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    1 <html>
    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>,
   30 <b>pcre16_exec()</b> and <b>pcre32_exec()</b> functions. These work in the same
   31 as as Perl's matching function, and provide a Perl-compatible matching operation.
   32 The just-in-time (JIT) optimization that is described in the
   33 <a href="pcrejit.html"><b>pcrejit</b></a>
   34 documentation is compatible with these functions.
   35 </P>
   36 <P>
   37 An alternative algorithm is provided by the <b>pcre_dfa_exec()</b>,
   38 <b>pcre16_dfa_exec()</b> and <b>pcre32_dfa_exec()</b> functions; they operate in
   39 a different way, and are not Perl-compatible. This alternative has advantages
   40 and disadvantages compared with the standard algorithm, and these are described
   41 below.
   42 </P>
   43 <P>
   44 When there is only one possible way in which a given subject string can match a
   45 pattern, the two algorithms give the same answer. A difference arises, however,
   46 when there are multiple possibilities. For example, if the pattern
   47 <pre>
   48   ^&#60;.*&#62;
   49 </pre>
   50 is matched against the string
   51 <pre>
   52   &#60;something&#62; &#60;something else&#62; &#60;something further&#62;
   53 </pre>
   54 there are three possible answers. The standard algorithm finds only one of
   55 them, whereas the alternative algorithm finds all three.
   56 </P>
   57 <br><a name="SEC2" href="#TOC1">REGULAR EXPRESSIONS AS TREES</a><br>
   58 <P>
   59 The set of strings that are matched by a regular expression can be represented
   60 as a tree structure. An unlimited repetition in the pattern makes the tree of
   61 infinite size, but it is still a tree. Matching the pattern to a given subject
   62 string (from a given starting point) can be thought of as a search of the tree.
   63 There are two ways to search a tree: depth-first and breadth-first, and these
   64 correspond to the two matching algorithms provided by PCRE.
   65 </P>
   66 <br><a name="SEC3" href="#TOC1">THE STANDARD MATCHING ALGORITHM</a><br>
   67 <P>
   68 In the terminology of Jeffrey Friedl's book "Mastering Regular
   69 Expressions", the standard algorithm is an "NFA algorithm". It conducts a
   70 depth-first search of the pattern tree. That is, it proceeds along a single
   71 path through the tree, checking that the subject matches what is required. When
   72 there is a mismatch, the algorithm tries any alternatives at the current point,
   73 and if they all fail, it backs up to the previous branch point in the tree, and
   74 tries the next alternative branch at that level. This often involves backing up
   75 (moving to the left) in the subject string as well. The order in which
   76 repetition branches are tried is controlled by the greedy or ungreedy nature of
   77 the quantifier.
   78 </P>
   79 <P>
   80 If a leaf node is reached, a matching string has been found, and at that point
   81 the algorithm stops. Thus, if there is more than one possible match, this
   82 algorithm returns the first one that it finds. Whether this is the shortest,
   83 the longest, or some intermediate length depends on the way the greedy and
   84 ungreedy repetition quantifiers are specified in the pattern.
   85 </P>
   86 <P>
   87 Because it ends up with a single path through the tree, it is relatively
   88 straightforward for this algorithm to keep track of the substrings that are
   89 matched by portions of the pattern in parentheses. This provides support for
   90 capturing parentheses and back references.
   91 </P>
   92 <br><a name="SEC4" href="#TOC1">THE ALTERNATIVE MATCHING ALGORITHM</a><br>
   93 <P>
   94 This algorithm conducts a breadth-first search of the tree. Starting from the
   95 first matching point in the subject, it scans the subject string from left to
   96 right, once, character by character, and as it does this, it remembers all the
   97 paths through the tree that represent valid matches. In Friedl's terminology,
   98 this is a kind of "DFA algorithm", though it is not implemented as a
   99 traditional finite state machine (it keeps multiple states active
  100 simultaneously).
  101 </P>
  102 <P>
  103 Although the general principle of this matching algorithm is that it scans the
  104 subject string only once, without backtracking, there is one exception: when a
  105 lookaround assertion is encountered, the characters following or preceding the
  106 current point have to be independently inspected.
  107 </P>
  108 <P>
  109 The scan continues until either the end of the subject is reached, or there are
  110 no more unterminated paths. At this point, terminated paths represent the
  111 different matching possibilities (if there are none, the match has failed).
  112 Thus, if there is more than one possible match, this algorithm finds all of
  113 them, and in particular, it finds the longest. The matches are returned in
  114 decreasing order of length. There is an option to stop the algorithm after the
  115 first match (which is necessarily the shortest) is found.
  116 </P>
  117 <P>
  118 Note that all the matches that are found start at the same point in the
  119 subject. If the pattern
  120 <pre>
  121   cat(er(pillar)?)?
  122 </pre>
  123 is matched against the string "the caterpillar catchment", the result will be
  124 the three strings "caterpillar", "cater", and "cat" that start at the fifth
  125 character of the subject. The algorithm does not automatically move on to find
  126 matches that start at later positions.
