225 lines
9.5 KiB
HTML
225 lines
9.5 KiB
HTML
<html>
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<head>
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<title>pcrematching specification</title>
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</head>
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<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
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<h1>pcrematching man page</h1>
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<p>
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Return to the <a href="index.html">PCRE index page</a>.
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</p>
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<p>
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This page is part of the PCRE HTML documentation. It was generated automatically
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from the original man page. If there is any nonsense in it, please consult the
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man page, in case the conversion went wrong.
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<br>
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<ul>
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<li><a name="TOC1" href="#SEC1">PCRE MATCHING ALGORITHMS</a>
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<li><a name="TOC2" href="#SEC2">REGULAR EXPRESSIONS AS TREES</a>
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<li><a name="TOC3" href="#SEC3">THE STANDARD MATCHING ALGORITHM</a>
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<li><a name="TOC4" href="#SEC4">THE ALTERNATIVE MATCHING ALGORITHM</a>
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<li><a name="TOC5" href="#SEC5">ADVANTAGES OF THE ALTERNATIVE ALGORITHM</a>
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<li><a name="TOC6" href="#SEC6">DISADVANTAGES OF THE ALTERNATIVE ALGORITHM</a>
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<li><a name="TOC7" href="#SEC7">AUTHOR</a>
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<li><a name="TOC8" href="#SEC8">REVISION</a>
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</ul>
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<br><a name="SEC1" href="#TOC1">PCRE MATCHING ALGORITHMS</a><br>
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<P>
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This document describes the two different algorithms that are available in PCRE
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for matching a compiled regular expression against a given subject string. The
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"standard" algorithm is the one provided by the <b>pcre_exec()</b> function.
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This works in the same was as Perl's matching function, and provides a
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Perl-compatible matching operation.
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</P>
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<P>
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An alternative algorithm is provided by the <b>pcre_dfa_exec()</b> function;
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this operates in a different way, and is not Perl-compatible. It has advantages
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and disadvantages compared with the standard algorithm, and these are described
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below.
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</P>
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<P>
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When there is only one possible way in which a given subject string can match a
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pattern, the two algorithms give the same answer. A difference arises, however,
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when there are multiple possibilities. For example, if the pattern
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<pre>
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^<.*>
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</pre>
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is matched against the string
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<pre>
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<something> <something else> <something further>
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</pre>
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there are three possible answers. The standard algorithm finds only one of
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them, whereas the alternative algorithm finds all three.
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</P>
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<br><a name="SEC2" href="#TOC1">REGULAR EXPRESSIONS AS TREES</a><br>
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<P>
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The set of strings that are matched by a regular expression can be represented
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as a tree structure. An unlimited repetition in the pattern makes the tree of
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infinite size, but it is still a tree. Matching the pattern to a given subject
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string (from a given starting point) can be thought of as a search of the tree.
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There are two ways to search a tree: depth-first and breadth-first, and these
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correspond to the two matching algorithms provided by PCRE.
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</P>
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<br><a name="SEC3" href="#TOC1">THE STANDARD MATCHING ALGORITHM</a><br>
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<P>
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In the terminology of Jeffrey Friedl's book "Mastering Regular
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Expressions", the standard algorithm is an "NFA algorithm". It conducts a
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depth-first search of the pattern tree. That is, it proceeds along a single
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path through the tree, checking that the subject matches what is required. When
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there is a mismatch, the algorithm tries any alternatives at the current point,
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and if they all fail, it backs up to the previous branch point in the tree, and
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tries the next alternative branch at that level. This often involves backing up
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(moving to the left) in the subject string as well. The order in which
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repetition branches are tried is controlled by the greedy or ungreedy nature of
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the quantifier.
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</P>
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<P>
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If a leaf node is reached, a matching string has been found, and at that point
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the algorithm stops. Thus, if there is more than one possible match, this
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algorithm returns the first one that it finds. Whether this is the shortest,
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the longest, or some intermediate length depends on the way the greedy and
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ungreedy repetition quantifiers are specified in the pattern.
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</P>
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<P>
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Because it ends up with a single path through the tree, it is relatively
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straightforward for this algorithm to keep track of the substrings that are
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matched by portions of the pattern in parentheses. This provides support for
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capturing parentheses and back references.
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</P>
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<br><a name="SEC4" href="#TOC1">THE ALTERNATIVE MATCHING ALGORITHM</a><br>
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<P>
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This algorithm conducts a breadth-first search of the tree. Starting from the
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first matching point in the subject, it scans the subject string from left to
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right, once, character by character, and as it does this, it remembers all the
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paths through the tree that represent valid matches. In Friedl's terminology,
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this is a kind of "DFA algorithm", though it is not implemented as a
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traditional finite state machine (it keeps multiple states active
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simultaneously).
