NAME | PCRE MATCHING ALGORITHMS | REGULAR EXPRESSIONS AS TREES | THE STANDARD MATCHING ALGORITHM | THE ALTERNATIVE MATCHING ALGORITHM | ADVANTAGES OF THE ALTERNATIVE ALGORITHM | DISADVANTAGES OF THE ALTERNATIVE ALGORITHM | AUTHOR | REVISION | COLOPHON

PCREMATCHING(3)           Library Functions Manual           PCREMATCHING(3)

NAME         top

       PCRE - Perl-compatible regular expressions

PCRE MATCHING ALGORITHMS         top


       This document describes the two different algorithms that are
       available in PCRE for matching 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 documentation 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 multiple 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         top


       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 provided by PCRE.

THE STANDARD MATCHING ALGORITHM         top


       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 algorithm 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
       references.

THE ALTERNATIVE MATCHING ALGORITHM         top


       This algorithm conducts a breadth-first search of the tree. Starting
       from the first matching 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 represent
       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 assertion 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 possibilities (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 subject. 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
       backtracking 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 repetition quantifiers is not relevant. Greedy and
       ungreedy quantifiers are treated in exactly the same way. However,
       possessive quantifiers can make a difference when what follows 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 standalone 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 straightforward 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 supported, 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         top


       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 partially matched substrings, it is more
       complicated. The pcrepartial documentation gives details of partial
       matching and discusses multi-segment matching.

DISADVANTAGES OF THE ALTERNATIVE ALGORITHM         top


       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 optimization.

       2. Capturing parentheses and back references are not supported.

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

AUTHOR         top


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

REVISION         top


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

COLOPHON         top

       This page is part of the PCRE (Perl Compatible Regular Expressions)
       project.  Information about the project can be found at 
       ⟨http://www.pcre.org/⟩.  If you have a bug report for this manual
       page, see ⟨http://bugs.exim.org/enter_bug.cgi?product=PCRE⟩.  This
       page was obtained from the tarball pcre-8.40.tar.gz fetched from 
       ⟨ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre/⟩ on
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PCRE 8.34                     12 November 2013               PCREMATCHING(3)