<P>
				125	There are a number of features of PCRE regular expressions that are not
				126	supported by the alternative matching algorithm. They are as follows:
				127	</P>
				128	<P>
				129	1. Because the algorithm finds all possible matches, the greedy or ungreedy
				130	nature of repetition quantifiers is not relevant. Greedy and ungreedy
				131	quantifiers are treated in exactly the same way. However, possessive
				132	quantifiers can make a difference when what follows could also match what is
				133	quantified, for example in a pattern like this:
				134	<pre>
				135	^a++\w!
				136	</pre>
				137	This pattern matches "aaab!" but not "aaa!", which would be matched by a
				138	non-possessive quantifier. Similarly, if an atomic group is present, it is
				139	matched as if it were a standalone pattern at the current point, and the
				140	longest match is then "locked in" for the rest of the overall pattern.
				141	</P>
				142	<P>
				143	2. When dealing with multiple paths through the tree simultaneously, it is not
				144	straightforward to keep track of captured substrings for the different matching
				145	possibilities, and PCRE's implementation of this algorithm does not attempt to
				146	do this. This means that no captured substrings are available.
				147	</P>
				148	<P>
				149	3. Because no substrings are captured, back references within the pattern are
				150	not supported, and cause errors if encountered.
				151	</P>
				152	<P>
				153	4. For the same reason, conditional expressions that use a backreference as the
				154	condition or test for a specific group recursion are not supported.
				155	</P>
				156	<P>
				157	5. Because many paths through the tree may be active, the \K escape sequence,
				158	which resets the start of the match when encountered (but may be on some paths
				159	and not on others), is not supported. It causes an error if encountered.
				160	</P>
				161	<P>
				162	6. Callouts are supported, but the value of the <i>capture_top</i> field is
				163	always 1, and the value of the <i>capture_last</i> field is always -1.
				164	</P>
				165	<P>
				166	7. The \C escape sequence, which (in the standard algorithm) matches a single
				167	byte, even in UTF-8 mode, is not supported in UTF-8 mode, because the
				168	alternative algorithm moves through the subject string one character at a time,
				169	for all active paths through the tree.
				170	</P>
				171	<P>
				172	8. Except for (FAIL), the backtracking control verbs such as (PRUNE) are not
				173	supported. (*FAIL) is supported, and behaves like a failing negative assertion.
				174	</P>
				175	<br><a name="SEC5" href="#TOC1">ADVANTAGES OF THE ALTERNATIVE ALGORITHM</a><br>
				176	<P>
				177	Using the alternative matching algorithm provides the following advantages:
				178	</P>
				179	<P>
				180	1. All possible matches (at a single point in the subject) are automatically
				181	found, and in particular, the longest match is found. To find more than one
				182	match using the standard algorithm, you have to do kludgy things with
				183	callouts.
				184	</P>
				185	<P>
				186	2. Because the alternative algorithm scans the subject string just once, and
				187	never needs to backtrack, it is possible to pass very long subject strings to
				188	the matching function in several pieces, checking for partial matching each
				189	time. Although it is possible to do multi-segment matching using the standard
				190	algorithm (<b>pcre_exec()</b>), by retaining partially matched substrings, it is
				191	more complicated. The
				192	<a href="pcrepartial.html"><b>pcrepartial</b></a>
				193	documentation gives details of partial matching and discusses multi-segment
				194	matching.
				195	</P>
				196	<br><a name="SEC6" href="#TOC1">DISADVANTAGES OF THE ALTERNATIVE ALGORITHM</a><br>
				197	<P>
				198	The alternative algorithm suffers from a number of disadvantages:
				199	</P>
				200	<P>
				201	1. It is substantially slower than the standard algorithm. This is partly
				202	because it has to search for all possible matches, but is also because it is
				203	less susceptible to optimization.
				204	</P>
				205	<P>
				206	2. Capturing parentheses and back references are not supported.
				207	</P>
				208	<P>
				209	3. Although atomic groups are supported, their use does not provide the
				210	performance advantage that it does for the standard algorithm.
				211	</P>
				212	<br><a name="SEC7" href="#TOC1">AUTHOR</a><br>
				213	<P>
				214	Philip Hazel
				215	<br>
				216	University Computing Service
				217	<br>
				218	Cambridge CB2 3QH, England.
				219	<br>
				220	</P>
				221	<br><a name="SEC8" href="#TOC1">REVISION</a><br>
				222	<P>
				223	Last updated: 19 November 2011
				224	<br>
				225	Copyright © 1997-2010 University of Cambridge.
				226	<br>
				227	<p>
				228	Return to the <a href="index.html">PCRE index page</a>.
				229	</p>