PCREMATCHING(3)           OpenBSD Programmer's 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 matching a compiled regular expression against a given
       subject string. The "standard" algorithm is the one provided by the
       pcre_exec() and pcre16_exec() functions. These work in the same was 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() and
       pcre16_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

       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

       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

       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.

       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 or UTF-16 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 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

       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

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

REVISION

       Last updated: 08 January 2012
       Copyright (c) 1997-2012 University of Cambridge.



                                                               PCREMATCHING(3)