  127 </P>
  128 <P>
  129 PCRE's "auto-possessification" optimization usually applies to character
  130 repeats at the end of a pattern (as well as internally). For example, the
  131 pattern "a\d+" is compiled as if it were "a\d++" because there is no point
  132 even considering the possibility of backtracking into the repeated digits. For
  133 DFA matching, this means that only one possible match is found. If you really
  134 do want multiple matches in such cases, either use an ungreedy repeat
  135 ("a\d+?") or set the PCRE_NO_AUTO_POSSESS option when compiling.
  136 </P>
  137 <P>
  138 There are a number of features of PCRE regular expressions that are not
  139 supported by the alternative matching algorithm. They are as follows:
  140 </P>
  141 <P>
  142 1. Because the algorithm finds all possible matches, the greedy or ungreedy
  143 nature of repetition quantifiers is not relevant. Greedy and ungreedy
  144 quantifiers are treated in exactly the same way. However, possessive
  145 quantifiers can make a difference when what follows could also match what is
  146 quantified, for example in a pattern like this:
  147 <pre>
  148   ^a++\w!
  149 </pre>
  150 This pattern matches "aaab!" but not "aaa!", which would be matched by a
  151 non-possessive quantifier. Similarly, if an atomic group is present, it is
  152 matched as if it were a standalone pattern at the current point, and the
  153 longest match is then "locked in" for the rest of the overall pattern.
  154 </P>
  155 <P>
  156 2. When dealing with multiple paths through the tree simultaneously, it is not
  157 straightforward to keep track of captured substrings for the different matching
  158 possibilities, and PCRE's implementation of this algorithm does not attempt to
  159 do this. This means that no captured substrings are available.
  160 </P>
  161 <P>
  162 3. Because no substrings are captured, back references within the pattern are
  163 not supported, and cause errors if encountered.
  164 </P>
  165 <P>
  166 4. For the same reason, conditional expressions that use a backreference as the
  167 condition or test for a specific group recursion are not supported.
  168 </P>
  169 <P>
  170 5. Because many paths through the tree may be active, the \K escape sequence,
  171 which resets the start of the match when encountered (but may be on some paths
  172 and not on others), is not supported. It causes an error if encountered.
  173 </P>
  174 <P>
  175 6. Callouts are supported, but the value of the <i>capture_top</i> field is
  176 always 1, and the value of the <i>capture_last</i> field is always -1.
  177 </P>
  178 <P>
  179 7. The \C escape sequence, which (in the standard algorithm) always matches a
  180 single data unit, even in UTF-8, UTF-16 or UTF-32 modes, is not supported in
  181 these modes, because the alternative algorithm moves through the subject string
  182 one character (not data unit) at a time, for all active paths through the tree.
  183 </P>
  184 <P>
  185 8. Except for (*FAIL), the backtracking control verbs such as (*PRUNE) are not
  186 supported. (*FAIL) is supported, and behaves like a failing negative assertion.
  187 </P>
  188 <br><a name="SEC5" href="#TOC1">ADVANTAGES OF THE ALTERNATIVE ALGORITHM</a><br>
  189 <P>
  190 Using the alternative matching algorithm provides the following advantages:
  191 </P>
  192 <P>
  193 1. All possible matches (at a single point in the subject) are automatically
  194 found, and in particular, the longest match is found. To find more than one
  195 match using the standard algorithm, you have to do kludgy things with
  196 callouts.
  197 </P>
  198 <P>
  199 2. Because the alternative algorithm scans the subject string just once, and
  200 never needs to backtrack (except for lookbehinds), it is possible to pass very
  201 long subject strings to the matching function in several pieces, checking for
  202 partial matching each time. Although it is possible to do multi-segment
  203 matching using the standard algorithm by retaining partially matched
  204 substrings, it is more complicated. The
  205 <a href="pcrepartial.html"><b>pcrepartial</b></a>
  206 documentation gives details of partial matching and discusses multi-segment
  207 matching.
  208 </P>
  209 <br><a name="SEC6" href="#TOC1">DISADVANTAGES OF THE ALTERNATIVE ALGORITHM</a><br>
  210 <P>
  211 The alternative algorithm suffers from a number of disadvantages:
  212 </P>
  213 <P>
  214 1. It is substantially slower than the standard algorithm. This is partly
  215 because it has to search for all possible matches, but is also because it is
  216 less susceptible to optimization.
  217 </P>
  218 <P>
  219 2. Capturing parentheses and back references are not supported.
  220 </P>
  221 <P>
  222 3. Although atomic groups are supported, their use does not provide the
  223 performance advantage that it does for the standard algorithm.
  224 </P>
  225 <br><a name="SEC7" href="#TOC1">AUTHOR</a><br>
  226 <P>
  227 Philip Hazel
  228 <br>
  229 University Computing Service
  230 <br>
  231 Cambridge CB2 3QH, England.
  232 <br>
  233 </P>
  234 <br><a name="SEC8" href="#TOC1">REVISION</a><br>
  235 <P>
  236 Last updated: 12 November 2013
  237 <br>
  238 Copyright &copy; 1997-2012 University of Cambridge.
  239 <br>
  240 <p>
  241 Return to the <a href="index.html">PCRE index page</a>.
  242 </p>