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<html><head><title>pcre2matching specification</title></head><body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB"><h1>pcre2matching man page</h1><p>Return to the <a href="index.html">PCRE2 index page</a>.</p><p>This page is part of the PCRE2 HTML documentation. It was generatedautomatically from the original man page. If there is any nonsense in it,please consult the man page, in case the conversion went wrong.<br><ul><li><a name="TOC1" href="#SEC1">PCRE2 MATCHING ALGORITHMS</a><li><a name="TOC2" href="#SEC2">REGULAR EXPRESSIONS AS TREES</a><li><a name="TOC3" href="#SEC3">THE STANDARD MATCHING ALGORITHM</a><li><a name="TOC4" href="#SEC4">THE ALTERNATIVE MATCHING ALGORITHM</a><li><a name="TOC5" href="#SEC5">ADVANTAGES OF THE ALTERNATIVE ALGORITHM</a><li><a name="TOC6" href="#SEC6">DISADVANTAGES OF THE ALTERNATIVE ALGORITHM</a><li><a name="TOC7" href="#SEC7">AUTHOR</a><li><a name="TOC8" href="#SEC8">REVISION</a></ul><br><a name="SEC1" href="#TOC1">PCRE2 MATCHING ALGORITHMS</a><br><P>This document describes the two different algorithms that are available inPCRE2 for matching a compiled regular expression against a given subjectstring. The "standard" algorithm is the one provided by the <b>pcre2_match()</b>function. This works in the same as Perl's matching function, and provide aPerl-compatible matching operation. The just-in-time (JIT) optimization that isdescribed in the<a href="pcre2jit.html"><b>pcre2jit</b></a>documentation is compatible with this function.</P><P>An alternative algorithm is provided by the <b>pcre2_dfa_match()</b> function;it operates in a different way, and is not Perl-compatible. This alternativehas advantages and disadvantages compared with the standard algorithm, andthese are described below.</P><P>When there is only one possible way in which a given subject string can match apattern, the two algorithms give the same answer. A difference arises, however,when there are multiple possibilities. For example, if the pattern<pre> ^<.*></pre>is matched against the string<pre> <something> <something else> <something further></pre>there are three possible answers. The standard algorithm finds only one ofthem, whereas the alternative algorithm finds all three.</P><br><a name="SEC2" href="#TOC1">REGULAR EXPRESSIONS AS TREES</a><br><P>The set of strings that are matched by a regular expression can be representedas a tree structure. An unlimited repetition in the pattern makes the tree ofinfinite size, but it is still a tree. Matching the pattern to a given subjectstring (from a given starting point) can be thought of as a search of the tree.There are two ways to search a tree: depth-first and breadth-first, and thesecorrespond to the two matching algorithms provided by PCRE2.</P><br><a name="SEC3" href="#TOC1">THE STANDARD MATCHING ALGORITHM</a><br><P>In the terminology of Jeffrey Friedl's book "Mastering Regular Expressions",the standard algorithm is an "NFA algorithm". It conducts a depth-first searchof the pattern tree. That is, it proceeds along a single path through the tree,checking that the subject matches what is required. When there is a mismatch,the algorithm tries any alternatives at the current point, and if they allfail, it backs up to the previous branch point in the tree, and tries the nextalternative branch at that level. This often involves backing up (moving to theleft) in the subject string as well. The order in which repetition branches aretried is controlled by the greedy or ungreedy nature of the quantifier.</P><P>If a leaf node is reached, a matching string has been found, and at that pointthe algorithm stops. Thus, if there is more than one possible match, thisalgorithm returns the first one that it finds. Whether this is the shortest,the longest, or some intermediate length depends on the way the alternationsand the greedy or ungreedy repetition quantifiers are specified in thepattern.</P><P>Because it ends up with a single path through the tree, it is relativelystraightforward for this algorithm to keep track of the substrings that arematched by portions of the pattern in parentheses. This provides support forcapturing parentheses and backreferences.</P><br><a name="SEC4" href="#TOC1">THE ALTERNATIVE MATCHING ALGORITHM</a><br><P>This algorithm conducts a breadth-first search of the tree. Starting from thefirst matching point in the subject, it scans the subject string from left toright, once, character by character, and as it does this, it remembers all thepaths through the tree that represent valid matches. In Friedl's terminology,this is a kind of "DFA algorithm", though it is not implemented as atraditional finite state machine (it keeps multiple states activesimultaneously).</P><P>Although the general principle of this matching algorithm is that it scans thesubject string only once, without backtracking, there is one exception: when alookaround assertion is encountered, the characters following or preceding thecurrent point have to be independently inspected.</P><P>The scan continues until either the end of the subject is reached, or there areno more unterminated paths. At this point, terminated paths represent thedifferent matching possibilities (if there are none, the match has failed).Thus, if there is more than one possible match, this algorithm finds all ofthem, and in particular, it finds the longest. The matches are returned inthe output vector in decreasing order of length. There is an option to stop thealgorithm after the first match (which is necessarily the shortest) is found.