2014-07-04 22:28:24 +00:00
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<title>pcrepartial specification</title>
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<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
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<h1>pcrepartial man page</h1>
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<p>
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Return to the <a href="index.html">PCRE index page</a>.
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</p>
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<p>
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This page is part of the PCRE HTML documentation. It was generated automatically
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from the original man page. If there is any nonsense in it, please consult the
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man page, in case the conversion went wrong.
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<br>
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<ul>
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<li><a name="TOC1" href="#SEC1">PARTIAL MATCHING IN PCRE</a>
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<li><a name="TOC2" href="#SEC2">PARTIAL MATCHING USING pcre_exec() OR pcre[16|32]_exec()</a>
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<li><a name="TOC3" href="#SEC3">PARTIAL MATCHING USING pcre_dfa_exec() OR pcre[16|32]_dfa_exec()</a>
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<li><a name="TOC4" href="#SEC4">PARTIAL MATCHING AND WORD BOUNDARIES</a>
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<li><a name="TOC5" href="#SEC5">FORMERLY RESTRICTED PATTERNS</a>
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<li><a name="TOC6" href="#SEC6">EXAMPLE OF PARTIAL MATCHING USING PCRETEST</a>
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<li><a name="TOC7" href="#SEC7">MULTI-SEGMENT MATCHING WITH pcre_dfa_exec() OR pcre[16|32]_dfa_exec()</a>
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<li><a name="TOC8" href="#SEC8">MULTI-SEGMENT MATCHING WITH pcre_exec() OR pcre[16|32]_exec()</a>
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<li><a name="TOC9" href="#SEC9">ISSUES WITH MULTI-SEGMENT MATCHING</a>
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<li><a name="TOC10" href="#SEC10">AUTHOR</a>
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<li><a name="TOC11" href="#SEC11">REVISION</a>
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</ul>
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<br><a name="SEC1" href="#TOC1">PARTIAL MATCHING IN PCRE</a><br>
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<P>
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In normal use of PCRE, if the subject string that is passed to a matching
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function matches as far as it goes, but is too short to match the entire
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pattern, PCRE_ERROR_NOMATCH is returned. There are circumstances where it might
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be helpful to distinguish this case from other cases in which there is no
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match.
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</P>
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<P>
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Consider, for example, an application where a human is required to type in data
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for a field with specific formatting requirements. An example might be a date
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in the form <i>ddmmmyy</i>, defined by this pattern:
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<pre>
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^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$
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</pre>
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If the application sees the user's keystrokes one by one, and can check that
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what has been typed so far is potentially valid, it is able to raise an error
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as soon as a mistake is made, by beeping and not reflecting the character that
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has been typed, for example. This immediate feedback is likely to be a better
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user interface than a check that is delayed until the entire string has been
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entered. Partial matching can also be useful when the subject string is very
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long and is not all available at once.
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</P>
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<P>
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PCRE supports partial matching by means of the PCRE_PARTIAL_SOFT and
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PCRE_PARTIAL_HARD options, which can be set when calling any of the matching
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functions. For backwards compatibility, PCRE_PARTIAL is a synonym for
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PCRE_PARTIAL_SOFT. The essential difference between the two options is whether
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or not a partial match is preferred to an alternative complete match, though
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the details differ between the two types of matching function. If both options
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are set, PCRE_PARTIAL_HARD takes precedence.
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</P>
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<P>
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If you want to use partial matching with just-in-time optimized code, you must
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call <b>pcre_study()</b>, <b>pcre16_study()</b> or <b>pcre32_study()</b> with one
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or both of these options:
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<pre>
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PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE
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PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE
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</pre>
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PCRE_STUDY_JIT_COMPILE should also be set if you are going to run non-partial
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matches on the same pattern. If the appropriate JIT study mode has not been set
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for a match, the interpretive matching code is used.
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</P>
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<P>
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Setting a partial matching option disables two of PCRE's standard
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optimizations. PCRE remembers the last literal data unit in a pattern, and
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abandons matching immediately if it is not present in the subject string. This
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optimization cannot be used for a subject string that might match only
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partially. If the pattern was studied, PCRE knows the minimum length of a
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matching string, and does not bother to run the matching function on shorter
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strings. This optimization is also disabled for partial matching.
