pcrepartial(3) — Linux manual page


PCREPARTIAL(3)            Library Functions Manual            PCREPARTIAL(3)

NAME         top

       PCRE - Perl-compatible regular expressions


       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:


       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.

       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_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

       Setting a partial matching option 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 pcre[16|32]_exec()         top

       A partial match occurs during a call to pcre_exec() or
       pcre[16|32]_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. 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
       assertions, or begin with \b or \B, characters before the one where
       matching started may have been inspected while carrying out the
       match. For example, consider this pattern:


       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
       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:


       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 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 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:


       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:


       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()         top

       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:


       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.


       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:


       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.


       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.


       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:

         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 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
       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):

         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.

       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 starting 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()         top

       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 start of what would be returned
       for a complete match, because it contains all the characters that
       were inspected during the partial match.


       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

       2. Lookbehind assertions that have already been obeyed are catered
       for in the offsets that are returned for a partial match. However a
       lookbehind assertion later in the pattern could require even earlier
       characters 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
       characters 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

       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 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

           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:


       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:


       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         top

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

REVISION         top

       Last updated: 02 July 2013
       Copyright (c) 1997-2013 University of Cambridge.

COLOPHON         top

       This page is part of the PCRE (Perl Compatible Regular Expressions)
       project.  Information about the project can be found at 
       ⟨http://www.pcre.org/⟩.  If you have a bug report for this manual
       page, see ⟨http://bugs.exim.org/enter_bug.cgi?product=PCRE⟩.  This
       page was obtained from the tarball pcre-8.44.tar.gz fetched from
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       2020-09-18.  If you discover any rendering problems in this HTML ver‐
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PCRE 8.34                       02 July 2013                  PCREPARTIAL(3)

Pages that refer to this page: pcreapi(3)