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       PCRE - Perl-compatible regular expressions


       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 algorithm finds all three.


       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.


       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.


       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

       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. 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


       is  matched  against the string "the caterpillar catchment", the result
       will be the three strings "cat", "cater", and "caterpillar" that  start
       at  the  fourth  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:


       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

       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) matches  a
       single  byte,  even  in  UTF-8  mode,  is  not  supported  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.


       Using   the  alternative  matching  algorithm  provides  the  following

       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.  The  pcrepartial  documentation  gives
       details of partial 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 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.


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


       Last updated: 29 September 2009
       Copyright (c) 1997-2009 University of Cambridge.