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</P>
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<br><a name="SEC2" href="#TOC1">PARTIAL MATCHING USING pcre_exec() OR pcre[16|32]_exec()</a><br>
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<P>
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A partial match occurs during a call to <b>pcre_exec()</b> or
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<b>pcre[16|32]_exec()</b> when the end of the subject string is reached
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successfully, but matching cannot continue because more characters are needed.
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However, at least one character in the subject must have been inspected. This
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character need not form part of the final matched string; lookbehind assertions
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and the \K escape sequence provide ways of inspecting characters before the
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start of a matched substring. The requirement for inspecting at least one
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character exists because an empty string can always be matched; without such a
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restriction there would always be a partial match of an empty string at the end
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of the subject.
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</P>
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<P>
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If there are at least two slots in the offsets vector when a partial match is
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returned, the first slot is set to the offset of the earliest character that
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was inspected. For convenience, the second offset points to the end of the
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subject so that a substring can easily be identified. If there are at least
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three slots in the offsets vector, the third slot is set to the offset of the
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character where matching started.
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</P>
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<P>
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For the majority of patterns, the contents of the first and third slots will be
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the same. However, for patterns that contain lookbehind assertions, or begin
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with \b or \B, characters before the one where matching started may have been
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inspected while carrying out the match. For example, consider this pattern:
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<pre>
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/(?<=abc)123/
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</pre>
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This pattern matches "123", but only if it is preceded by "abc". If the subject
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string is "xyzabc12", the first two offsets after a partial match are for the
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substring "abc12", because all these characters were inspected. However, the
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third offset is set to 6, because that is the offset where matching began.
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</P>
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<P>
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What happens when a partial match is identified depends on which of the two
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partial matching options are set.
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</P>
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<br><b>
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PCRE_PARTIAL_SOFT WITH pcre_exec() OR pcre[16|32]_exec()
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</b><br>
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<P>
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If PCRE_PARTIAL_SOFT is set when <b>pcre_exec()</b> or <b>pcre[16|32]_exec()</b>
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identifies a partial match, the partial match is remembered, but matching
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continues as normal, and other alternatives in the pattern are tried. If no
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complete match can be found, PCRE_ERROR_PARTIAL is returned instead of
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PCRE_ERROR_NOMATCH.
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</P>
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<P>
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This option is "soft" because it prefers a complete match over a partial match.
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All the various matching items in a pattern behave as if the subject string is
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potentially complete. For example, \z, \Z, and $ match at the end of the
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subject, as normal, and for \b and \B the end of the subject is treated as a
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non-alphanumeric.
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</P>
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<P>
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If there is more than one partial match, the first one that was found provides
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the data that is returned. Consider this pattern:
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<pre>
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/123\w+X|dogY/
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</pre>
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If this is matched against the subject string "abc123dog", both
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alternatives fail to match, but the end of the subject is reached during
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matching, so PCRE_ERROR_PARTIAL is returned. The offsets are set to 3 and 9,
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identifying "123dog" as the first partial match that was found. (In this
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example, there are two partial matches, because "dog" on its own partially
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matches the second alternative.)
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</P>
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<br><b>
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PCRE_PARTIAL_HARD WITH pcre_exec() OR pcre[16|32]_exec()
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</b><br>
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<P>
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If PCRE_PARTIAL_HARD is set for <b>pcre_exec()</b> or <b>pcre[16|32]_exec()</b>,
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PCRE_ERROR_PARTIAL is returned as soon as a partial match is found, without
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continuing to search for possible complete matches. This option is "hard"
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because it prefers an earlier partial match over a later complete match. For
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this reason, the assumption is made that the end of the supplied subject string
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may not be the true end of the available data, and so, if \z, \Z, \b, \B,
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or $ are encountered at the end of the subject, the result is
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PCRE_ERROR_PARTIAL, provided that at least one character in the subject has
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been inspected.
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</P>
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<P>
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Setting PCRE_PARTIAL_HARD also affects the way UTF-8 and UTF-16
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subject strings are checked for validity. Normally, an invalid sequence
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causes the error PCRE_ERROR_BADUTF8 or PCRE_ERROR_BADUTF16. However, in the
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special case of a truncated character at the end of the subject,
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PCRE_ERROR_SHORTUTF8 or PCRE_ERROR_SHORTUTF16 is returned when
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PCRE_PARTIAL_HARD is set.
