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1 <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 <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> and
30 <b>pcre16_exec()</b> functions. These work in the same was as Perl's matching
31 function, and provide a Perl-compatible matching operation. The just-in-time
32 (JIT) optimization that is described in the
33 <a href="pcrejit.html"><b>pcrejit</b></a>
34 documentation is compatible with these functions.
35 </P>
36 <P>
37 An alternative algorithm is provided by the <b>pcre_dfa_exec()</b> and
38 <b>pcre16_dfa_exec()</b> functions; they operate in a different way, and are not
39 Perl-compatible. This alternative has advantages and disadvantages compared
40 with the standard algorithm, and these are described below.
41 </P>
42 <P>
43 When there is only one possible way in which a given subject string can match a
44 pattern, the two algorithms give the same answer. A difference arises, however,
45 when there are multiple possibilities. For example, if the pattern
46 <pre>
47 ^&#60;.*&#62;
48 </pre>
49 is matched against the string
50 <pre>
51 &#60;something&#62; &#60;something else&#62; &#60;something further&#62;
52 </pre>
53 there are three possible answers. The standard algorithm finds only one of
54 them, whereas the alternative algorithm finds all three.
55 </P>
56 <br><a name="SEC2" href="#TOC1">REGULAR EXPRESSIONS AS TREES</a><br>
57 <P>
58 The set of strings that are matched by a regular expression can be represented
59 as a tree structure. An unlimited repetition in the pattern makes the tree of
60 infinite size, but it is still a tree. Matching the pattern to a given subject
61 string (from a given starting point) can be thought of as a search of the tree.
62 There are two ways to search a tree: depth-first and breadth-first, and these
63 correspond to the two matching algorithms provided by PCRE.
64 </P>
65 <br><a name="SEC3" href="#TOC1">THE STANDARD MATCHING ALGORITHM</a><br>
66 <P>
67 In the terminology of Jeffrey Friedl's book "Mastering Regular
68 Expressions", the standard algorithm is an "NFA algorithm". It conducts a
69 depth-first search of the pattern tree. That is, it proceeds along a single
70 path through the tree, checking that the subject matches what is required. When
71 there is a mismatch, the algorithm tries any alternatives at the current point,
72 and if they all fail, it backs up to the previous branch point in the tree, and
73 tries the next alternative branch at that level. This often involves backing up
74 (moving to the left) in the subject string as well. The order in which
75 repetition branches are tried is controlled by the greedy or ungreedy nature of
76 the quantifier.
77 </P>
78 <P>
79 If a leaf node is reached, a matching string has been found, and at that point
80 the algorithm stops. Thus, if there is more than one possible match, this
81 algorithm returns the first one that it finds. Whether this is the shortest,
82 the longest, or some intermediate length depends on the way the greedy and
83 ungreedy repetition quantifiers are specified in the pattern.
84 </P>
85 <P>
86 Because it ends up with a single path through the tree, it is relatively
87 straightforward for this algorithm to keep track of the substrings that are
88 matched by portions of the pattern in parentheses. This provides support for
89 capturing parentheses and back references.
90 </P>
91 <br><a name="SEC4" href="#TOC1">THE ALTERNATIVE MATCHING ALGORITHM</a><br>
92 <P>
93 This algorithm conducts a breadth-first search of the tree. Starting from the
94 first matching point in the subject, it scans the subject string from left to
95 right, once, character by character, and as it does this, it remembers all the
96 paths through the tree that represent valid matches. In Friedl's terminology,
97 this is a kind of "DFA algorithm", though it is not implemented as a
98 traditional finite state machine (it keeps multiple states active
99 simultaneously).
100 </P>
101 <P>
102 Although the general principle of this matching algorithm is that it scans the
103 subject string only once, without backtracking, there is one exception: when a
104 lookaround assertion is encountered, the characters following or preceding the
105 current point have to be independently inspected.
106 </P>
107 <P>
108 The scan continues until either the end of the subject is reached, or there are
109 no more unterminated paths. At this point, terminated paths represent the
110 different matching possibilities (if there are none, the match has failed).
111 Thus, if there is more than one possible match, this algorithm finds all of
112 them, and in particular, it finds the longest. The matches are returned in
113 decreasing order of length. There is an option to stop the algorithm after the
114 first match (which is necessarily the shortest) is found.
115 </P>
116 <P>
117 Note that all the matches that are found start at the same point in the
118 subject. If the pattern
119 <pre>
120 cat(er(pillar)?)?
