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1 .TH PCREMATCHING 3
2 .SH NAME
3 PCRE - Perl-compatible regular expressions
4 .SH "PCRE MATCHING ALGORITHMS"
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 function.
10 This works in the same was as Perl's matching function, and provides a
11 Perl-compatible matching operation.
12 .P
13 An alternative algorithm is provided by the \fBpcre_dfa_exec()\fP function;
14 this operates in a different way, and is not Perl-compatible. It has advantages
15 and disadvantages compared with the standard algorithm, and these are described
16 below.
17 .P
18 When there is only one possible way in which a given subject string can match a
19 pattern, the two algorithms give the same answer. A difference arises, however,
20 when there are multiple possibilities. For example, if the pattern
21 .sp
22 ^<.*>
23 .sp
24 is matched against the string
25 .sp
26 <something> <something else> <something further>
27 .sp
28 there are three possible answers. The standard algorithm finds only one of
29 them, whereas the alternative algorithm finds all three.
30 .
31 .SH "REGULAR EXPRESSIONS AS TREES"
32 .rs
33 .sp
34 The set of strings that are matched by a regular expression can be represented
35 as a tree structure. An unlimited repetition in the pattern makes the tree of
36 infinite size, but it is still a tree. Matching the pattern to a given subject
37 string (from a given starting point) can be thought of as a search of the tree.
38 There are two ways to search a tree: depth-first and breadth-first, and these
39 correspond to the two matching algorithms provided by PCRE.
40 .
41 .SH "THE STANDARD MATCHING ALGORITHM"
42 .rs
43 .sp
44 In the terminology of Jeffrey Friedl's book \fIMastering Regular
45 Expressions\fP, the standard algorithm is an "NFA algorithm". It conducts a
46 depth-first search of the pattern tree. That is, it proceeds along a single
47 path through the tree, checking that the subject matches what is required. When
48 there is a mismatch, the algorithm tries any alternatives at the current point,
49 and if they all fail, it backs up to the previous branch point in the tree, and
50 tries the next alternative branch at that level. This often involves backing up
51 (moving to the left) in the subject string as well. The order in which
52 repetition branches are tried is controlled by the greedy or ungreedy nature of
53 the quantifier.
54 .P
55 If a leaf node is reached, a matching string has been found, and at that point
56 the algorithm stops. Thus, if there is more than one possible match, this
57 algorithm returns the first one that it finds. Whether this is the shortest,
58 the longest, or some intermediate length depends on the way the greedy and
59 ungreedy repetition quantifiers are specified in the pattern.
60 .P
61 Because it ends up with a single path through the tree, it is relatively
62 straightforward for this algorithm to keep track of the substrings that are
63 matched by portions of the pattern in parentheses. This provides support for
64 capturing parentheses and back references.
65 .
66 .SH "THE ALTERNATIVE MATCHING ALGORITHM"
67 .rs
68 .sp
69 This algorithm conducts a breadth-first search of the tree. Starting from the
70 first matching point in the subject, it scans the subject string from left to
71 right, once, character by character, and as it does this, it remembers all the
72 paths through the tree that represent valid matches. In Friedl's terminology,
73 this is a kind of "DFA algorithm", though it is not implemented as a
74 traditional finite state machine (it keeps multiple states active
75 simultaneously).
76 .P
77 The scan continues until either the end of the subject is reached, or there are
78 no more unterminated paths. At this point, terminated paths represent the
79 different matching possibilities (if there are none, the match has failed).
80 Thus, if there is more than one possible match, this algorithm finds all of
81 them, and in particular, it finds the longest. In PCRE, there is an option to
82 stop the algorithm after the first match (which is necessarily the shortest)
83 has been found.
84 .P
85 Note that all the matches that are found start at the same point in the
86 subject. If the pattern
87 .sp
88 cat(er(pillar)?)
89 .sp
90 is matched against the string "the caterpillar catchment", the result will be
91 the three strings "cat", "cater", and "caterpillar" that start at the fourth
92 character of the subject. The algorithm does not automatically move on to find
93 matches that start at later positions.
94 .P
95 There are a number of features of PCRE regular expressions that are not
96 supported by the alternative matching algorithm. They are as follows:
97 .P
98 1. Because the algorithm finds all possible matches, the greedy or ungreedy
99 nature of repetition quantifiers is not relevant. Greedy and ungreedy
100 quantifiers are treated in exactly the same way. However, possessive
101 quantifiers can make a difference when what follows could also match what is
102 quantified, for example in a pattern like this:
103 .sp
104 ^a++\ew!
105 .sp
106 This pattern matches "aaab!" but not "aaa!", which would be matched by a
107 non-possessive quantifier. Similarly, if an atomic group is present, it is
108 matched as if it were a standalone pattern at the current point, and the
109 longest match is then "locked in" for the rest of the overall pattern.
110 .P
111 2. When dealing with multiple paths through the tree simultaneously, it is not
112 straightforward to keep track of captured substrings for the different matching
113 possibilities, and PCRE's implementation of this algorithm does not attempt to
114 do this. This means that no captured substrings are available.
115 .P
116 3. Because no substrings are captured, back references within the pattern are
117 not supported, and cause errors if encountered.
118 .P
119 4. For the same reason, conditional expressions that use a backreference as the
120 condition or test for a specific group recursion are not supported.
121 .P
122 5. Callouts are supported, but the value of the \fIcapture_top\fP field is
123 always 1, and the value of the \fIcapture_last\fP field is always -1.
124 .P
125 6.
126 The \eC escape sequence, which (in the standard algorithm) matches a single
127 byte, even in UTF-8 mode, is not supported because the alternative algorithm
128 moves through the subject string one character at a time, for all active paths
129 through the tree.
130 .
131 .SH "ADVANTAGES OF THE ALTERNATIVE ALGORITHM"
132 .rs
133 .sp
134 Using the alternative matching algorithm provides the following advantages:
135 .P
136 1. All possible matches (at a single point in the subject) are automatically
137 found, and in particular, the longest match is found. To find more than one
138 match using the standard algorithm, you have to do kludgy things with
139 callouts.
140 .P
141 2. There is much better support for partial matching. The restrictions on the
142 content of the pattern that apply when using the standard algorithm for partial
143 matching do not apply to the alternative algorithm. For non-anchored patterns,
144 the starting position of a partial match is available.
145 .P
146 3. Because the alternative algorithm scans the subject string just once, and
147 never needs to backtrack, it is possible to pass very long subject strings to
148 the matching function in several pieces, checking for partial matching each
149 time.
150 .
151 .SH "DISADVANTAGES OF THE ALTERNATIVE ALGORITHM"
152 .rs
153 .sp
154 The alternative algorithm suffers from a number of disadvantages:
155 .P
156 1. It is substantially slower than the standard algorithm. This is partly
157 because it has to search for all possible matches, but is also because it is
158 less susceptible to optimization.
159 .P
160 2. Capturing parentheses and back references are not supported.
161 .P
162 3. Although atomic groups are supported, their use does not provide the
163 performance advantage that it does for the standard algorithm.
164 .P
165 .in 0
166 Last updated: 24 November 2006
167 .br
168 Copyright (c) 1997-2006 University of Cambridge.

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