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</P>
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<br><b>
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Comparing hard and soft partial matching
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</b><br>
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<P>
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The difference between the two partial matching options can be illustrated by a
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pattern such as:
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<pre>
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/dog(sbody)?/
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</pre>
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This matches either "dog" or "dogsbody", greedily (that is, it prefers the
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longer string if possible). If it is matched against the string "dog" with
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PCRE_PARTIAL_SOFT, it yields a complete match for "dog". However, if
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PCRE_PARTIAL_HARD is set, the result is PCRE_ERROR_PARTIAL. On the other hand,
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if the pattern is made ungreedy the result is different:
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<pre>
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/dog(sbody)??/
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</pre>
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In this case the result is always a complete match because that is found first,
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and matching never continues after finding a complete match. It might be easier
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to follow this explanation by thinking of the two patterns like this:
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<pre>
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/dog(sbody)?/ is the same as /dogsbody|dog/
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/dog(sbody)??/ is the same as /dog|dogsbody/
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</pre>
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The second pattern will never match "dogsbody", because it will always find the
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shorter match first.
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</P>
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<br><a name="SEC3" href="#TOC1">PARTIAL MATCHING USING pcre_dfa_exec() OR pcre[16|32]_dfa_exec()</a><br>
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<P>
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The DFA functions move along the subject string character by character, without
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backtracking, searching for all possible matches simultaneously. If the end of
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the subject is reached before the end of the pattern, there is the possibility
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of a partial match, again provided that at least one character has been
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inspected.
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</P>
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<P>
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When PCRE_PARTIAL_SOFT is set, PCRE_ERROR_PARTIAL is returned only if there
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have been no complete matches. Otherwise, the complete matches are returned.
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However, if PCRE_PARTIAL_HARD is set, a partial match takes precedence over any
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complete matches. The portion of the string that was inspected when the longest
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partial match was found is set as the first matching string, provided there are
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at least two slots in the offsets vector.
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</P>
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<P>
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Because the DFA functions always search for all possible matches, and there is
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no difference between greedy and ungreedy repetition, their behaviour is
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different from the standard functions when PCRE_PARTIAL_HARD is set. Consider
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the string "dog" matched against the ungreedy pattern shown above:
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<pre>
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/dog(sbody)??/
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</pre>
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Whereas the standard functions stop as soon as they find the complete match for
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"dog", the DFA functions also find the partial match for "dogsbody", and so
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return that when PCRE_PARTIAL_HARD is set.
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</P>
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<br><a name="SEC4" href="#TOC1">PARTIAL MATCHING AND WORD BOUNDARIES</a><br>
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<P>
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If a pattern ends with one of sequences \b or \B, which test for word
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boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter-intuitive
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results. Consider this pattern:
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<pre>
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/\bcat\b/
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</pre>
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This matches "cat", provided there is a word boundary at either end. If the
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subject string is "the cat", the comparison of the final "t" with a following
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character cannot take place, so a partial match is found. However, normal
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matching carries on, and \b matches at the end of the subject when the last
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character is a letter, so a complete match is found. The result, therefore, is
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<i>not</i> PCRE_ERROR_PARTIAL. Using PCRE_PARTIAL_HARD in this case does yield
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PCRE_ERROR_PARTIAL, because then the partial match takes precedence.
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</P>
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<br><a name="SEC5" href="#TOC1">FORMERLY RESTRICTED PATTERNS</a><br>
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<P>
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For releases of PCRE prior to 8.00, because of the way certain internal
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optimizations were implemented in the <b>pcre_exec()</b> function, the
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PCRE_PARTIAL option (predecessor of PCRE_PARTIAL_SOFT) could not be used with
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all patterns. From release 8.00 onwards, the restrictions no longer apply, and
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partial matching with can be requested for any pattern.
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</P>
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<P>
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Items that were formerly restricted were repeated single characters and
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repeated metasequences. If PCRE_PARTIAL was set for a pattern that did not
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conform to the restrictions, <b>pcre_exec()</b> returned the error code
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PCRE_ERROR_BADPARTIAL (-13). This error code is no longer in use. The
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PCRE_INFO_OKPARTIAL call to <b>pcre_fullinfo()</b> to find out if a compiled
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pattern can be used for partial matching now always returns 1.
