# Contents of /code/trunk/doc/pcrematching.3

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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 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 "Mastering Regular 45 Expressions", 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 Although the general principle of this matching algorithm is that it scans the 78 subject string only once, without backtracking, there is one exception: when a 79 lookaround assertion is encountered, the characters following or preceding the 80 current point have to be independently inspected. 81 .P 82 The scan continues until either the end of the subject is reached, or there are 83 no more unterminated paths. At this point, terminated paths represent the 84 different matching possibilities (if there are none, the match has failed). 85 Thus, if there is more than one possible match, this algorithm finds all of 86 them, and in particular, it finds the longest. There is an option to stop the 87 algorithm after the first match (which is necessarily the shortest) is found. 88 .P 89 Note that all the matches that are found start at the same point in the 90 subject. If the pattern 91 .sp 92 cat(er(pillar)?) 93 .sp 94 is matched against the string "the caterpillar catchment", the result will be 95 the three strings "cat", "cater", and "caterpillar" that start at the fourth 96 character of the subject. The algorithm does not automatically move on to find 97 matches that start at later positions. 98 .P 99 There are a number of features of PCRE regular expressions that are not 100 supported by the alternative matching algorithm. They are as follows: 101 .P 102 1. Because the algorithm finds all possible matches, the greedy or ungreedy 103 nature of repetition quantifiers is not relevant. Greedy and ungreedy 104 quantifiers are treated in exactly the same way. However, possessive 105 quantifiers can make a difference when what follows could also match what is 106 quantified, for example in a pattern like this: 107 .sp 108 ^a++\ew! 109 .sp 110 This pattern matches "aaab!" but not "aaa!", which would be matched by a 111 non-possessive quantifier. Similarly, if an atomic group is present, it is 112 matched as if it were a standalone pattern at the current point, and the 113 longest match is then "locked in" for the rest of the overall pattern. 114 .P 115 2. When dealing with multiple paths through the tree simultaneously, it is not 116 straightforward to keep track of captured substrings for the different matching 117 possibilities, and PCRE's implementation of this algorithm does not attempt to 118 do this. This means that no captured substrings are available. 119 .P 120 3. Because no substrings are captured, back references within the pattern are 121 not supported, and cause errors if encountered. 122 .P 123 4. For the same reason, conditional expressions that use a backreference as the 124 condition or test for a specific group recursion are not supported. 125 .P 126 5. Because many paths through the tree may be active, the \eK escape sequence, 127 which resets the start of the match when encountered (but may be on some paths 128 and not on others), is not supported. It causes an error if encountered. 129 .P 130 6. Callouts are supported, but the value of the \fIcapture_top\fP field is 131 always 1, and the value of the \fIcapture_last\fP field is always -1. 132 .P 133 7. The \eC escape sequence, which (in the standard algorithm) matches a single 134 byte, even in UTF-8 mode, is not supported because the alternative algorithm 135 moves through the subject string one character at a time, for all active paths 136 through the tree. 137 .P 138 8. Except for (*FAIL), the backtracking control verbs such as (*PRUNE) are not 139 supported. (*FAIL) is supported, and behaves like a failing negative assertion. 140 . 141 .SH "ADVANTAGES OF THE ALTERNATIVE ALGORITHM" 142 .rs 143 .sp 144 Using the alternative matching algorithm provides the following advantages: 145 .P 146 1. All possible matches (at a single point in the subject) are automatically 147 found, and in particular, the longest match is found. To find more than one 148 match using the standard algorithm, you have to do kludgy things with 149 callouts. 150 .P 151 2. Because the alternative algorithm scans the subject string just once, and 152 never needs to backtrack, it is possible to pass very long subject strings to 153 the matching function in several pieces, checking for partial matching each 154 time. The 155 .\" HREF 156 \fBpcrepartial\fP 157 .\" 158 documentation gives details of partial matching. 159 . 160 . 161 .SH "DISADVANTAGES OF THE ALTERNATIVE ALGORITHM" 162 .rs 163 .sp 164 The alternative algorithm suffers from a number of disadvantages: 165 .P 166 1. It is substantially slower than the standard algorithm. This is partly 167 because it has to search for all possible matches, but is also because it is 168 less susceptible to optimization. 169 .P 170 2. Capturing parentheses and back references are not supported. 171 .P 172 3. Although atomic groups are supported, their use does not provide the 173 performance advantage that it does for the standard algorithm. 174 . 175 . 176 .SH AUTHOR 177 .rs 178 .sp 179 .nf 180 Philip Hazel 181 University Computing Service 182 Cambridge CB2 3QH, England. 183 .fi 184 . 185 . 186 .SH REVISION 187 .rs 188 .sp 189 .nf 190 Last updated: 29 September 2009 191 Copyright (c) 1997-2009 University of Cambridge. 192 .fi

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