Annotation of embedaddon/pcre/doc/html/pcrematching.html, revision 1.1
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! 2: <head>
! 3: <title>pcrematching specification</title>
! 4: </head>
! 5: <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
! 6: <h1>pcrematching man page</h1>
! 7: <p>
! 8: Return to the <a href="index.html">PCRE index page</a>.
! 9: </p>
! 10: <p>
! 11: This page is part of the PCRE HTML documentation. It was generated automatically
! 12: from the original man page. If there is any nonsense in it, please consult the
! 13: man page, in case the conversion went wrong.
! 14: <br>
! 15: <ul>
! 16: <li><a name="TOC1" href="#SEC1">PCRE MATCHING ALGORITHMS</a>
! 17: <li><a name="TOC2" href="#SEC2">REGULAR EXPRESSIONS AS TREES</a>
! 18: <li><a name="TOC3" href="#SEC3">THE STANDARD MATCHING ALGORITHM</a>
! 19: <li><a name="TOC4" href="#SEC4">THE ALTERNATIVE MATCHING ALGORITHM</a>
! 20: <li><a name="TOC5" href="#SEC5">ADVANTAGES OF THE ALTERNATIVE ALGORITHM</a>
! 21: <li><a name="TOC6" href="#SEC6">DISADVANTAGES OF THE ALTERNATIVE ALGORITHM</a>
! 22: <li><a name="TOC7" href="#SEC7">AUTHOR</a>
! 23: <li><a name="TOC8" href="#SEC8">REVISION</a>
! 24: </ul>
! 25: <br><a name="SEC1" href="#TOC1">PCRE MATCHING ALGORITHMS</a><br>
! 26: <P>
! 27: This document describes the two different algorithms that are available in PCRE
! 28: for matching a compiled regular expression against a given subject string. The
! 29: "standard" algorithm is the one provided by the <b>pcre_exec()</b> function.
! 30: This works in the same was as Perl's matching function, and provides a
! 31: Perl-compatible matching operation.
! 32: </P>
! 33: <P>
! 34: An alternative algorithm is provided by the <b>pcre_dfa_exec()</b> function;
! 35: this operates in a different way, and is not Perl-compatible. It has advantages
! 36: and disadvantages compared with the standard algorithm, and these are described
! 37: below.
! 38: </P>
! 39: <P>
! 40: When there is only one possible way in which a given subject string can match a
! 41: pattern, the two algorithms give the same answer. A difference arises, however,
! 42: when there are multiple possibilities. For example, if the pattern
! 43: <pre>
! 44: ^<.*>
! 45: </pre>
! 46: is matched against the string
! 47: <pre>
! 48: <something> <something else> <something further>
! 49: </pre>
! 50: there are three possible answers. The standard algorithm finds only one of
! 51: them, whereas the alternative algorithm finds all three.
! 52: </P>
! 53: <br><a name="SEC2" href="#TOC1">REGULAR EXPRESSIONS AS TREES</a><br>
! 54: <P>
! 55: The set of strings that are matched by a regular expression can be represented
! 56: as a tree structure. An unlimited repetition in the pattern makes the tree of
! 57: infinite size, but it is still a tree. Matching the pattern to a given subject
! 58: string (from a given starting point) can be thought of as a search of the tree.
! 59: There are two ways to search a tree: depth-first and breadth-first, and these
! 60: correspond to the two matching algorithms provided by PCRE.
! 61: </P>
! 62: <br><a name="SEC3" href="#TOC1">THE STANDARD MATCHING ALGORITHM</a><br>
! 63: <P>
! 64: In the terminology of Jeffrey Friedl's book "Mastering Regular
! 65: Expressions", the standard algorithm is an "NFA algorithm". It conducts a
! 66: depth-first search of the pattern tree. That is, it proceeds along a single
! 67: path through the tree, checking that the subject matches what is required. When
! 68: there is a mismatch, the algorithm tries any alternatives at the current point,
! 69: and if they all fail, it backs up to the previous branch point in the tree, and
! 70: tries the next alternative branch at that level. This often involves backing up
! 71: (moving to the left) in the subject string as well. The order in which
! 72: repetition branches are tried is controlled by the greedy or ungreedy nature of
! 73: the quantifier.
