PCREMATCHING(3) 					     Library Functions 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() 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 differ-
       ence 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 quan-
       tifier.

       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 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).

       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. In PCRE, there is an option to stop the algorithm after the
       first match (which is necessarily the shortest) has been 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 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 fol-
       lows:

       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 sub-
       strings 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 alter-
       native 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.  There  is  much  better support for partial matching. The restrictions on the content of the pattern that apply when using the standard
       algorithm for partial matching do not apply to the alternative algorithm. For non-anchored patterns, the starting  position  of	a  partial
       match is available.

       3.  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.

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: 19 April 2008
       Copyright (c) 1997-2008 University of Cambridge.

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