doc/html/pcrematching.html

<html>
<head>
<title>pcrematching specification</title>
</head>
<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
<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>
<li><a name="TOC7" href="#SEC7">AUTHOR</a>
<li><a name="TOC8" href="#SEC8">REVISION</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 "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 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>
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 
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 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 "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>
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. 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. It causes an error if encountered.
</P>
<P>
6. 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>
7. 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>
<P>
8. Except for (*FAIL), the backtracking control verbs such as (*PRUNE) are not
supported. (*FAIL) is supported, and behaves like a failing negative assertion.
</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. 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. Although it is possible to do multi-segment matching using the standard
algorithm (<b>pcre_exec()</b>), by retaining partially matched substrings, it is
more complicated. The
<a href="pcrepartial.html"><b>pcrepartial</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 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>
<br><a name="SEC7" href="#TOC1">AUTHOR</a><br>
<P>
Philip Hazel
<br>
University Computing Service
<br>
Cambridge CB2 3QH, England.
<br>
</P>
<br><a name="SEC8" href="#TOC1">REVISION</a><br>
<P>
Last updated: 17 November 2010
<br>
Copyright &copy; 1997-2010 University of Cambridge.
<br>
<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	ph10	111	<p>
11	nigel	77	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	ph10	111	<br>
15	nigel	77	<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	ph10	99	<li><a name="TOC7" href="#SEC7">AUTHOR</a>
23			<li><a name="TOC8" href="#SEC8">REVISION</a>
24	nigel	77	</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	nigel	93	them, whereas the alternative algorithm finds all three.
52	nigel	77	</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	nigel	91	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	nigel	77	</P>
62			<br><a name="SEC3" href="#TOC1">THE STANDARD MATCHING ALGORITHM</a><br>
63			<P>
64	ph10	148	In the terminology of Jeffrey Friedl's book "Mastering Regular
65			Expressions", the standard algorithm is an "NFA algorithm". It conducts a
66	nigel	77	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	nigel	93	<br><a name="SEC4" href="#TOC1">THE ALTERNATIVE MATCHING ALGORITHM</a><br>
89	nigel	77	<P>
90	nigel	93	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	nigel	77	</P>
98			<P>
99	ph10	461	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	nigel	77	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	ph10	572	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	nigel	77	</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	ph10	572	cat(er(pillar)?)?
118	nigel	77	</pre>
119			is matched against the string "the caterpillar catchment", the result will be
120	ph10	572	the three strings "caterpillar", "cater", and "cat" that start at the fifth
121	nigel	77	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	nigel	93	supported by the alternative matching algorithm. They are as follows:
127	nigel	77	</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	nigel	93	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	nigel	77	</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	nigel	93	condition or test for a specific group recursion are not supported.
155	nigel	77	</P>
156			<P>
157	ph10	172	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	nigel	77	always 1, and the value of the <i>capture_last</i> field is always -1.
164			</P>
165			<P>
166	ph10	211	7. The \C escape sequence, which (in the standard algorithm) matches a single
167	nigel	93	byte, even in UTF-8 mode, is not supported because the alternative algorithm
168			moves through the subject string one character at a time, for all active paths
169	nigel	77	through the tree.
170			</P>
171	ph10	211	<P>
172	ph10	345	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	ph10	211	</P>
175	nigel	93	<br><a name="SEC5" href="#TOC1">ADVANTAGES OF THE ALTERNATIVE ALGORITHM</a><br>
176	nigel	77	<P>
177	nigel	93	Using the alternative matching algorithm provides the following advantages:
178	nigel	77	</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	ph10	429	2. Because the alternative algorithm scans the subject string just once, and
187	nigel	93	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	ph10	572	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	ph10	461	<a href="pcrepartial.html"><b>pcrepartial</b></a>
193	ph10	567	documentation gives details of partial matching and discusses multi-segment
194			matching.
195	nigel	77	</P>
196	nigel	93	<br><a name="SEC6" href="#TOC1">DISADVANTAGES OF THE ALTERNATIVE ALGORITHM</a><br>
197	nigel	77	<P>
198	nigel	93	The alternative algorithm suffers from a number of disadvantages:
199	nigel	77	</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	nigel	93	3. Although atomic groups are supported, their use does not provide the
210			performance advantage that it does for the standard algorithm.
211	nigel	77	</P>
212	ph10	99	<br><a name="SEC7" href="#TOC1">AUTHOR</a><br>
213	nigel	77	<P>
214	ph10	99	Philip Hazel
215	nigel	77	<br>
216	ph10	99	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	ph10	572	Last updated: 17 November 2010
224	ph10	99	<br>
225	ph10	567	Copyright © 1997-2010 University of Cambridge.
226	ph10	99	<br>
227	nigel	77	<p>
228			Return to the <a href="index.html">PCRE index page</a>.
229			</p>
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