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PCREPARTIAL(3)                                                  PCREPARTIAL(3)



NAME
       PCRE - Perl-compatible regular expressions

PARTIAL MATCHING IN PCRE

       In normal use of PCRE, if the subject string that is passed to pcre_exec() or pcre_dfa_exec() matches as far as
       it goes, but is too short to match the entire pattern, PCRE_ERROR_NOMATCH is returned. There are  circumstances
       where it might be helpful to distinguish this case from other cases in which there is no match.

       Consider,  for example, an application where a human is required to type in data for a field with specific for-
       matting requirements. An example might be a date in the form ddmmmyy, defined by this pattern:

         ^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$

       If the application sees the user's keystrokes one by one, and can check that what has  been  typed  so  far  is
       potentially valid, it is able to raise an error as soon as a mistake is made, by beeping and not reflecting the
       character that has been typed, for example. This immediate feedback is likely to be  a  better  user  interface
       than a check that is delayed until the entire string has been entered. Partial matching can also be useful when
       the subject string is very long and is not all available at once.

       PCRE supports partial matching by means of the PCRE_PARTIAL_SOFT and PCRE_PARTIAL_HARD options,  which  can  be
       set  when  calling  pcre_exec()  or pcre_dfa_exec(). For backwards compatibility, PCRE_PARTIAL is a synonym for
       PCRE_PARTIAL_SOFT. The essential difference between the two options is whether or not a partial match  is  pre-
       ferred  to an alternative complete match, though the details differ between the two matching functions. If both
       options are set, PCRE_PARTIAL_HARD takes precedence.

       Setting a partial matching option for pcre_exec() disables the use of any just-in-time code that was set up  by
       calling  pcre_study() with the PCRE_STUDY_JIT_COMPILE option. It also disables two of PCRE's standard optimiza-
       tions. PCRE remembers the last literal byte in a pattern, and abandons matching immediately if such a  byte  is
       not  present in the subject string. This optimization cannot be used for a subject string that might match only
       partially. If the pattern was studied, PCRE knows the minimum length of a matching string, and does not  bother
       to run the matching function on shorter strings. This optimization is also disabled for partial matching.

PARTIAL MATCHING USING pcre_exec()

       A partial match occurs during a call to pcre_exec() when the end of the subject string is reached successfully,
       but matching cannot continue because more characters are needed. However, at least one character in the subject
       must  have been inspected. This character need not form part of the final matched string; lookbehind assertions
       and the \K escape sequence provide ways of inspecting characters before the start of a matched  substring.  The
       requirement for inspecting at least one character exists because an empty string can always be matched; without
       such a restriction there would always be a partial match of an empty string at the end of the subject.

       If there are at least two slots in the offsets vector when pcre_exec() returns with a partial match, the  first
       slot  is  set  to the offset of the earliest character that was inspected when the partial match was found. For
       convenience, the second offset points to the end of the subject so that a substring can easily be identified.

       For the majority of patterns, the first offset identifies the start of the partially matched  string.  However,
       for  patterns  that  contain lookbehind assertions, or \K, or begin with \b or \B, earlier characters have been
       inspected while carrying out the match. For example:

         /(?<=abc)123/

       This pattern matches "123", but only if it is preceded by "abc". If the subject string is "xyzabc12", the  off-
       sets  after  a  partial match are for the substring "abc12", because all these characters are needed if another
       match is tried with extra characters added to the subject.

       What happens when a partial match is identified depends on which of the two partial matching options are set.

   PCRE_PARTIAL_SOFT with pcre_exec()

       If PCRE_PARTIAL_SOFT is set when pcre_exec() identifies a partial match, the partial match is  remembered,  but
       matching  continues  as  normal,  and  other alternatives in the pattern are tried. If no complete match can be
       found, pcre_exec() returns PCRE_ERROR_PARTIAL instead of PCRE_ERROR_NOMATCH.

       This option is "soft" because it prefers a complete match over a partial match.  All the various matching items
       in  a  pattern behave as if the subject string is potentially complete. For example, \z, \Z, and $ match at the
       end of the subject, as normal, and for \b and \B the end of the subject is treated as a non-alphanumeric.