</P><P>Note that the size of vector needed to contain all the results depends on thenumber of simultaneous matches, not on the number of parentheses in thepattern. Using <b>pcre2_match_data_create_from_pattern()</b> to create the matchdata block is therefore not advisable when doing DFA matching.</P><P>Note also that all the matches that are found start at the same point in thesubject. If the pattern<pre> cat(er(pillar)?)?</pre>is matched against the string "the caterpillar catchment", the result is thethree strings "caterpillar", "cater", and "cat" that start at the fifthcharacter of the subject. The algorithm does not automatically move on to findmatches that start at later positions.</P><P>PCRE2's "auto-possessification" optimization usually applies to characterrepeats at the end of a pattern (as well as internally). For example, thepattern "a\d+" is compiled as if it were "a\d++" because there is no pointeven considering the possibility of backtracking into the repeated digits. ForDFA matching, this means that only one possible match is found. If you reallydo want multiple matches in such cases, either use an ungreedy repeat("a\d+?") or set the PCRE2_NO_AUTO_POSSESS option when compiling.</P><P>There are a number of features of PCRE2 regular expressions that are notsupported or behave differently in the alternative matching function. Thosethat are not supported cause an error if encountered.</P><P>1. Because the algorithm finds all possible matches, the greedy or ungreedynature of repetition quantifiers is not relevant (though it may affectauto-possessification, as just described). During matching, greedy and ungreedyquantifiers are treated in exactly the same way. However, possessivequantifiers can make a difference when what follows could also match what isquantified, for example in a pattern like this:<pre> ^a++\w!</pre>This pattern matches "aaab!" but not "aaa!", which would be matched by anon-possessive quantifier. Similarly, if an atomic group is present, it ismatched as if it were a standalone pattern at the current point, and thelongest match is then "locked in" for the rest of the overall pattern.</P><P>2. When dealing with multiple paths through the tree simultaneously, it is notstraightforward to keep track of captured substrings for the different matchingpossibilities, and PCRE2's implementation of this algorithm does not attempt todo this. This means that no captured substrings are available.</P><P>3. Because no substrings are captured, backreferences within the pattern arenot supported.</P><P>4. For the same reason, conditional expressions that use a backreference as thecondition or test for a specific group recursion are not supported.</P><P>5. Again for the same reason, script runs are not supported.</P><P>6. Because many paths through the tree may be active, the \K escape sequence,which resets the start of the match when encountered (but may be on some pathsand not on others), is not supported.</P><P>7. Callouts are supported, but the value of the <i>capture_top</i> field isalways 1, and the value of the <i>capture_last</i> field is always 0.</P><P>8. The \C escape sequence, which (in the standard algorithm) always matches asingle code unit, even in a UTF mode, is not supported in these modes, becausethe alternative algorithm moves through the subject string one character (notcode unit) at a time, for all active paths through the tree.</P><P>9. Except for (*FAIL), the backtracking control verbs such as (*PRUNE) are notsupported. (*FAIL) is supported, and behaves like a failing negative assertion.</P><P>10. The PCRE2_MATCH_INVALID_UTF option for <b>pcre2_compile()</b> is notsupported by <b>pcre2_dfa_match()</b>.</P><br><a name="SEC5" href="#TOC1">ADVANTAGES OF THE ALTERNATIVE ALGORITHM</a><br><P>The main advantage of the alternative algorithm is that all possible matches(at a single point in the subject) are automatically found, and in particular,the longest match is found. To find more than one match at the same point usingthe standard algorithm, you have to do kludgy things with callouts.</P><P>Partial matching is possible with this algorithm, though it has somelimitations. The<a href="pcre2partial.html"><b>pcre2partial</b></a>documentation gives details of partial matching and discusses multi-segmentmatching.</P><br><a name="SEC6" href="#TOC1">DISADVANTAGES OF THE ALTERNATIVE ALGORITHM</a><br><P>The alternative algorithm suffers from a number of disadvantages:</P><P>1. It is substantially slower than the standard algorithm. This is partlybecause it has to search for all possible matches, but is also because it isless susceptible to optimization.</P><P>2. Capturing parentheses, backreferences, script runs, and matching withininvalid UTF string are not supported.</P><P>3. Although atomic groups are supported, their use does not provide theperformance advantage that it does for the standard algorithm.</P><P>4. JIT optimization is not supported.</P><br><a name="SEC7" href="#TOC1">AUTHOR</a><br><P>Philip Hazel<br>Retired from University Computing Service<br>Cambridge, England.<br></P><br><a name="SEC8" href="#TOC1">REVISION</a><br><P>Last updated: 19 January 2024<br>Copyright © 1997-2024 University of Cambridge.<br><p>Return to the <a href="index.html">PCRE2 index page</a>.</p>
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