doc/html/pcrematching.html

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<title>pcrematching specification</title>
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<h1>pcrematching man page</h1>
<p>
Return to the <a href="index.html">PCRE index page</a>.
</p>
<p>
This page is part of the PCRE 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">PCRE 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>
</ul>
<br><a name="SEC1" href="#TOC1">PCRE MATCHING ALGORITHMS</a><br>
<P>
This document describes the two different algorithms that are available in PCRE
for matching a compiled regular expression against a given subject string. The
"standard" algorithm is the one provided by the <b>pcre_exec()</b> function.
This works in the same was as Perl's matching function, and provides a
Perl-compatible matching operation.
</P>
<P>
An alternative algorithm is provided by the <b>pcre_dfa_exec()</b> function;
this operates in a different way, and is not Perl-compatible. It 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 PCRE.
</P>
<br><a name="SEC3" href="#TOC1">THE STANDARD MATCHING ALGORITHM</a><br>
<P>
In the terminology of Jeffrey Friedl's book \fIMastering Regular
Expressions\fP, 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 greedy and
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 back references.
</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>
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. In PCRE, there is an option to
stop the algorithm after the first match (which is necessarily the shortest)
has been found.
</P>
<P>
Note 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 will be
the three strings "cat", "cater", and "caterpillar" that start at the fourth
character of the subject. The algorithm does not automatically move on to find
matches that start at later positions.
</P>
<P>
There are a number of features of PCRE regular expressions that are not
supported by the alternative matching algorithm. They are as follows:
</P>
<P>
1. Because the algorithm finds all possible matches, the greedy or ungreedy
nature of repetition quantifiers is not relevant. 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 PCRE'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, back references within the pattern are
not supported, and cause errors if encountered.
</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. 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 -1.
</P>
<P>
6.
The \C escape sequence, which (in the standard algorithm) matches a single
byte, even in UTF-8 mode, is not supported because the alternative algorithm
moves through the subject string one character at a time, for all active paths
through the tree.
</P>
<br><a name="SEC5" href="#TOC1">ADVANTAGES OF THE ALTERNATIVE ALGORITHM</a><br>
<P>
Using the alternative matching algorithm provides the following advantages:
</P>
<P>
1. 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 using the standard algorithm, you have to do kludgy things with
callouts.
</P>
<P>
2. There is much better support for partial matching. The restrictions on the
content of the pattern that apply when using the standard algorithm for partial
matching do not apply to the alternative algorithm. For non-anchored patterns,
the starting position of a partial match is available.
</P>
<P>
3. Because the alternative algorithm scans the subject string just once, and
never needs to backtrack, it is possible to pass very long subject strings to
the matching function in several pieces, checking for partial matching each
time.
</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 and back references 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>
Last updated: 24 November 2006
<br>
Copyright &copy; 1997-2006 University of Cambridge.
<p>
Return to the <a href="index.html">PCRE index page</a>.
</p>
1	nigel	77	<html>
2			<head>
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	nigel	93	<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	nigel	77	</ul>
23			<br><a name="SEC1" href="#TOC1">PCRE MATCHING ALGORITHMS</a><br>
24			<P>
25			This document describes the two different algorithms that are available in PCRE
26			for matching a compiled regular expression against a given subject string. The
27			"standard" algorithm is the one provided by the <b>pcre_exec()</b> function.
28			This works in the same was as Perl's matching function, and provides a
29			Perl-compatible matching operation.
30			</P>
31			<P>
32			An alternative algorithm is provided by the <b>pcre_dfa_exec()</b> function;
33			this operates in a different way, and is not Perl-compatible. It has advantages
34			and disadvantages compared with the standard algorithm, and these are described
35			below.
36			</P>
37			<P>
38			When there is only one possible way in which a given subject string can match a
39			pattern, the two algorithms give the same answer. A difference arises, however,
40			when there are multiple possibilities. For example, if the pattern
41			<pre>
42			^<.*>
43			</pre>
44			is matched against the string
45			<pre>
46			<something> <something else> <something further>
47			</pre>
48			there are three possible answers. The standard algorithm finds only one of
49	nigel	93	them, whereas the alternative algorithm finds all three.
50	nigel	77	</P>
51			<br><a name="SEC2" href="#TOC1">REGULAR EXPRESSIONS AS TREES</a><br>
52			<P>
53			The set of strings that are matched by a regular expression can be represented
54			as a tree structure. An unlimited repetition in the pattern makes the tree of
55			infinite size, but it is still a tree. Matching the pattern to a given subject
56			string (from a given starting point) can be thought of as a search of the tree.
57	nigel	91	There are two ways to search a tree: depth-first and breadth-first, and these
58			correspond to the two matching algorithms provided by PCRE.
59	nigel	77	</P>
60			<br><a name="SEC3" href="#TOC1">THE STANDARD MATCHING ALGORITHM</a><br>
61			<P>
62			In the terminology of Jeffrey Friedl's book \fIMastering Regular
63			Expressions\fP, the standard algorithm is an "NFA algorithm". It conducts a
64			depth-first search of the pattern tree. That is, it proceeds along a single
65			path through the tree, checking that the subject matches what is required. When
66			there is a mismatch, the algorithm tries any alternatives at the current point,
67			and if they all fail, it backs up to the previous branch point in the tree, and
68			tries the next alternative branch at that level. This often involves backing up
69			(moving to the left) in the subject string as well. The order in which
70			repetition branches are tried is controlled by the greedy or ungreedy nature of
71			the quantifier.
72			</P>
73			<P>
74			If a leaf node is reached, a matching string has been found, and at that point
75			the algorithm stops. Thus, if there is more than one possible match, this
76			algorithm returns the first one that it finds. Whether this is the shortest,
77			the longest, or some intermediate length depends on the way the greedy and
78			ungreedy repetition quantifiers are specified in the pattern.
