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PCREMATCHING(3)                      Library Functions Manual                     PCREMATCHING(3)



NAME
       PCRE - Perl-compatible regular expressions

PCRE MATCHING ALGORITHMS

       This document describes the two different algorithms that are available in PCRE for match‐
       ing a compiled regular expression against a given subject string. The "standard" algorithm
       is  the  one provided by the pcre_exec(), pcre16_exec() and pcre32_exec() functions. These
       work in the same as as Perl's matching function, and provide  a  Perl-compatible  matching
       operation.   The just-in-time (JIT) optimization that is described in the pcrejit documen‐
       tation is compatible with these functions.

       An alternative  algorithm  is  provided  by  the  pcre_dfa_exec(),  pcre16_dfa_exec()  and
       pcre32_dfa_exec() functions; they operate in a different way, and are not Perl-compatible.
       This alternative has advantages and disadvantages compared with  the  standard  algorithm,
       and these are described below.

       When  there  is only one possible way in which a given subject string can match a pattern,
       the two algorithms give the same answer. A difference arises, however, when there are mul‐
       tiple possibilities. For example, if the pattern

         ^<.*>

       is matched against the string

         <something> <something else> <something further>

       there  are  three possible answers. The standard algorithm finds only one of them, whereas
       the alternative algorithm finds all three.

REGULAR EXPRESSIONS AS TREES

       The set of strings that are matched by a regular expression can be represented as  a  tree
       structure.  An unlimited repetition in the pattern makes the tree of infinite size, but it
       is still a tree. Matching the pattern to a given subject string  (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 these correspond to the two  matching  algorithms  pro‐
       vided by PCRE.

THE STANDARD MATCHING ALGORITHM

       In  the terminology of Jeffrey Friedl's book "Mastering Regular Expressions", the standard
       algorithm is an "NFA algorithm". It conducts a depth-first search  of  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 all fail, it backs up to the previous branch point in
       the tree, and tries the next alternative branch at that level. This often involves backing
       up  (moving  to  the  left)  in  the subject string as well. The order in which repetition
       branches are tried is controlled by the greedy or ungreedy nature of the quantifier.

       If a leaf node is reached, a matching string has been found, and at that point  the  algo‐
       rithm  stops.  Thus,  if there is more than one possible match, this algorithm returns the
       first one that it finds. Whether this is the shortest, the longest, or  some  intermediate
       length  depends on the way the greedy and ungreedy repetition quantifiers are specified in
       the pattern.

       Because it ends up with a single path through the tree, it is  relatively  straightforward
       for  this  algorithm  to  keep track of the substrings that are matched by portions of the
       pattern in parentheses. This provides support for capturing parentheses  and  back  refer‐
       ences.

THE ALTERNATIVE MATCHING ALGORITHM

       This algorithm conducts a breadth-first search of the tree. Starting from the first match‐
       ing point in the subject, it scans the subject string from left to right, once,  character
       by character, and as it does this, it remembers all the paths through the tree that repre‐
       sent valid matches. In Friedl's terminology, this is a kind of "DFA algorithm", though  it
       is  not implemented as a traditional finite state machine (it keeps multiple states active
       simultaneously).

       Although the general principle of this matching algorithm is that  it  scans  the  subject
       string  only  once, without backtracking, there is one exception: when a lookaround asser‐
       tion is encountered, the characters following or preceding the current point  have  to  be
       independently inspected.

       The  scan  continues  until either the end of the subject is reached, or there are no more
       unterminated paths. At this point, terminated paths represent the different matching  pos‐
       sibilities  (if  there  are  none, the match has failed).  Thus, if there is more than one
       possible match, this algorithm finds all of them, and in particular, it finds the longest.
       The  matches  are  returned  in decreasing order of length. There is an option to stop the
       algorithm after the first match (which is necessarily the shortest) is found.

