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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 a match-
       ing function 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 formatting 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 immedi-
       ate  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 any of the
       matching functions. For backwards compatibility, PCRE_PARTIAL is a syn-
       onym  for  PCRE_PARTIAL_SOFT.  The essential difference between the two
       options is whether or not a partial match is preferred to  an  alterna-
       tive complete match, though the details differ between the two types of
       matching function. If both options  are  set,  PCRE_PARTIAL_HARD  takes
       precedence.

       If  you  want to use partial matching with just-in-time optimized code,
       you must call pcre_study(), pcre16_study() or  pcre32_study() with  one
       or both of these options:

         PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE
         PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE

       PCRE_STUDY_JIT_COMPILE  should also be set if you are going to run non-
       partial matches on the same pattern. If the appropriate JIT study  mode
       has not been set for a match, the interpretive matching code is used.

       Setting a partial matching option disables two of PCRE's standard opti-
       mizations. PCRE remembers the last literal data unit in a pattern,  and
       abandons  matching  immediately  if  it  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 dis-
       abled for partial matching.


PARTIAL MATCHING USING pcre_exec() OR pcre[16|32]_exec()


       A  partial   match   occurs   during   a   call   to   pcre_exec()   or
       pcre[16|32]_exec()  when  the end of the subject string is reached suc-
       cessfully, 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  a  partial
       match  is returned, the first slot is set to the offset of the earliest
       character that was inspected. For convenience, the second offset points
       to the end of the subject so that a substring can easily be identified.
       If there are at least three slots in the offsets vector, the third slot
       is set to the offset of the character where matching started.

       For the majority of patterns, the contents of the first and third slots
       will be the same. However, for patterns that contain lookbehind  asser-
       tions, or begin with \b or \B, characters before the one where matching
       started may have been inspected while carrying out the match. For exam-
       ple, consider this pattern:

         /(?<=abc)123/

       This pattern matches "123", but only if it is preceded by "abc". If the
       subject string is "xyzabc12", the first two  offsets  after  a  partial
       match  are for the substring "abc12", because all these characters were
       inspected. However, the third offset is set to 6, because that  is  the
       offset where matching began.

       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() OR pcre[16|32]_exec()

       If PCRE_PARTIAL_SOFT is  set  when  pcre_exec()  or  pcre[16|32]_exec()
       identifies a partial match, the partial match is remembered, but match-
       ing continues as normal, and other  alternatives  in  the  pattern  are
       tried.  If  no  complete  match  can  be  found,  PCRE_ERROR_PARTIAL is
       returned instead of PCRE_ERROR_NOMATCH.

       This option is "soft" because it prefers a complete match over  a  par-
       tial  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. Consider this pattern:

         /123\w+X|dogY/

       If  this is matched against the subject string "abc123dog", both alter-
       natives 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, identifying "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() OR pcre[16|32]_exec()

       If PCRE_PARTIAL_HARD is  set  for  pcre_exec()  or  pcre[16|32]_exec(),
       PCRE_ERROR_PARTIAL  is  returned  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 com-
       plete 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, provided that at least one
       character in the subject has been inspected.

       Setting PCRE_PARTIAL_HARD also affects the way UTF-8 and UTF-16 subject
       strings  are checked for validity. Normally, an invalid sequence causes
       the error PCRE_ERROR_BADUTF8 or PCRE_ERROR_BADUTF16.  However,  in  the
       special  case  of  a  truncated  character  at  the end of the subject,
       PCRE_ERROR_SHORTUTF8  or   PCRE_ERROR_SHORTUTF16   is   returned   when
       PCRE_PARTIAL_HARD is set.

   Comparing hard and soft partial matching

       The  difference  between the two partial matching options can be illus-
       trated 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  dif-
       ferent:

         /dog(sbody)??/

       In  this  case  the  result  is always a complete match because that is
       found first, and matching never  continues  after  finding  a  complete
       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", because it will  always
       find the shorter match first.


PARTIAL MATCHING USING pcre_dfa_exec() OR pcre[16|32]_dfa_exec()


       The DFA functions move along the subject string character by character,
       without backtracking, searching for  all  possible  matches  simultane-
       ously.  If the end of the subject is reached before the end of the pat-
       tern, 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. Otherwise,  the  complete  matches
       are  returned.   However,  if PCRE_PARTIAL_HARD is set, a partial match
       takes precedence 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  the  DFA functions always search for all possible matches, and
       there is no difference between greedy and  ungreedy  repetition,  their
       behaviour  is  different  from  the  standard  functions when PCRE_PAR-
       TIAL_HARD is  set.  Consider  the  string  "dog"  matched  against  the
       ungreedy pattern shown above:

         /dog(sbody)??/

       Whereas  the  standard functions stop as soon as they find the complete
       match for "dog", the DFA functions also  find  the  partial  match  for
       "dogsbody", and so return 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, normal matching carries on, and \b matches at the end  of  the
       subject  when  the  last  character is a letter, so a complete match is
       found.  The  result,  therefore,  is  not   PCRE_ERROR_PARTIAL.   Using
       PCRE_PARTIAL_HARD  in  this case does yield PCRE_ERROR_PARTIAL, because
       then the partial match takes precedence.


