pcre2partial

PCRE2PARTIAL(3)            Library Functions Manual            PCRE2PARTIAL(3)

NAME
       PCRE2 - Perl-compatible regular expressions

PARTIAL MATCHING IN PCRE2

       In  normal use of PCRE2, if there is a match up to the end of a subject
       string, but more characters are needed to  match  the  entire  pattern,
       PCRE2_ERROR_NOMATCH  is  returned,  just  like any other failing match.
       There are circumstances where it might be helpful to  distinguish  this
       "partial match" case.

       One  example  is  an application where the subject string is very long,
       and not all available at once. The requirement here is to be able to do
       the  matching  segment  by segment, but special action is needed when a
       matched substring spans the boundary between two segments.

       Another example is checking a user input string  as  it  is  typed,  to
       ensure that it conforms to a required format. Invalid characters can be
       immediately diagnosed and rejected, giving instant feedback.

       Partial matching is a PCRE2-specific feature; it is  not  Perl-compati-
       ble.  It  is  requested  by  setting  one  of the PCRE2_PARTIAL_HARD or
       PCRE2_PARTIAL_SOFT options when calling a matching function.  The  dif-
       ference  between  the  two options is whether or not a partial match is
       preferred to an alternative complete match, though the  details  differ
       between  the  two  types of matching function. If both options are set,
       PCRE2_PARTIAL_HARD takes precedence.

       If you want to use partial matching with just-in-time  optimized  code,
       as  well  as  setting a partial match option for the matching function,
       you must also call  pcre2_jit_compile()  with  one  or  both  of  these
       options:

         PCRE2_JIT_PARTIAL_HARD
         PCRE2_JIT_PARTIAL_SOFT

       PCRE2_JIT_COMPLETE  should also be set if you are going to run non-par-
       tial matches on the same pattern. Separate code is  compiled  for  each
       mode.  If  the appropriate JIT mode has not been compiled, interpretive
       matching code is used.

       Setting a partial matching option  disables  two  of  PCRE2's  standard
       optimization  hints.  PCRE2  remembers  the last literal code 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. PCRE2 also remembers a 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.

REQUIREMENTS FOR A PARTIAL MATCH

       A  possible  partial  match  occurs during matching when the end of the
       subject string is reached successfully, but either more characters  are
       needed  to complete the match, or the addition of more characters might
       change what is matched.

       Example 1: if the pattern is /abc/ and the subject is "ab", more  char-
       acters  are  definitely  needed  to complete a match. In this case both
       hard and soft matching options yield a partial match.

       Example 2: if the pattern is /ab+/ and the subject is "ab", a  complete
       match  can  be  found, but the addition of more characters might change
       what is matched. In this case, only PCRE2_PARTIAL_HARD returns  a  par-
       tial match; PCRE2_PARTIAL_SOFT returns the complete match.

       On  reaching the end of the subject, when PCRE2_PARTIAL_HARD is set, if
       the next pattern item is \z, \Z, \b, \B, or $ there is always a partial
       match.   Otherwise, for both options, the next pattern item must be one
       that inspects a character, and at least one of the  following  must  be
       true:

       (1)  At  least  one  character has already been inspected. An inspected
       character need not form part of the final  matched  string;  lookbehind
       assertions  and the \K escape sequence provide ways of inspecting char-
       acters before the start of a matched string.

       (2) The pattern contains one or more lookbehind assertions. This condi-
       tion  exists  in  case  there  is a lookbehind that inspects characters
       before the start of the match.

       (3) There is a special case when the whole pattern can match  an  empty
       string.   When  the  starting  point  is at the end of the subject, the
       empty string match is a possibility, and if PCRE2_PARTIAL_SOFT  is  set
       and  neither  of the above conditions is true, it is returned. However,
       because adding more characters  might  result  in  a  non-empty  match,
       PCRE2_PARTIAL_HARD  returns  a  partial match, which in this case means
       "there is going to be a match at this point, but until some more  char-
       acters are added, we do not know if it will be an empty string or some-
       thing longer".

PARTIAL MATCHING USING pcre2_match()

       When  a  partial  matching  option  is  set,  the  result  of   calling
       pcre2_match() can be one of the following:

       A successful match
         A complete match has been found, starting and ending within this sub-
         ject.