       If there is more than one partial match, the first one that was found provides the data that is returned.  Con-
       sider this pattern:

         /123\w+X|dogY/

       If  this is matched against the subject string "abc123dog", both alternatives fail to match, but the end of the
       subject is reached during matching, so PCRE_ERROR_PARTIAL is returned. The offsets are set to 3 and 9,  identi-
       fying  "123dog"  as  the  first  partial match that was found. (In this example, there are two partial matches,
       because "dog" on its own partially matches the second alternative.)

   PCRE_PARTIAL_HARD with pcre_exec()

       If PCRE_PARTIAL_HARD is set for pcre_exec(), it returns PCRE_ERROR_PARTIAL as soon as a partial match is found,
       without continuing to search for possible complete matches. This option is "hard" because it prefers an earlier
       partial match over a later complete match. For this reason, the assumption is made that the end of the supplied
       subject  string  may not be the true end of the available data, and so, if \z, \Z, \b, \B, or $ are encountered
       at the end of the subject, the result is PCRE_ERROR_PARTIAL.

       Setting PCRE_PARTIAL_HARD also affects the way pcre_exec() checks UTF-8 subject strings for validity. Normally,
       an  invalid  UTF-8  sequence  causes  the error PCRE_ERROR_BADUTF8. However, in the special case of a truncated
       UTF-8 character at the end of the subject, PCRE_ERROR_SHORTUTF8 is returned when PCRE_PARTIAL_HARD is set.

   Comparing hard and soft partial matching

       The difference between the two partial matching options can be illustrated by a pattern such as:

         /dog(sbody)?/

       This matches either "dog" or "dogsbody", greedily (that is, it prefers the longer string if possible). If it is
       matched  against  the  string  "dog"  with PCRE_PARTIAL_SOFT, it yields a complete match for "dog". However, if
       PCRE_PARTIAL_HARD is set, the result is PCRE_ERROR_PARTIAL. On the other hand, if the pattern is made  ungreedy
       the result is different:

         /dog(sbody)??/

       In this case the result is always a complete match because pcre_exec() finds that first, and it never continues
       after finding a match. It might be easier to follow this explanation by thinking of the two patterns like this:

         /dog(sbody)?/    is the same as  /dogsbody|dog/
         /dog(sbody)??/   is the same as  /dog|dogsbody/

       The  second  pattern  will  never  match  "dogsbody"  when pcre_exec() is used, because it will always find the
       shorter match first.

PARTIAL MATCHING USING pcre_dfa_exec()

       The pcre_dfa_exec() function moves along the subject  string  character  by  character,  without  backtracking,
       searching  for  all possible matches simultaneously. If the end of the subject is reached before the end of the
       pattern, there is the possibility of a partial match, again provided that  at  least  one  character  has  been
       inspected.

       When PCRE_PARTIAL_SOFT is set, PCRE_ERROR_PARTIAL is returned only if there have been no complete matches. Oth-
       erwise, the complete matches are returned.  However, if PCRE_PARTIAL_HARD is set, a partial match takes  prece-
       dence  over  any  complete matches. The portion of the string that was inspected when the longest partial match
       was found is set as the first matching string, provided there are at least two slots in the offsets vector.

       Because pcre_dfa_exec() always searches for all possible matches, and there is no difference between greedy and
       ungreedy  repetition,  its  behaviour  is  different from pcre_exec when PCRE_PARTIAL_HARD is set. Consider the
       string "dog" matched against the ungreedy pattern shown above:

         /dog(sbody)??/

       Whereas pcre_exec() stops as soon as it finds the complete match for "dog", pcre_dfa_exec() also finds the par-
       tial match for "dogsbody", and so returns that when PCRE_PARTIAL_HARD is set.

PARTIAL MATCHING AND WORD BOUNDARIES

       If  a  pattern  ends  with  one  of  sequences  \b or \B, which test for word boundaries, partial matching with
       PCRE_PARTIAL_SOFT can give counter-intuitive results. Consider this pattern:

         /\bcat\b/

       This matches "cat", provided there is a word boundary at either end. If the subject string is  "the  cat",  the
       comparison of the final "t" with a following character cannot take place, so a partial match is found. However,
       pcre_exec() carries on with normal matching, which matches \b at the end of the subject when the last character
       is  a  letter,  thus finding a complete match. The result, therefore, is not PCRE_ERROR_PARTIAL. The same thing
       happens with pcre_dfa_exec(), because it also finds the complete match.

