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 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 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 preferred 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 disables two of PCRE's optimizations.
       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 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()

       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. 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()

       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. 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 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 partial 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 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 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 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
       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 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 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 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 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: 07 November 2010
       Copyright (c) 1997-2010 University of Cambridge.



                                                                PCREPARTIAL(3)