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 can be requested for  any  pat-
       tern.

       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 com-
       plete pattern, but the first two are partial matches. Similar output is
       obtained if DFA matching 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() OR pcre[16|32]_dfa_exec()


       When a partial match has been found using a DFA matching  function,  it
       is  possible to continue the match by providing additional subject data
       and calling the function again with the same compiled  regular  expres-
       sion,  this time setting the PCRE_DFA_RESTART option. You must pass the
       same working space as before, because this is where details of the pre-
       vious  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 the DFA matching function):

           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 match-
       ing; the second call  has  "n05"  as  the  subject  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.

       That means that, for an unanchored pattern, if a continued match fails,
       it  is  not  possible  to  try  again at a new starting point. All this
       facility is capable of doing is  continuing  with  the  previous  match
       attempt.  In  the previous example, if the second set of data is "ug23"
       the result is no match, even though there would be a match for  "aug23"
       if  the entire string were given at once. Depending on the application,
       this may or may not be what you want.  The only way to allow for start-
       ing  again  at  the next character is to retain the matched part of the
       subject and try a new complete match.

       You can set the PCRE_PARTIAL_SOFT  or  PCRE_PARTIAL_HARD  options  with
       PCRE_DFA_RESTART  to  continue partial matching over multiple segments.
       This facility can be used to pass very long subject strings to the  DFA
       matching functions.


MULTI-SEGMENT MATCHING WITH pcre_exec() OR pcre[16|32]_exec()


       From  release 8.00, the standard matching functions can also be used to
       do multi-segment matching. Unlike the DFA functions, it is not possible
       to  restart the previous match with a new segment of data. Instead, new
       data must be added to the previous subject string, and the entire match
       re-run,  starting from the point where the partial match occurred. Ear-
       lier 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  pattern  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  the  matching  function
       again.  Unlike  the  DFA matching functions, 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 includes
       characters  that precede the start of what would be returned for a com-
       plete match, because it contains all the characters that were inspected
       during the partial match.


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 that have already been obeyed are catered  for
       in the offsets that are returned for a partial match. However a lookbe-
       hind assertion later in the pattern could require even earlier  charac-
       ters   to  be  inspected.  You  can  handle  this  case  by  using  the
       PCRE_INFO_MAXLOOKBEHIND    option    of    the    pcre_fullinfo()    or
       pcre[16|32]_fullinfo()  functions  to  obtain the length of the longest
       lookbehind in the pattern. This length  is  given  in  characters,  not
       bytes.  If  you  always retain at least that many characters before the
       partially matched string, all should be  well.  (Of  course,  near  the
       start of the subject, fewer characters may be present; in that case all
       characters should be retained.)

       From release 8.33, there is a more accurate way of deciding which char-
       acters  to  retain.  Instead  of  subtracting the length of the longest
       lookbehind from the  earliest  inspected  character  (offsets[0]),  the
       match  start  position  (offsets[2]) should be used, and the next match
       attempt started at the offsets[2] character by setting the  startoffset
       argument of pcre_exec() or pcre_dfa_exec().

       For  example, if the pattern "(?<=123)abc" is partially matched against
       the string "xx123a", the three offset values returned are 2, 6, and  5.
       This  indicates  that  the  matching  process that gave a partial match
       started at offset 5, but the characters "123a" were all inspected.  The
       maximum  lookbehind  for  that pattern is 3, so taking that away from 5
       shows that we need only keep "123a", and the next match attempt can  be
       started at offset 3 (that is, at "a") when further characters have been
       added. When the match start is not the  earliest  inspected  character,
       pcretest shows it explicitly:

           re> "(?<=123)abc"
         data> xx123a\P\P
         Partial match at offset 5: 123a

       3.  Because a partial match must always contain at least one character,
       what might be considered a partial match of an  empty  string  actually
       gives a "no match" result. For example:

           re> /c(?<=abc)x/
         data> ab\P
         No match

       If the next segment begins "cx", a match should be found, but this will
       only happen if characters from the previous segment are  retained.  For
       this  reason,  a  "no  match"  result should be interpreted as "partial
       match of an empty string" when the pattern contains lookbehinds.

       4. 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  possi-
       ble. 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 a standard matching
       function, 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 a DFA matching function 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,  a  DFA  matching  function
       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  differ-
       ently:

           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

       5. 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. 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 a standard matching  function  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 the DFA  matching  func-
       tions.  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: 02 July 2013
       Copyright (c) 1997-2013 University of Cambridge.



PCRE 8.34                        02 July 2013                   pcrepartial(3)

pcre 8.34 - Generated Sun Jan 12 18:37:33 CST 2014
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