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Revision 1398 - (show annotations)
Tue Nov 12 15:20:26 2013 UTC (7 years, 8 months ago) by ph10
File size: 9472 byte(s)
Error occurred while calculating annotation data.
Rename the (as yet unreleased) PCRE_NO_AUTO_POSSESSIFY option to 
PCRE_NO_AUTO_POSSESS - no need to be so long - and to match (*NO_AUTO_POSSESS).
1 .TH PCREMATCHING 3 "12 November 2013" "PCRE 8.34"
3 PCRE - Perl-compatible regular expressions
5 .rs
6 .sp
7 This document describes the two different algorithms that are available in PCRE
8 for matching a compiled regular expression against a given subject string. The
9 "standard" algorithm is the one provided by the \fBpcre_exec()\fP,
10 \fBpcre16_exec()\fP and \fBpcre32_exec()\fP functions. These work in the same
11 as as Perl's matching function, and provide a Perl-compatible matching operation.
12 The just-in-time (JIT) optimization that is described in the
13 .\" HREF
14 \fBpcrejit\fP
15 .\"
16 documentation is compatible with these functions.
17 .P
18 An alternative algorithm is provided by the \fBpcre_dfa_exec()\fP,
19 \fBpcre16_dfa_exec()\fP and \fBpcre32_dfa_exec()\fP functions; they operate in
20 a different way, and are not Perl-compatible. This alternative has advantages
21 and disadvantages compared with the standard algorithm, and these are described
22 below.
23 .P
24 When there is only one possible way in which a given subject string can match a
25 pattern, the two algorithms give the same answer. A difference arises, however,
26 when there are multiple possibilities. For example, if the pattern
27 .sp
28 ^<.*>
29 .sp
30 is matched against the string
31 .sp
32 <something> <something else> <something further>
33 .sp
34 there are three possible answers. The standard algorithm finds only one of
35 them, whereas the alternative algorithm finds all three.
36 .
37 .
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41 The set of strings that are matched by a regular expression can be represented
42 as a tree structure. An unlimited repetition in the pattern makes the tree of
43 infinite size, but it is still a tree. Matching the pattern to a given subject
44 string (from a given starting point) can be thought of as a search of the tree.
45 There are two ways to search a tree: depth-first and breadth-first, and these
46 correspond to the two matching algorithms provided by PCRE.
47 .
48 .
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52 In the terminology of Jeffrey Friedl's book "Mastering Regular
53 Expressions", the standard algorithm is an "NFA algorithm". It conducts a
54 depth-first search of the pattern tree. That is, it proceeds along a single
55 path through the tree, checking that the subject matches what is required. When
56 there is a mismatch, the algorithm tries any alternatives at the current point,
57 and if they all fail, it backs up to the previous branch point in the tree, and
58 tries the next alternative branch at that level. This often involves backing up
59 (moving to the left) in the subject string as well. The order in which
60 repetition branches are tried is controlled by the greedy or ungreedy nature of
61 the quantifier.
62 .P
63 If a leaf node is reached, a matching string has been found, and at that point
64 the algorithm stops. Thus, if there is more than one possible match, this
65 algorithm returns the first one that it finds. Whether this is the shortest,
66 the longest, or some intermediate length depends on the way the greedy and
67 ungreedy repetition quantifiers are specified in the pattern.
68 .P
69 Because it ends up with a single path through the tree, it is relatively
70 straightforward for this algorithm to keep track of the substrings that are
71 matched by portions of the pattern in parentheses. This provides support for
72 capturing parentheses and back references.
73 .
74 .
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77 .sp
78 This algorithm conducts a breadth-first search of the tree. Starting from the
79 first matching point in the subject, it scans the subject string from left to
80 right, once, character by character, and as it does this, it remembers all the
81 paths through the tree that represent valid matches. In Friedl's terminology,
82 this is a kind of "DFA algorithm", though it is not implemented as a
83 traditional finite state machine (it keeps multiple states active
84 simultaneously).
85 .P
86 Although the general principle of this matching algorithm is that it scans the
87 subject string only once, without backtracking, there is one exception: when a
88 lookaround assertion is encountered, the characters following or preceding the
89 current point have to be independently inspected.
90 .P
91 The scan continues until either the end of the subject is reached, or there are
92 no more unterminated paths. At this point, terminated paths represent the
93 different matching possibilities (if there are none, the match has failed).
94 Thus, if there is more than one possible match, this algorithm finds all of
95 them, and in particular, it finds the longest. The matches are returned in
96 decreasing order of length. There is an option to stop the algorithm after the
97 first match (which is necessarily the shortest) is found.
