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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 sub-
       ject  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.

REGULAR EXPRESSIONS AS TREES

       The set of strings that are matched by a regular expression can be rep-
       resented	 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  Expres-
       sions",	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  alterna-
       tives  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.

								1

PCREMATCHING(3)					  PCREMATCHING(3)

       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 possi-
       ble 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 rela-
       tively straightforward for this algorithm to keep  track	 of  the  sub-
       strings	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. 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 "cat", "cater", and "caterpillar" that	 start
       at the fourth character of the subject. The algorithm does not automat-
       ically 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, pos-
       sessive quantifiers can make a difference when what follows could  also
       match what is quantified, for example in a pattern like this:

								2

PCREMATCHING(3)					  PCREMATCHING(3)

	 ^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 sub-
       strings are available.

       3.  Because no substrings are captured, back references within the pat-
       tern are not supported, and cause errors if encountered.

       4. For the same reason, conditional expressions that use	 a  backrefer-
       ence  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 alterna-
       tive 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 advan-
       tages:

       1. All possible matches (at a single point in the subject) are automat-
       ically  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.

								3

PCREMATCHING(3)					  PCREMATCHING(3)

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

								4

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