       Using PCRE_PARTIAL_HARD in this case does yield PCRE_ERROR_PARTIAL, because then the partial match takes prece-
       dence.

FORMERLY RESTRICTED PATTERNS

       For  releases  of PCRE prior to 8.00, because of the way certain internal optimizations were implemented in the
       pcre_exec() function, the PCRE_PARTIAL option (predecessor of PCRE_PARTIAL_SOFT) could not  be  used  with  all
       patterns.  From  release  8.00 onwards, the restrictions no longer apply, and partial matching with pcre_exec()
       can be requested for any pattern.

       Items that were formerly restricted were repeated single characters and repeated metasequences. If PCRE_PARTIAL
       was  set  for  a  pattern  that  did  not  conform  to  the  restrictions,  pcre_exec() returned the error code
       PCRE_ERROR_BADPARTIAL  (-13).  This  error  code  is  no  longer  in  use.  The  PCRE_INFO_OKPARTIAL  call   to
       pcre_fullinfo() to find out if a compiled pattern can be used for partial matching now always returns 1.

EXAMPLE OF PARTIAL MATCHING USING PCRETEST

       If  the  escape  sequence  \P  is present in a pcretest data line, the PCRE_PARTIAL_SOFT option is used for the
       match. Here is a run of pcretest that uses the date example quoted above:

           re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
         data> 25jun04\P
          0: 25jun04
          1: jun
         data> 25dec3\P
         Partial match: 23dec3
         data> 3ju\P
         Partial match: 3ju
         data> 3juj\P
         No match
         data> j\P
         No match

       The first data string is matched completely, so pcretest shows  the  matched  substrings.  The  remaining  four
       strings  do  not  match the complete pattern, but the first two are partial matches. Similar output is obtained
       when pcre_dfa_exec() is used.

       If the escape sequence \P is present more than once in a pcretest data line, the  PCRE_PARTIAL_HARD  option  is
       set for the match.

MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()

       When  a  partial  match has been found using pcre_dfa_exec(), it is possible to continue the match by providing
       additional subject data and calling pcre_dfa_exec() again with the same compiled regular expression, this  time
       setting  the  PCRE_DFA_RESTART  option.  You  must pass the same working space as before, because this is where
       details of the previous partial match are stored. Here is an  example  using  pcretest,  using  the  \R  escape
       sequence to set the PCRE_DFA_RESTART option (\D specifies the use of pcre_dfa_exec()):

           re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
         data> 23ja\P\D
         Partial match: 23ja
         data> n05\R\D
          0: n05

       The  first call has "23ja" as the subject, and requests partial matching; the second call has "n05" as the sub-
       ject for the continued (restarted) match.  Notice that when the match is complete, only the last part is shown;
       PCRE  does not retain the previously partially-matched string. It is up to the calling program to do that if it
       needs to.

       You can set the PCRE_PARTIAL_SOFT or PCRE_PARTIAL_HARD options with PCRE_DFA_RESTART to continue partial match-
       ing over multiple segments. This facility can be used to pass very long subject strings to pcre_dfa_exec().

MULTI-SEGMENT MATCHING WITH pcre_exec()

       From release 8.00, pcre_exec() can also be used to do multi-segment matching. Unlike pcre_dfa_exec(), it is not
       possible to restart the previous match with a new segment of data. Instead, new data must be added to the  pre-
       vious  subject  string,  and the entire match re-run, starting from the point where the partial match occurred.
       Earlier data can be discarded. It is best to use PCRE_PARTIAL_HARD in this situation, because it does not treat
       the end of a segment as the end of the subject when matching \z, \Z, \b, \B, and $. Consider an unanchored pat-
       tern that matches dates:

           re> /\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d/
         data> The date is 23ja\P\P
         Partial match: 23ja

       At this stage, an application could discard the text preceding "23ja", add on text from the next  segment,  and
       call  pcre_exec()  again.  Unlike pcre_dfa_exec(), the entire matching string must always be available, and the
       complete matching process occurs for each call, so more memory and more processing time is needed.