98 .P
99 Note that all the matches that are found start at the same point in the
100 subject. If the pattern
101 .sp
102 cat(er(pillar)?)?
103 .sp
104 is matched against the string "the caterpillar catchment", the result will be
105 the three strings "caterpillar", "cater", and "cat" that start at the fifth
106 character of the subject. The algorithm does not automatically move on to find
107 matches that start at later positions.
108 .P
109 PCRE's "auto-possessification" optimization usually applies to character
110 repeats at the end of a pattern (as well as internally). For example, the
111 pattern "a\ed+" is compiled as if it were "a\ed++" because there is no point
112 even considering the possibility of backtracking into the repeated digits. For
113 DFA matching, this means that only one possible match is found. If you really
114 do want multiple matches in such cases, either use an ungreedy repeat
115 ("a\ed+?") or set the PCRE_NO_AUTO_POSSESS option when compiling.
116 .P
117 There are a number of features of PCRE regular expressions that are not
118 supported by the alternative matching algorithm. They are as follows:
119 .P
120 1. Because the algorithm finds all possible matches, the greedy or ungreedy
121 nature of repetition quantifiers is not relevant. Greedy and ungreedy
122 quantifiers are treated in exactly the same way. However, possessive
123 quantifiers can make a difference when what follows could also match what is
124 quantified, for example in a pattern like this:
125 .sp
126 ^a++\ew!
127 .sp
128 This pattern matches "aaab!" but not "aaa!", which would be matched by a
129 non-possessive quantifier. Similarly, if an atomic group is present, it is
130 matched as if it were a standalone pattern at the current point, and the
131 longest match is then "locked in" for the rest of the overall pattern.
132 .P
133 2. When dealing with multiple paths through the tree simultaneously, it is not
134 straightforward to keep track of captured substrings for the different matching
135 possibilities, and PCRE's implementation of this algorithm does not attempt to
136 do this. This means that no captured substrings are available.
137 .P
138 3. Because no substrings are captured, back references within the pattern are
139 not supported, and cause errors if encountered.
140 .P
141 4. For the same reason, conditional expressions that use a backreference as the
142 condition or test for a specific group recursion are not supported.
143 .P
144 5. Because many paths through the tree may be active, the \eK escape sequence,
145 which resets the start of the match when encountered (but may be on some paths
146 and not on others), is not supported. It causes an error if encountered.
147 .P
148 6. Callouts are supported, but the value of the \fIcapture_top\fP field is
149 always 1, and the value of the \fIcapture_last\fP field is always -1.
150 .P
151 7. The \eC escape sequence, which (in the standard algorithm) always matches a
152 single data unit, even in UTF-8, UTF-16 or UTF-32 modes, is not supported in
153 these modes, because the alternative algorithm moves through the subject string
154 one character (not data unit) at a time, for all active paths through the tree.
155 .P
156 8. Except for (*FAIL), the backtracking control verbs such as (*PRUNE) are not
157 supported. (*FAIL) is supported, and behaves like a failing negative assertion.
158 .
159 .
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163 Using the alternative matching algorithm provides the following advantages:
164 .P
165 1. All possible matches (at a single point in the subject) are automatically
166 found, and in particular, the longest match is found. To find more than one
167 match using the standard algorithm, you have to do kludgy things with
168 callouts.
169 .P
170 2. Because the alternative algorithm scans the subject string just once, and
171 never needs to backtrack (except for lookbehinds), it is possible to pass very
172 long subject strings to the matching function in several pieces, checking for
173 partial matching each time. Although it is possible to do multi-segment
174 matching using the standard algorithm by retaining partially matched
175 substrings, it is more complicated. The
176 .\" HREF
177 \fBpcrepartial\fP
178 .\"
179 documentation gives details of partial matching and discusses multi-segment
180 matching.
181 .
182 .
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186 The alternative algorithm suffers from a number of disadvantages:
187 .P
188 1. It is substantially slower than the standard algorithm. This is partly
189 because it has to search for all possible matches, but is also because it is
190 less susceptible to optimization.
191 .P
192 2. Capturing parentheses and back references are not supported.
193 .P
194 3. Although atomic groups are supported, their use does not provide the
195 performance advantage that it does for the standard algorithm.
196 .
197 .
199 .rs
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201 .nf
202 Philip Hazel
203 University Computing Service
204 Cambridge CB2 3QH, England.
205 .fi
206 .
207 .
209 .rs
210 .sp
211 .nf
212 Last updated: 12 November 2013
213 Copyright (c) 1997-2012 University of Cambridge.
214 .fi


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