Annotation of embedaddon/pcre/doc/pcrematching.3, revision 1.1
1.1 ! misho 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
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