! 74: </P>
! 75: <P>
! 76: If a leaf node is reached, a matching string has been found, and at that point
! 77: the algorithm stops. Thus, if there is more than one possible match, this
! 78: algorithm returns the first one that it finds. Whether this is the shortest,
! 79: the longest, or some intermediate length depends on the way the greedy and
! 80: ungreedy repetition quantifiers are specified in the pattern.
! 81: </P>
! 82: <P>
! 83: Because it ends up with a single path through the tree, it is relatively
! 84: straightforward for this algorithm to keep track of the substrings that are
! 85: matched by portions of the pattern in parentheses. This provides support for
! 86: capturing parentheses and back references.
! 87: </P>
! 88: <br><a name="SEC4" href="#TOC1">THE ALTERNATIVE MATCHING ALGORITHM</a><br>
! 89: <P>
! 90: This algorithm conducts a breadth-first search of the tree. Starting from the
! 91: first matching point in the subject, it scans the subject string from left to
! 92: right, once, character by character, and as it does this, it remembers all the
! 93: paths through the tree that represent valid matches. In Friedl's terminology,
! 94: this is a kind of "DFA algorithm", though it is not implemented as a
! 95: traditional finite state machine (it keeps multiple states active
! 96: simultaneously).
! 97: </P>
! 98: <P>
! 99: Although the general principle of this matching algorithm is that it scans the
! 100: subject string only once, without backtracking, there is one exception: when a
! 101: lookaround assertion is encountered, the characters following or preceding the
! 102: current point have to be independently inspected.
! 103: </P>
! 104: <P>
! 105: The scan continues until either the end of the subject is reached, or there are
! 106: no more unterminated paths. At this point, terminated paths represent the
! 107: different matching possibilities (if there are none, the match has failed).
! 108: Thus, if there is more than one possible match, this algorithm finds all of
! 109: them, and in particular, it finds the longest. The matches are returned in
! 110: decreasing order of length. There is an option to stop the algorithm after the
! 111: first match (which is necessarily the shortest) is found.
! 112: </P>
! 113: <P>
! 114: Note that all the matches that are found start at the same point in the
! 115: subject. If the pattern
! 116: <pre>
! 117: cat(er(pillar)?)?
! 118: </pre>
! 119: is matched against the string "the caterpillar catchment", the result will be
! 120: the three strings "caterpillar", "cater", and "cat" that start at the fifth
! 121: character of the subject. The algorithm does not automatically move on to find
! 122: matches that start at later positions.
! 123: </P>
! 124: <P>
! 125: There are a number of features of PCRE regular expressions that are not
! 126: supported by the alternative matching algorithm. They are as follows:
! 127: </P>
! 128: <P>
! 129: 1. Because the algorithm finds all possible matches, the greedy or ungreedy
! 130: nature of repetition quantifiers is not relevant. Greedy and ungreedy
! 131: quantifiers are treated in exactly the same way. However, possessive
! 132: quantifiers can make a difference when what follows could also match what is
! 133: quantified, for example in a pattern like this:
! 134: <pre>
! 135: ^a++\w!
! 136: </pre>
! 137: This pattern matches "aaab!" but not "aaa!", which would be matched by a
! 138: non-possessive quantifier. Similarly, if an atomic group is present, it is
! 139: matched as if it were a standalone pattern at the current point, and the
! 140: longest match is then "locked in" for the rest of the overall pattern.
! 141: </P>
! 142: <P>
! 143: 2. When dealing with multiple paths through the tree simultaneously, it is not
! 144: straightforward to keep track of captured substrings for the different matching
! 145: possibilities, and PCRE's implementation of this algorithm does not attempt to
! 146: do this. This means that no captured substrings are available.