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</P>
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<P>
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The scan continues until either the end of the subject is reached, or there are
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no more unterminated paths. At this point, terminated paths represent the
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different matching possibilities (if there are none, the match has failed).
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Thus, if there is more than one possible match, this algorithm finds all of
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them, and in particular, it finds the longest. In PCRE, there is an option to
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stop the algorithm after the first match (which is necessarily the shortest)
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has been found.
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</P>
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<P>
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Note that all the matches that are found start at the same point in the
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subject. If the pattern
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<pre>
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cat(er(pillar)?)
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</pre>
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is matched against the string "the caterpillar catchment", the result will be
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the three strings "cat", "cater", and "caterpillar" that start at the fourth
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character of the subject. The algorithm does not automatically move on to find
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matches that start at later positions.
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</P>
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<P>
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There are a number of features of PCRE regular expressions that are not
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supported by the alternative matching algorithm. They are as follows:
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</P>
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<P>
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1. Because the algorithm finds all possible matches, the greedy or ungreedy
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nature of repetition quantifiers is not relevant. Greedy and ungreedy
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quantifiers are treated in exactly the same way. However, possessive
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quantifiers can make a difference when what follows could also match what is
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quantified, for example in a pattern like this:
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<pre>
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^a++\w!
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</pre>
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This pattern matches "aaab!" but not "aaa!", which would be matched by a
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non-possessive quantifier. Similarly, if an atomic group is present, it is
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matched as if it were a standalone pattern at the current point, and the
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longest match is then "locked in" for the rest of the overall pattern.
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</P>
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<P>
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2. When dealing with multiple paths through the tree simultaneously, it is not
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straightforward to keep track of captured substrings for the different matching
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possibilities, and PCRE's implementation of this algorithm does not attempt to
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do this. This means that no captured substrings are available.
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</P>
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<P>
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3. Because no substrings are captured, back references within the pattern are
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not supported, and cause errors if encountered.
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</P>
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<P>
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4. For the same reason, conditional expressions that use a backreference as the
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condition or test for a specific group recursion are not supported.
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</P>
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<P>
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5. Because many paths through the tree may be active, the \K escape sequence,
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which resets the start of the match when encountered (but may be on some paths
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and not on others), is not supported. It causes an error if encountered.
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</P>
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<P>
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6. Callouts are supported, but the value of the <i>capture_top</i> field is
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always 1, and the value of the <i>capture_last</i> field is always -1.
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</P>
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<P>
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7. The \C escape sequence, which (in the standard algorithm) matches a single
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byte, even in UTF-8 mode, is not supported because the alternative algorithm
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moves through the subject string one character at a time, for all active paths
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through the tree.
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</P>
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<P>
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8. Except for (*FAIL), the backtracking control verbs such as (*PRUNE) are not
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supported. (*FAIL) is supported, and behaves like a failing negative assertion.
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</P>
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<br><a name="SEC5" href="#TOC1">ADVANTAGES OF THE ALTERNATIVE ALGORITHM</a><br>
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<P>
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Using the alternative matching algorithm provides the following advantages:
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</P>
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<P>
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1. All possible matches (at a single point in the subject) are automatically
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found, and in particular, the longest match is found. To find more than one
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match using the standard algorithm, you have to do kludgy things with
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callouts.
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</P>
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<P>
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2. There is much better support for partial matching. The restrictions on the
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content of the pattern that apply when using the standard algorithm for partial
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matching do not apply to the alternative algorithm. For non-anchored patterns,
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the starting position of a partial match is available.
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</P>
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<P>
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3. Because the alternative algorithm scans the subject string just once, and
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never needs to backtrack, it is possible to pass very long subject strings to
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the matching function in several pieces, checking for partial matching each
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time.
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</P>
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<br><a name="SEC6" href="#TOC1">DISADVANTAGES OF THE ALTERNATIVE ALGORITHM</a><br>
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<P>
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The alternative algorithm suffers from a number of disadvantages:
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</P>
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<P>
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1. It is substantially slower than the standard algorithm. This is partly
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because it has to search for all possible matches, but is also because it is
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less susceptible to optimization.
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</P>
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<P>
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2. Capturing parentheses and back references are not supported.
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</P>
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<P>
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3. Although atomic groups are supported, their use does not provide the
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performance advantage that it does for the standard algorithm.
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</P>
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<br><a name="SEC7" href="#TOC1">AUTHOR</a><br>
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<P>
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Philip Hazel
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<br>
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University Computing Service
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<br>
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Cambridge CB2 3QH, England.
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<br>
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</P>
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<br><a name="SEC8" href="#TOC1">REVISION</a><br>
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<P>
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Last updated: 19 April 2008
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<br>
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Copyright © 1997-2008 University of Cambridge.
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<br>
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<p>
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Return to the <a href="index.html">PCRE index page</a>.
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</p>
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