121 </pre>
122 is matched against the string "the caterpillar catchment", the result will be
123 the three strings "caterpillar", "cater", and "cat" that start at the fifth
124 character of the subject. The algorithm does not automatically move on to find
125 matches that start at later positions.
126 </P>
127 <P>
128 There are a number of features of PCRE regular expressions that are not
129 supported by the alternative matching algorithm. They are as follows:
130 </P>
131 <P>
132 1. Because the algorithm finds all possible matches, the greedy or ungreedy
133 nature of repetition quantifiers is not relevant. Greedy and ungreedy
134 quantifiers are treated in exactly the same way. However, possessive
135 quantifiers can make a difference when what follows could also match what is
136 quantified, for example in a pattern like this:
137 <pre>
138 ^a++\w!
139 </pre>
140 This pattern matches "aaab!" but not "aaa!", which would be matched by a
141 non-possessive quantifier. Similarly, if an atomic group is present, it is
142 matched as if it were a standalone pattern at the current point, and the
143 longest match is then "locked in" for the rest of the overall pattern.
144 </P>
145 <P>
146 2. When dealing with multiple paths through the tree simultaneously, it is not
147 straightforward to keep track of captured substrings for the different matching
148 possibilities, and PCRE's implementation of this algorithm does not attempt to
149 do this. This means that no captured substrings are available.
150 </P>
151 <P>
152 3. Because no substrings are captured, back references within the pattern are
153 not supported, and cause errors if encountered.
154 </P>
155 <P>
156 4. For the same reason, conditional expressions that use a backreference as the
157 condition or test for a specific group recursion are not supported.
158 </P>
159 <P>
160 5. Because many paths through the tree may be active, the \K escape sequence,
161 which resets the start of the match when encountered (but may be on some paths
162 and not on others), is not supported. It causes an error if encountered.
163 </P>
164 <P>
165 6. Callouts are supported, but the value of the <i>capture_top</i> field is
166 always 1, and the value of the <i>capture_last</i> field is always -1.
167 </P>
168 <P>
169 7. The \C escape sequence, which (in the standard algorithm) always matches a
170 single data unit, even in UTF-8 or UTF-16 modes, is not supported in these
171 modes, because the alternative algorithm moves through the subject string one
172 character (not data unit) at a time, for all active paths through the tree.
173 </P>
174 <P>
175 8. Except for (*FAIL), the backtracking control verbs such as (*PRUNE) are not
176 supported. (*FAIL) is supported, and behaves like a failing negative assertion.
177 </P>
178 <br><a name="SEC5" href="#TOC1">ADVANTAGES OF THE ALTERNATIVE ALGORITHM</a><br>
179 <P>
180 Using the alternative matching algorithm provides the following advantages:
181 </P>
182 <P>
183 1. All possible matches (at a single point in the subject) are automatically
184 found, and in particular, the longest match is found. To find more than one
185 match using the standard algorithm, you have to do kludgy things with
186 callouts.
187 </P>
188 <P>
189 2. Because the alternative algorithm scans the subject string just once, and
190 never needs to backtrack (except for lookbehinds), it is possible to pass very
191 long subject strings to the matching function in several pieces, checking for
192 partial matching each time. Although it is possible to do multi-segment
193 matching using the standard algorithm by retaining partially matched
194 substrings, it is more complicated. The
195 <a href="pcrepartial.html"><b>pcrepartial</b></a>
196 documentation gives details of partial matching and discusses multi-segment
197 matching.
198 </P>
199 <br><a name="SEC6" href="#TOC1">DISADVANTAGES OF THE ALTERNATIVE ALGORITHM</a><br>
200 <P>
201 The alternative algorithm suffers from a number of disadvantages:
202 </P>
203 <P>
204 1. It is substantially slower than the standard algorithm. This is partly
205 because it has to search for all possible matches, but is also because it is
206 less susceptible to optimization.
207 </P>
208 <P>
209 2. Capturing parentheses and back references are not supported.
210 </P>
211 <P>
212 3. Although atomic groups are supported, their use does not provide the
213 performance advantage that it does for the standard algorithm.
214 </P>
215 <br><a name="SEC7" href="#TOC1">AUTHOR</a><br>
216 <P>
217 Philip Hazel
218 <br>
219 University Computing Service
220 <br>
221 Cambridge CB2 3QH, England.
222 <br>
223 </P>
224 <br><a name="SEC8" href="#TOC1">REVISION</a><br>
225 <P>
226 Last updated: 08 January 2012
227 <br>
228 Copyright &copy; 1997-2012 University of Cambridge.
229 <br>
230 <p>
231 Return to the <a href="index.html">PCRE index page</a>.
232 </p>

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