       PCRE2_ERROR_NOMATCH
         No match can start anywhere in this subject.

       PCRE2_ERROR_PARTIAL
         Adding more characters may result in a complete match that  uses  one
         or more characters from the end of this subject.

       When a partial match is returned, the first two elements in the ovector
       point to the portion of the subject that was matched, but the values in
       the rest of the ovector are undefined. The appearance of \K in the pat-
       tern has no effect for a partial match. Consider this pattern:

         /abc\K123/

       If it is matched against "456abc123xyz" the result is a complete match,
       and  the ovector defines the matched string as "123", because \K resets
       the "start of match" point. However, if a partial  match  is  requested
       and  the subject string is "456abc12", a partial match is found for the
       string "abc12", because all these characters are needed  for  a  subse-
       quent re-match with additional characters.

       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 PCRE2_ERROR_PARTIAL is returned. The offsets are set to  3
       and  9, identifying "123dog" as the first partial match. (In this exam-
       ple, there are two partial matches, because "dog" on its own  partially
       matches the second alternative.)

   How a partial match is processed by pcre2_match()

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

       If PCRE2_PARTIAL_HARD is set, PCRE2_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 complete match. For this reason, the assump-
       tion is made that the end of the supplied subject  string  is  not  the
       true  end  of  the  available  data, which is why \z, \Z, \b, \B, and $
       always give a partial match.

       If PCRE2_PARTIAL_SOFT is set, 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,  PCRE2_ERROR_PARTIAL  is
       returned  instead of PCRE2_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 poten-
       tially complete; \z, \Z, and $ match at the end of the subject, as nor-
       mal,  and  for  \b  and  \B the end of the subject is treated as a non-
       alphanumeric.

       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  PCRE2_PARTIAL_SOFT,  it yields a complete match for "dog".
       However, if PCRE2_PARTIAL_HARD is set, the result  is  PCRE2_ERROR_PAR-
       TIAL.  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  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.

   Example of partial matching using pcre2test

       The pcre2test data modifiers partial_hard (or ph) and partial_soft  (or
       ps)  set  PCRE2_PARTIAL_HARD and PCRE2_PARTIAL_SOFT, respectively, when
       calling pcre2_match(). Here is a run of pcre2test using a pattern  that
       matches the whole subject in the form of a date:

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

       This  example  gives  the  same  results for both hard and soft partial
       matching options. Here is an example where there is a difference:

           re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
         data> 25jun04\=ps
          0: 25jun04
          1: jun
         data> 25jun04\=ph
         Partial match: 25jun04

       With  PCRE2_PARTIAL_SOFT,  the  subject  is  matched  completely.   For
       PCRE2_PARTIAL_HARD, however, the subject is assumed not to be complete,
       so there is only a partial match.

MULTI-SEGMENT MATCHING WITH pcre2_match()

       PCRE was not originally designed with multi-segment matching  in  mind.
       However,  over  time,  features  (including partial matching) that make
       multi-segment matching possible have been added. A very long string can
       be  searched  segment  by  segment by calling pcre2_match() repeatedly,
       with the aim of achieving the same results that  would  happen  if  the
       entire  string  was available for searching all the time. Normally, the
       strings that are being sought are much  shorter  than  each  individual
       segment,  and are in the middle of very long strings, so the pattern is
       normally not anchored.

       Special logic must be implemented to handle a  matched  substring  that
       spans a segment boundary. PCRE2_PARTIAL_HARD should be used, because it
       returns a partial match at the end of a segment whenever there  is  the
       possibility  of  changing  the  match  by  adding  more characters. The
       PCRE2_NOTBOL option should also be set for all but the first segment.

       When a partial match occurs, the next segment must be added to the cur-
       rent  subject  and  the match re-run, using the startoffset argument of
       pcre2_match() to begin at the point where the  partial  match  started.
       For example:

           re> /\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d/
         data> ...the date is 23ja\=ph
         Partial match: 23ja
         data> ...the date is 23jan19 and on that day...\=offset=15
          0: 23jan19
          1: jan

       Note  the  use  of the offset modifier to start the new match where the
       partial match was found. In this example, the next segment was added to
       the  one  in  which  the  partial  match  was  found.  This is the most
       straightforward approach, typically using a memory buffer that is twice
       the  size of each segment. After a partial match, the first half of the
       buffer is discarded, the second half is moved to the start of the  buf-
       fer,  and  a  new segment is added before repeating the match as in the
       example above. After a no match, the entire buffer can be discarded.

