Tristan Matthews | 0461646 | 2013-11-14 16:09:34 -0500 | [diff] [blame] | 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 "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. The matches are returned in |
| 87 | decreasing order of length. There is an option to stop the algorithm after the |
| 88 | first match (which is necessarily the shortest) is found. |
| 89 | .P |
| 90 | Note that all the matches that are found start at the same point in the |
| 91 | subject. If the pattern |
| 92 | .sp |
| 93 | cat(er(pillar)?)? |
| 94 | .sp |
| 95 | is matched against the string "the caterpillar catchment", the result will be |
| 96 | the three strings "caterpillar", "cater", and "cat" that start at the fifth |
| 97 | character of the subject. The algorithm does not automatically move on to find |
| 98 | matches that start at later positions. |
| 99 | .P |
| 100 | There are a number of features of PCRE regular expressions that are not |
| 101 | supported by the alternative matching algorithm. They are as follows: |
| 102 | .P |
| 103 | 1. Because the algorithm finds all possible matches, the greedy or ungreedy |
| 104 | nature of repetition quantifiers is not relevant. Greedy and ungreedy |
| 105 | quantifiers are treated in exactly the same way. However, possessive |
| 106 | quantifiers can make a difference when what follows could also match what is |
| 107 | quantified, for example in a pattern like this: |
| 108 | .sp |
| 109 | ^a++\ew! |
| 110 | .sp |
| 111 | This pattern matches "aaab!" but not "aaa!", which would be matched by a |
| 112 | non-possessive quantifier. Similarly, if an atomic group is present, it is |
| 113 | matched as if it were a standalone pattern at the current point, and the |
| 114 | longest match is then "locked in" for the rest of the overall pattern. |
| 115 | .P |
| 116 | 2. When dealing with multiple paths through the tree simultaneously, it is not |
| 117 | straightforward to keep track of captured substrings for the different matching |
| 118 | possibilities, and PCRE's implementation of this algorithm does not attempt to |
| 119 | do this. This means that no captured substrings are available. |
| 120 | .P |
| 121 | 3. Because no substrings are captured, back references within the pattern are |
| 122 | not supported, and cause errors if encountered. |
| 123 | .P |
| 124 | 4. For the same reason, conditional expressions that use a backreference as the |
| 125 | condition or test for a specific group recursion are not supported. |
| 126 | .P |
| 127 | 5. Because many paths through the tree may be active, the \eK escape sequence, |
| 128 | which resets the start of the match when encountered (but may be on some paths |
| 129 | and not on others), is not supported. It causes an error if encountered. |
| 130 | .P |
| 131 | 6. Callouts are supported, but the value of the \fIcapture_top\fP field is |
| 132 | always 1, and the value of the \fIcapture_last\fP field is always -1. |
| 133 | .P |
| 134 | 7. The \eC escape sequence, which (in the standard algorithm) matches a single |
| 135 | byte, even in UTF-8 mode, is not supported in UTF-8 mode, because the |
| 136 | alternative algorithm moves through the subject string one character at a time, |
| 137 | for all active paths through the tree. |
| 138 | .P |
| 139 | 8. Except for (*FAIL), the backtracking control verbs such as (*PRUNE) are not |
| 140 | supported. (*FAIL) is supported, and behaves like a failing negative assertion. |
| 141 | . |
| 142 | .SH "ADVANTAGES OF THE ALTERNATIVE ALGORITHM" |
| 143 | .rs |
| 144 | .sp |
| 145 | Using the alternative matching algorithm provides the following advantages: |
| 146 | .P |
| 147 | 1. All possible matches (at a single point in the subject) are automatically |
| 148 | found, and in particular, the longest match is found. To find more than one |
| 149 | match using the standard algorithm, you have to do kludgy things with |
| 150 | callouts. |
| 151 | .P |
| 152 | 2. Because the alternative algorithm scans the subject string just once, and |
| 153 | never needs to backtrack, it is possible to pass very long subject strings to |
| 154 | the matching function in several pieces, checking for partial matching each |
| 155 | time. Although it is possible to do multi-segment matching using the standard |
| 156 | algorithm (\fBpcre_exec()\fP), by retaining partially matched substrings, it is |
| 157 | more complicated. The |
| 158 | .\" HREF |
| 159 | \fBpcrepartial\fP |
| 160 | .\" |
| 161 | documentation gives details of partial matching and discusses multi-segment |
| 162 | matching. |
| 163 | . |
| 164 | . |
| 165 | .SH "DISADVANTAGES OF THE ALTERNATIVE ALGORITHM" |
| 166 | .rs |
| 167 | .sp |
| 168 | The alternative algorithm suffers from a number of disadvantages: |
| 169 | .P |
| 170 | 1. It is substantially slower than the standard algorithm. This is partly |
| 171 | because it has to search for all possible matches, but is also because it is |
| 172 | less susceptible to optimization. |
| 173 | .P |
| 174 | 2. Capturing parentheses and back references are not supported. |
| 175 | .P |
| 176 | 3. Although atomic groups are supported, their use does not provide the |
| 177 | performance advantage that it does for the standard algorithm. |
| 178 | . |
| 179 | . |
| 180 | .SH AUTHOR |
| 181 | .rs |
| 182 | .sp |
| 183 | .nf |
| 184 | Philip Hazel |
| 185 | University Computing Service |
| 186 | Cambridge CB2 3QH, England. |
| 187 | .fi |
| 188 | . |
| 189 | . |
| 190 | .SH REVISION |
| 191 | .rs |
| 192 | .sp |
| 193 | .nf |
| 194 | Last updated: 19 November 2011 |
| 195 | Copyright (c) 1997-2010 University of Cambridge. |
| 196 | .fi |