pcrematching(3) — Linux manual page

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-8.45.tar.gz fetched from
       ⟨ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre/⟩ on
       2021-06-20.  If you discover any rendering problems in this HTML
       version of the page, or you believe there is a better or more up-
       to-date source for the page, or you have corrections or
       improvements to the information in this COLOPHON (which is not
       part of the original manual page), send a mail to
       man-pages@man7.org

PCRE 8.34                   12 November 2013             PCREMATCHING(3)

Pages that refer to this page: pcretest(1)pcreapi(3)pcrepattern(3)pcresyntax(3)