pcre2matching(3) — Linux manual page


PCRE2MATCHING(3)        Library Functions Manual        PCRE2MATCHING(3)

NAME         top

       PCRE2 - Perl-compatible regular expressions (revised API)


       This document describes the two different algorithms that are
       available in PCRE2 for matching a compiled regular expression
       against a given subject string. The "standard" algorithm is the
       one provided by the pcre2_match() function. This works 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 pcre2jit documentation is
       compatible with this function.

       An alternative algorithm is provided by the pcre2_dfa_match()
       function; it operates in a different way, and is not Perl-
       compatible. This alternative has advantages and disadvantages
       compared with the standard algorithm, and these are described

       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


       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 PCRE2.


       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 alternations and the
       greedy or ungreedy repetition quantifiers are specified in the

       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


       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 the output vector 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 the size of vector needed to contain all the results
       depends on the number of simultaneous matches, not on the number
       of parentheses in the pattern. Using
       pcre2_match_data_create_from_pattern() to create the match data
       block is therefore not advisable when doing DFA matching.

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


       is matched against the string "the caterpillar catchment", the
       result is 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.

       PCRE2'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 PCRE2_NO_AUTO_POSSESS option
       when compiling.

       There are a number of features of PCRE2 regular expressions that
       are not supported or behave differently in the alternative
       matching function. Those that are not supported cause an error if

       1. Because the algorithm finds all possible matches, the greedy
       or ungreedy nature of repetition quantifiers is not relevant
       (though it may affect auto-possessification, as just described).
       During matching, 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:


       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
       PCRE2'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, backreferences within the
       pattern are not supported.

       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. Again for the same reason, script runs are not supported.

       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 paths and not on others), is not

       7. Callouts are supported, but the value of the capture_top field
       is always 1, and the value of the capture_last field is always 0.

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

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

       10. The PCRE2_MATCH_INVALID_UTF option for pcre2_compile() is not
       supported by pcre2_dfa_match().


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

       Partial matching is possible with this algorithm, though it has
       some limitations. The pcre2partial documentation gives details of
       partial matching and discusses multi-segment matching.


       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, backreferences, script runs, and
       matching within invalid UTF string are not supported.

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

       4. JIT optimization is not supported.

AUTHOR         top

       Philip Hazel
       Retired from University Computing Service
       Cambridge, England.

REVISION         top

       Last updated: 28 August 2021
       Copyright (c) 1997-2021 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 fetched from
       ⟨https://github.com/PhilipHazel/pcre2.git⟩ on 2023-12-22.  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

PCRE2 10.38                  28 August 2021             PCRE2MATCHING(3)

Pages that refer to this page: pcre2test(1)pcre2api(3)pcre2pattern(3)pcre2syntax(3)