79			</P>
80			<P>
81			Because it ends up with a single path through the tree, it is relatively
82			straightforward for this algorithm to keep track of the substrings that are
83			matched by portions of the pattern in parentheses. This provides support for
84			capturing parentheses and back references.
85			</P>
86	nigel	93	<br><a name="SEC4" href="#TOC1">THE ALTERNATIVE MATCHING ALGORITHM</a><br>
87	nigel	77	<P>
88	nigel	93	This algorithm conducts a breadth-first search of the tree. Starting from the
89			first matching point in the subject, it scans the subject string from left to
90			right, once, character by character, and as it does this, it remembers all the
91			paths through the tree that represent valid matches. In Friedl's terminology,
92			this is a kind of "DFA algorithm", though it is not implemented as a
93			traditional finite state machine (it keeps multiple states active
94			simultaneously).
95	nigel	77	</P>
96			<P>
97			The scan continues until either the end of the subject is reached, or there are
98			no more unterminated paths. At this point, terminated paths represent the
99			different matching possibilities (if there are none, the match has failed).
100			Thus, if there is more than one possible match, this algorithm finds all of
101			them, and in particular, it finds the longest. In PCRE, there is an option to
102			stop the algorithm after the first match (which is necessarily the shortest)
103			has been found.
104			</P>
105			<P>
106			Note that all the matches that are found start at the same point in the
107			subject. If the pattern
108			<pre>
109			cat(er(pillar)?)
110			</pre>
111			is matched against the string "the caterpillar catchment", the result will be
112			the three strings "cat", "cater", and "caterpillar" that start at the fourth
113			character of the subject. The algorithm does not automatically move on to find
114			matches that start at later positions.
115			</P>
116			<P>
117			There are a number of features of PCRE regular expressions that are not
118	nigel	93	supported by the alternative matching algorithm. They are as follows:
119	nigel	77	</P>
120			<P>
121			1. Because the algorithm finds all possible matches, the greedy or ungreedy
122			nature of repetition quantifiers is not relevant. Greedy and ungreedy
123	nigel	93	quantifiers are treated in exactly the same way. However, possessive
124			quantifiers can make a difference when what follows could also match what is
125			quantified, for example in a pattern like this:
126			<pre>
127			^a++\w!
128			</pre>
129			This pattern matches "aaab!" but not "aaa!", which would be matched by a
130			non-possessive quantifier. Similarly, if an atomic group is present, it is
131			matched as if it were a standalone pattern at the current point, and the
132			longest match is then "locked in" for the rest of the overall pattern.
133	nigel	77	</P>
134			<P>
135			2. When dealing with multiple paths through the tree simultaneously, it is not
136			straightforward to keep track of captured substrings for the different matching
137			possibilities, and PCRE's implementation of this algorithm does not attempt to
138			do this. This means that no captured substrings are available.
139			</P>
140			<P>
141			3. Because no substrings are captured, back references within the pattern are
142			not supported, and cause errors if encountered.
143			</P>
144			<P>
145			4. For the same reason, conditional expressions that use a backreference as the
146	nigel	93	condition or test for a specific group recursion are not supported.
147	nigel	77	</P>
148			<P>
149			5. Callouts are supported, but the value of the <i>capture_top</i> field is
150			always 1, and the value of the <i>capture_last</i> field is always -1.
151			</P>
152			<P>
153			6.
154			The \C escape sequence, which (in the standard algorithm) matches a single
155	nigel	93	byte, even in UTF-8 mode, is not supported because the alternative algorithm
156			moves through the subject string one character at a time, for all active paths
157	nigel	77	through the tree.
158			</P>
159	nigel	93	<br><a name="SEC5" href="#TOC1">ADVANTAGES OF THE ALTERNATIVE ALGORITHM</a><br>
160	nigel	77	<P>
161	nigel	93	Using the alternative matching algorithm provides the following advantages:
162	nigel	77	</P>
163			<P>
164			1. All possible matches (at a single point in the subject) are automatically
165			found, and in particular, the longest match is found. To find more than one
166			match using the standard algorithm, you have to do kludgy things with
167			callouts.
168			</P>
169			<P>
170			2. There is much better support for partial matching. The restrictions on the
171			content of the pattern that apply when using the standard algorithm for partial
172	nigel	93	matching do not apply to the alternative algorithm. For non-anchored patterns,
173			the starting position of a partial match is available.
174	nigel	77	</P>
175			<P>
176	nigel	93	3. Because the alternative algorithm scans the subject string just once, and
177			never needs to backtrack, it is possible to pass very long subject strings to
178			the matching function in several pieces, checking for partial matching each
179			time.
180	nigel	77	</P>
181	nigel	93	<br><a name="SEC6" href="#TOC1">DISADVANTAGES OF THE ALTERNATIVE ALGORITHM</a><br>
182	nigel	77	<P>
183	nigel	93	The alternative algorithm suffers from a number of disadvantages:
184	nigel	77	</P>
185			<P>
186			1. It is substantially slower than the standard algorithm. This is partly
187			because it has to search for all possible matches, but is also because it is
188			less susceptible to optimization.
189			</P>
190			<P>
191			2. Capturing parentheses and back references are not supported.
192			</P>
193			<P>
194	nigel	93	3. Although atomic groups are supported, their use does not provide the
195			performance advantage that it does for the standard algorithm.
196	nigel	77	</P>
197			<P>
198	nigel	93	Last updated: 24 November 2006
199	nigel	77	<br>
200	nigel	91	Copyright © 1997-2006 University of Cambridge.
201	nigel	77	<p>
202			Return to the <a href="index.html">PCRE index page</a>.
203			</p>