pcrepartial(3) — Linux manual page

NAME | PARTIAL MATCHING IN PCRE | PARTIAL MATCHING USING pcre_exec() OR pcre[16|32]_exec() | | PARTIAL MATCHING AND WORD BOUNDARIES | FORMERLY RESTRICTED PATTERNS | EXAMPLE OF PARTIAL MATCHING USING PCRETEST | | MULTI-SEGMENT MATCHING WITH pcre_exec() OR pcre[16|32]_exec() | ISSUES WITH MULTI-SEGMENT MATCHING | AUTHOR | REVISION | COLOPHON

PCREPARTIAL(3)          Library Functions Manual          PCREPARTIAL(3)

NAME         top

       PCRE - Perl-compatible regular expressions

PARTIAL MATCHING IN PCRE         top


       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.

       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
       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:

         /(?<=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 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 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:

         /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 pcre[16|32]_dfa_exec()

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       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         top


       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         top


       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         top


       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 pcre[16|32]_dfa_exec()

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       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.

       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.

ISSUES WITH MULTI-SEGMENT MATCHING         top


       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 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 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 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

       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 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-8.45.tar.gz fetched from
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       to-date source for the page, or you have corrections or
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       man-pages@man7.org

PCRE 8.34                     02 July 2013                PCREPARTIAL(3)

Pages that refer to this page: pcreapi(3)