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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 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) 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.

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  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: 17 November 2010
       Copyright (c) 1997-2010 University of Cambridge.

                                                                                  PCREMATCHING(3)