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PCREMATCHING(3)                                                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()
       function.   This  works  in  the  same was as Perl's matching function, and provides a Perl-compatible matching
       operation.

       An alternative algorithm is provided by the pcre_dfa_exec() function; this operates in a different way, and  is
       not  Perl-compatible.  It  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  algo-
       rithm 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 unlim-
       ited 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 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 quan-
       tifiers 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  sub-
       ject,  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,  with-
       out  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  quanti-
       fiers  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 spe-
       cific 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  cap-
       ture_last field is always -1.

       7.  The \C escape sequence, which (in the standard algorithm) matches a single byte, even in UTF-8 mode, is not
       supported in UTF-8 mode, because the alternative algorithm moves through the subject string one character 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,  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
       (pcre_exec()), 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 pos-
       sible 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: 19 November 2011
       Copyright (c) 1997-2010 University of Cambridge.



                                                               PCREMATCHING(3)