       Note: If the pattern contains lookbehind assertions, or \K, or starts  with  \b  or  \B,  the  string  that  is
       returned  for a partial match will include characters that precede the partially matched string itself, because
       these must be retained when adding on more characters for a subsequent matching attempt.

ISSUES WITH MULTI-SEGMENT MATCHING

       Certain types of pattern may give problems with multi-segment matching, whichever matching function is used.

       1. If the pattern contains a test for the beginning of a line, you need to pass the PCRE_NOTBOL option when the
       subject  string  for any call does start at the beginning of a line. There is also a PCRE_NOTEOL option, but in
       practice when doing multi-segment matching you should be using PCRE_PARTIAL_HARD, which includes the effect  of
       PCRE_NOTEOL.

       2.  Lookbehind assertions at the start of a pattern are catered for in the offsets that are returned for a par-
       tial match. However, in theory, a lookbehind assertion later in the pattern could require even earlier  charac-
       ters  to  be  inspected,  and  it  might not have been reached when a partial match occurs. This is probably an
       extremely unlikely case; you could guard against it to a certain extent by always including extra characters at
       the start.

       3.  Matching  a  subject  string  that  is split into multiple segments may not always produce exactly the same
       result as matching over one single long string, especially when PCRE_PARTIAL_SOFT is used. The section "Partial
       Matching  and  Word Boundaries" above describes an issue that arises if the pattern ends with \b or \B. Another
       kind of difference may occur when there are multiple matching possibilities, because (for PCRE_PARTIAL_SOFT)  a
       partial match result is given only when there are no completed matches. This means that as soon as the shortest
       match has been found, continuation to a new subject segment is no longer possible. Consider again this pcretest
       example:

           re> /dog(sbody)?/
         data> dogsb\P
          0: dog
         data> do\P\D
         Partial match: do
         data> gsb\R\P\D
          0: g
         data> dogsbody\D
          0: dogsbody
          1: dog

       The  first  data  line passes the string "dogsb" to pcre_exec(), setting the PCRE_PARTIAL_SOFT option. Although
       the string is a partial match for "dogsbody", the result is not PCRE_ERROR_PARTIAL, because the shorter  string
       "dog"  is  a complete match. Similarly, when the subject is presented to pcre_dfa_exec() in several parts ("do"
       and "gsb" being the first two) the match stops when "dog" has been found, and it is not possible  to  continue.
       On the other hand, if "dogsbody" is presented as a single string, pcre_dfa_exec() finds both matches.

       Because  of  these  problems, it is best to use PCRE_PARTIAL_HARD when matching multi-segment data. The example
       above then behaves differently:

           re> /dog(sbody)?/
         data> dogsb\P\P
         Partial match: dogsb
         data> do\P\D
         Partial match: do
         data> gsb\R\P\P\D
         Partial match: gsb

       4. Patterns that contain alternatives at the top level which do not all start with the same  pattern  item  may
       not work as expected when PCRE_DFA_RESTART is used with pcre_dfa_exec(). For example, consider this pattern:

         1234|3789

       If  the  first  part of the subject is "ABC123", a partial match of the first alternative is found at offset 3.
       There is no partial match for the second alternative, because such a match does not start at the same point  in
       the  subject  string.  Attempting  to continue with the string "7890" does not yield a match because only those
       alternatives that match at one point in the subject are remembered. The problem arises because the start of the
       second alternative matches within the first alternative. There is no problem with anchored patterns or patterns
       such as:

         1234|ABCD

       where no string can be a partial match for both alternatives. This is not a problem  if  pcre_exec()  is  used,
       because the entire match has to be rerun each time:

           re> /1234|3789/
         data> ABC123\P\P
         Partial match: 123
         data> 1237890
          0: 3789

       Of  course,  instead  of  using PCRE_DFA_RESTART, the same technique of re-running the entire match can also be
       used with pcre_dfa_exec(). Another possibility is to work with two buffers. If a partial match at offset  n  in
       the first buffer is followed by "no match" when PCRE_DFA_RESTART is used on the second buffer, you can then try
       a new match starting at offset n+1 in the first buffer.

AUTHOR

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

REVISION

       Last updated: 26 August 2011
       Copyright (c) 1997-2011 University of Cambridge.



                                                                PCREPARTIAL(3)