# pcrepartial man page

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- PARTIAL MATCHING IN PCRE
- RESTRICTED PATTERNS FOR PCRE_PARTIAL
- EXAMPLE OF PARTIAL MATCHING USING PCRETEST

PARTIAL MATCHING IN PCRE

In normal use of PCRE, if the subject string that is passed to pcre_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, possibly beeping and not reflecting the character that has been typed. This immediate feedback is likely to be a better user interface than a check that is delayed until the entire string has been entered.

PCRE supports the concept of partial matching by means of the PCRE_PARTIAL option, which can be set when calling pcre_exec(). When this is done, the return code PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if at any time during the matching process the entire subject string matched part of the pattern. No captured data is set when this occurs.

Using PCRE_PARTIAL disables one 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.

RESTRICTED PATTERNS FOR PCRE_PARTIAL

Because of the way certain internal optimizations are implemented in PCRE, the PCRE_PARTIAL option cannot be used with all patterns. Repeated single characters such as

      a{2,4}

and repeated single metasequences such as

      \d+

are not permitted if the maximum number of occurrences is greater than one. Optional items such as \d? (where the maximum is one) are permitted. Quantifiers with any values are permitted after parentheses, so the invalid examples above can be coded thus:

      (a){2,4}
      (\d)+

These constructions run more slowly, but for the kinds of application that are envisaged for this facility, this is not felt to be a major restriction.

If PCRE_PARTIAL is set for a pattern that does not conform to the restrictions, pcre_exec() returns the error code PCRE_ERROR_BADPARTIAL (-13).

EXAMPLE OF PARTIAL MATCHING USING PCRETEST

If the escape sequence \P is present in a pcretest data line, the PCRE_PARTIAL flag 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
      data> 3ju\P
      Partial match
      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.

Last updated: 08 September 2004
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