       If there are memory constraints, you may want to discard text that pre-
       cedes  a  partial  match before adding the next segment. Unfortunately,
       this is not at present straightforward. In cases  such  as  the  above,
       where the pattern does not contain any lookbehinds, it is sufficient to
       retain only the partially matched substring. However,  if  the  pattern
       contains  a  lookbehind assertion, characters that precede the start of
       the partial match may have been inspected during the matching  process.
       When  pcre2test displays a partial match, it indicates these characters
       with '<' if the allusedtext modifier is set:

           re> "(?<=123)abc"
         data> xx123ab\=ph,allusedtext
         Partial match: 123ab
                        <<<

       However, the allusedtext modifier is not available  for  JIT  matching,
       because  JIT  matching  does  not  record the first (or last) consulted
       characters.  For this reason, this information is not available via the
       API. It is therefore not possible in general to obtain the exact number
       of characters that must be retained in order to  get  the  right  match
       result.  If  you  cannot  retain the entire segment, you must find some
       heuristic way of choosing.

       If you know the approximate length of the matching substrings, you  can
       use  that to decide how much text to retain. The only lookbehind infor-
       mation that is currently available via the API is  the  length  of  the
       longest  individual lookbehind in a pattern, but this can be misleading
       if  there  are  nested  lookbehinds.  The  value  returned  by  calling
       pcre2_pattern_info()  with  the  PCRE2_INFO_MAXLOOKBEHIND option is the
       maximum number of characters (not code units) that any individual look-
       behind   moves   back   when   it  is  processed.  A  pattern  such  as
       "(?<=(?<!b)a)" has a maximum lookbehind value of one, but inspects  two
       characters before its starting point.

       In  a  non-UTF or a 32-bit case, moving back is just a subtraction, but
       in UTF-8 or UTF-16 you have  to  count  characters  while  moving  back
       through the code units.

PARTIAL MATCHING USING pcre2_dfa_match()

       The DFA function moves 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.

       When PCRE2_PARTIAL_SOFT is set, PCRE2_ERROR_PARTIAL is returned only if
       there  have  been  no complete matches. Otherwise, the complete matches
       are returned.  If PCRE2_PARTIAL_HARD is  set,  a  partial  match  takes
       precedence  over  any  complete matches. The portion of the string that
       was matched when the longest partial match was  found  is  set  as  the
       first matching string.

       Because  the DFA function always searches for all possible matches, and
       there is no difference between greedy and ungreedy repetition, its  be-
       haviour  is different from the pcre2_match(). Consider the string "dog"
       matched against this ungreedy pattern:

         /dog(sbody)??/

       Whereas the standard function stops as soon as it  finds  the  complete
       match  for  "dog",  the  DFA  function also finds the partial match for
       "dogsbody", and so returns that when PCRE2_PARTIAL_HARD is set.

MULTI-SEGMENT MATCHING WITH pcre2_dfa_match()

       When a partial match has been found using the 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 PCRE2_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. You can set the PCRE2_PARTIAL_SOFT or
       PCRE2_PARTIAL_HARD options with PCRE2_DFA_RESTART to  continue  partial
       matching over multiple segments. Here is an example using pcre2test:

           re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
         data> 23ja\=dfa,ps
         Partial match: 23ja
         data> n05\=dfa,dfa_restart
          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; PCRE2 does not  retain  the  previously  partially-
       matched  string. It is up to the calling program to do that if it needs
       to. This 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. 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.  Depending  on  the
       application, this may or may not be what you want.

       If  you  do want to allow for starting again at the next character, one
       way of doing it is to retain some or all of the segment and try  a  new
       complete match, as described for pcre2_match() above. Another possibil-
       ity is to work with two buffers. If a partial match at offset n in  the
       first  buffer  is followed by "no match" when PCRE2_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, England.

REVISION

       Last updated: 04 September 2019
       Copyright (c) 1997-2019 University of Cambridge.

PCRE2 10.34                    04 September 2019               PCRE2PARTIAL(3)
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