# pcre2partial

## NAME



           PCRE2 - Perl-compatible regular expressions (revised API)
    


## 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-
           compatible. It is requested by setting one of the
           PCRE2_PARTIAL_HARD or PCRE2_PARTIAL_SOFT options when calling a
           matching function. The 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, 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-
           partial 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
           characters 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 partial 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 characters before the start of a matched
           string.
    
           (2) The pattern contains one or more lookbehind assertions. This
           condition 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 characters are added, we
           do not know if it will be an empty string or something 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
             subject.
    
           **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 pattern 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 subsequent 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
           alternatives 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 example, 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 assumption 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 potentially complete; \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.
    
           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 PCRE2_PARTIAL_SOFT, it yields a complete
           match for "dog". However, if PCRE2_PARTIAL_HARD is set, the result
           is PCRE2_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.
    
       **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
           current 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 buffer, 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
           precedes 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 information 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 lookbehind 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 simultaneously. If the end of the subject is reached
           before the end of the pattern, 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 behaviour 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 expression, this time setting the
           PCRE2_DFA_RESTART option. You must pass the same working space as
           before, because this is where details of the previous 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
           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; 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 possibility 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.
    


## REVISION



    
           Last updated: 27 November 2024
           Copyright (c) 1997-2019 University of Cambridge.
    


* * *