bionic (3) pcrematching.3.gz

Provided by: libpcre3-dev_8.39-9ubuntu0.1_amd64 bug

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(),  pcre16_exec()  and  pcre32_exec()  functions.  These work in the same as as Perl's matching
       function, and provide a Perl-compatible matching operation.  The just-in-time (JIT) optimization that  is
       described in the pcrejit documentation is compatible with these functions.

       An  alternative  algorithm  is  provided  by the pcre_dfa_exec(), pcre16_dfa_exec() and pcre32_dfa_exec()
       functions; they operate in a different way, and are not Perl-compatible. This alternative 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.

       PCRE's  "auto-possessification" optimization usually applies to character repeats at the end of a pattern
       (as well as internally). For example, the pattern "a\d+" is compiled as if it were "a\d++" because  there
       is  no point even considering the possibility of backtracking into the repeated digits. For DFA matching,
       this means that only one possible match is found. If you really do want multiple matches in  such  cases,
       either use an ungreedy repeat ("a\d+?") or set the PCRE_NO_AUTO_POSSESS option when compiling.

       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) always matches a single data unit,  even  in
       UTF-8,  UTF-16  or UTF-32 modes, is not supported in these modes, because the alternative algorithm moves
       through the subject string one character (not data unit) 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
       (except  for  lookbehinds),  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 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: 12 November 2013
       Copyright (c) 1997-2012 University of Cambridge.