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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>
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     ^&#60;.*&#62;
45     </pre>
46     is matched against the string
47     <pre>
48     &#60;something&#62; &#60;something else&#62; &#60;something further&#62;
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 &copy; 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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