! 147: </P>
! 148: <P>
! 149: 3. Because no substrings are captured, back references within the pattern are
! 150: not supported, and cause errors if encountered.
! 151: </P>
! 152: <P>
! 153: 4. For the same reason, conditional expressions that use a backreference as the
! 154: condition or test for a specific group recursion are not supported.
! 155: </P>
! 156: <P>
! 157: 5. Because many paths through the tree may be active, the \K escape sequence,
! 158: which resets the start of the match when encountered (but may be on some paths
! 159: and not on others), is not supported. It causes an error if encountered.
! 160: </P>
! 161: <P>
! 162: 6. Callouts are supported, but the value of the <i>capture_top</i> field is
! 163: always 1, and the value of the <i>capture_last</i> field is always -1.
! 164: </P>
! 165: <P>
! 166: 7. The \C escape sequence, which (in the standard algorithm) matches a single
! 167: byte, even in UTF-8 mode, is not supported in UTF-8 mode, because the
! 168: alternative algorithm moves through the subject string one character at a time,
! 169: for all active paths through the tree.
! 170: </P>
! 171: <P>
! 172: 8. Except for (*FAIL), the backtracking control verbs such as (*PRUNE) are not
! 173: supported. (*FAIL) is supported, and behaves like a failing negative assertion.
! 174: </P>
! 175: <br><a name="SEC5" href="#TOC1">ADVANTAGES OF THE ALTERNATIVE ALGORITHM</a><br>
! 176: <P>
! 177: Using the alternative matching algorithm provides the following advantages:
! 178: </P>
! 179: <P>
! 180: 1. All possible matches (at a single point in the subject) are automatically
! 181: found, and in particular, the longest match is found. To find more than one
! 182: match using the standard algorithm, you have to do kludgy things with
! 183: callouts.
! 184: </P>
! 185: <P>
! 186: 2. Because the alternative algorithm scans the subject string just once, and
! 187: never needs to backtrack, it is possible to pass very long subject strings to
! 188: the matching function in several pieces, checking for partial matching each
! 189: time. Although it is possible to do multi-segment matching using the standard
! 190: algorithm (<b>pcre_exec()</b>), by retaining partially matched substrings, it is
! 191: more complicated. The
! 192: <a href="pcrepartial.html"><b>pcrepartial</b></a>
! 193: documentation gives details of partial matching and discusses multi-segment
! 194: matching.
! 195: </P>
! 196: <br><a name="SEC6" href="#TOC1">DISADVANTAGES OF THE ALTERNATIVE ALGORITHM</a><br>
! 197: <P>
! 198: The alternative algorithm suffers from a number of disadvantages:
! 199: </P>
! 200: <P>
! 201: 1. It is substantially slower than the standard algorithm. This is partly
! 202: because it has to search for all possible matches, but is also because it is
! 203: less susceptible to optimization.
! 204: </P>
! 205: <P>
! 206: 2. Capturing parentheses and back references are not supported.
! 207: </P>
! 208: <P>
! 209: 3. Although atomic groups are supported, their use does not provide the
! 210: performance advantage that it does for the standard algorithm.
! 211: </P>
! 212: <br><a name="SEC7" href="#TOC1">AUTHOR</a><br>
! 213: <P>
! 214: Philip Hazel
! 215: <br>
! 216: University Computing Service
! 217: <br>
! 218: Cambridge CB2 3QH, England.
! 219: <br>
! 220: </P>
! 221: <br><a name="SEC8" href="#TOC1">REVISION</a><br>
! 222: <P>
! 223: Last updated: 19 November 2011
! 224: <br>
! 225: Copyright © 1997-2010 University of Cambridge.
! 226: <br>
! 227: <p>
! 228: Return to the <a href="index.html">PCRE index page</a>.
! 229: </p>
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