PCREPARTIAL(3)            OpenBSD Programmer's Manual           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
       matching 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
       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 any of the
       matching functions. 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 preferred to an
       alternative complete match, though the details differ between the two
       types of matching function. If both options are set, PCRE_PARTIAL_HARD
       takes precedence.

       Setting a partial matching option disables the use of any just-in-time
       code that was set up by studying the compiled pattern with the
       PCRE_STUDY_JIT_COMPILE option. It also disables two of PCRE's standard
       optimizations. 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
       disabled for partial matching.

PARTIAL MATCHING USING pcre_exec() OR pcre16_exec()

       A partial match occurs during a call to pcre_exec() or pcre16_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 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.

       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 offsets 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() OR pcre16_exec()

       If PCRE_PARTIAL_SOFT is set when pcre_exec() or pcre16_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_ERROR_PARTIAL is
       returned 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. Consider 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, 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 pcre16_exec()

       If PCRE_PARTIAL_HARD is set for pcre_exec() or pcre16_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
       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, 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
       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 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 pcre16_dfa_exec()

       The DFA functions move 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. 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_PARTIAL_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
       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 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 pcre16_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
       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 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
       matching; 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.

       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 pcre16_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.
       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 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 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 partial match. However, in theory,
       a lookbehind assertion later in the pattern could require even earlier
       characters 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 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
       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. 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
       functions. 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: 21 January 2012
       Copyright (c) 1997-2012 University of Cambridge.



                                                                PCREPARTIAL(3)