       Note that all the matches that are found start at the same point in the  subject.  If  the
       pattern

         cat(er(pillar)?)?

       is  matched  against  the string "the caterpillar catchment", the result will be the three
       strings "caterpillar", "cater", and "cat" that start at the fifth character  of  the  sub‐
       ject.  The  algorithm  does  not automatically move on to find matches that start at later
       positions.

       PCRE's "auto-possessification" optimization usually applies to character  repeats  at  the
       end  of  a pattern (as well as internally). For example, the pattern "a\d+" is compiled as
       if it were "a\d++" because there is no point even considering  the  possibility  of  back‐
       tracking  into  the  repeated  digits. For DFA matching, this means that only one possible
       match is found. If you really do want multiple  matches  in  such  cases,  either  use  an
       ungreedy repeat ("a\d+?") or set the PCRE_NO_AUTO_POSSESS option when compiling.

       There  are  a number of features of PCRE regular expressions that are not supported by the
       alternative matching algorithm. They are as follows:

       1. Because the algorithm finds all possible matches, the greedy or ungreedy nature of rep‐
       etition  quantifiers  is  not  relevant.  Greedy  and  ungreedy quantifiers are treated in
       exactly the same way. However, possessive quantifiers can make a difference when what fol‐
       lows could also match what is quantified, for example in a pattern like this:

         ^a++\w!

       This  pattern  matches  "aaab!" but not "aaa!", which would be matched by a non-possessive
       quantifier. Similarly, if an atomic group is present, it is matched as if it were a stand‐
       alone pattern at the current point, and the longest match is then "locked in" for the rest
       of the overall pattern.

       2. When dealing with multiple paths through the tree simultaneously, it is  not  straight‐
       forward to keep track of captured substrings for the different matching possibilities, and
       PCRE's implementation of this algorithm does not attempt to do this. This  means  that  no
       captured substrings are available.

       3.  Because  no  substrings  are captured, back references within the pattern are not sup‐
       ported, and cause errors if encountered.

       4. For the same reason, conditional expressions that use a backreference as the  condition
       or test for a specific group recursion are not supported.

       5. 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 paths and not on  others),  is
       not supported. It causes an error if encountered.

       6.  Callouts  are  supported,  but the value of the capture_top field is always 1, and the
       value of the capture_last field is always -1.

       7. The \C escape sequence, which (in the standard algorithm) always matches a single  data
       unit,  even in UTF-8, UTF-16 or UTF-32 modes, is not supported in these modes, because the
       alternative algorithm moves through the subject string one character (not data unit) at  a
       time, for all active paths through the tree.

       8.  Except for (*FAIL), the backtracking control verbs such as (*PRUNE) are not supported.
       (*FAIL) is supported, and behaves like a failing negative assertion.

ADVANTAGES OF THE ALTERNATIVE ALGORITHM

       Using the alternative matching algorithm provides the following advantages:

       1. 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 using the standard
       algorithm, you have to do kludgy things with callouts.

       2. Because the alternative algorithm scans the subject string just once, and  never  needs
       to backtrack (except for lookbehinds), it is possible to pass very long subject strings to
       the matching function in several pieces, checking for partial matching each time. Although
       it is possible to do multi-segment matching using the standard algorithm by retaining par‐
       tially matched substrings, it is more complicated.  The  pcrepartial  documentation  gives
       details of partial matching and discusses multi-segment matching.

DISADVANTAGES OF THE ALTERNATIVE ALGORITHM

       The alternative algorithm suffers from a number of disadvantages:

       1.  It  is substantially slower than the standard algorithm. This is partly because it has
       to search for all possible matches, but is also because it is less  susceptible  to  opti‐
       mization.

       2. Capturing parentheses and back references are not supported.

       3. Although atomic groups are supported, their use does not provide the performance advan‐
       tage that it does for the standard algorithm.

AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.

REVISION

       Last updated: 12 November 2013
       Copyright (c) 1997-2012 University of Cambridge.



PCRE 8.34                                12 November 2013                         PCREMATCHING(3)


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