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</P>
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<br><a name="SEC6" href="#TOC1">EXAMPLE OF PARTIAL MATCHING USING PCRETEST</a><br>
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<P>
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If the escape sequence \P is present in a <b>pcretest</b> data line, the
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PCRE_PARTIAL_SOFT option is used for the match. Here is a run of <b>pcretest</b>
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that uses the date example quoted above:
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<pre>
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re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
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data> 25jun04\P
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0: 25jun04
|
|
|
|
|
1: jun
|
|
|
|
|
data> 25dec3\P
|
|
|
|
|
Partial match: 23dec3
|
|
|
|
|
data> 3ju\P
|
|
|
|
|
Partial match: 3ju
|
|
|
|
|
data> 3juj\P
|
|
|
|
|
No match
|
|
|
|
|
data> j\P
|
|
|
|
|
No match
|
|
|
|
|
</pre>
|
|
|
|
|
The first data string is matched completely, so <b>pcretest</b> shows the
|
|
|
|
|
matched substrings. The remaining four strings do not match the complete
|
|
|
|
|
pattern, but the first two are partial matches. Similar output is obtained
|
|
|
|
|
if DFA matching is used.
|
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|
|
|
</P>
|
|
|
|
|
<P>
|
|
|
|
|
If the escape sequence \P is present more than once in a <b>pcretest</b> data
|
|
|
|
|
line, the PCRE_PARTIAL_HARD option is set for the match.
|
|
|
|
|
</P>
|
|
|
|
|
<br><a name="SEC7" href="#TOC1">MULTI-SEGMENT MATCHING WITH pcre_dfa_exec() OR pcre[16|32]_dfa_exec()</a><br>
|
|
|
|
|
<P>
|
|
|
|
|
When a partial match has been found using a DFA matching function, it is
|
|
|
|
|
possible to continue the match by providing additional subject data and calling
|
|
|
|
|
the function again with the same compiled regular expression, this time setting
|
|
|
|
|
the PCRE_DFA_RESTART option. You must pass the same working space as before,
|
|
|
|
|
because this is where details of the previous partial match are stored. Here is
|
|
|
|
|
an example using <b>pcretest</b>, using the \R escape sequence to set the
|
|
|
|
|
PCRE_DFA_RESTART option (\D specifies the use of the DFA matching function):
|
|
|
|
|
<pre>
|
|
|
|
|
re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
|
|
|
|
|
data> 23ja\P\D
|
|
|
|
|
Partial match: 23ja
|
|
|
|
|
data> n05\R\D
|
|
|
|
|
0: n05
|
|
|
|
|
</pre>
|
|
|
|
|
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.
|
|
|
|
|
</P>
|
|
|
|
|
<P>
|
2014-07-05 11:53:30 +00:00
|
|
|
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.
|
|
|
|
|
</P>
|
|
|
|
|
<P>
|
2014-07-04 22:28:24 +00:00
|
|
|
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.
|
|
|
|
|
</P>
|
|
|
|
|
<br><a name="SEC8" href="#TOC1">MULTI-SEGMENT MATCHING WITH pcre_exec() OR pcre[16|32]_exec()</a><br>
|
|
|
|
|
<P>
|
|
|
|
|
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.
|
|
|
|
|
</P>
|
|
|
|
|
<P>
|
|
|
|
|
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:
|
|
|
|
|
<pre>
|
|
|
|
|
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
|
|
|
|
|
</pre>
|
|
|
|
|
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.
|
|
|
|
|
</P>
|
|
|
|
|
<P>
|
|
|
|
|
<b>Note:</b> If the pattern contains lookbehind assertions, or \K, or starts
|
|
|
|
|
with \b or \B, the string that is returned for a partial match includes
|
2014-07-05 11:53:30 +00:00
|
|
|
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.
|
2014-07-04 22:28:24 +00:00
|
|
|
</P>
|
|
|
|
|
<br><a name="SEC9" href="#TOC1">ISSUES WITH MULTI-SEGMENT MATCHING</a><br>
|
|
|
|
|
<P>
|
|
|
|
|
Certain types of pattern may give problems with multi-segment matching,
|
|
|
|
|
whichever matching function is used.
|
|
|
|
|
</P>
|
|
|
|
|
<P>
|
|
|
|
|
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.
|
|
|
|
|
</P>
|
|
|
|
|
<P>
|
|
|
|
|
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
|
2014-07-05 11:53:30 +00:00
|
|
|
<b>pcre_fullinfo()</b> or <b>pcre[16|32]_fullinfo()</b> 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.)
|
|
|
|
|
</P>
|
|
|
|
|
<P>
|
|
|
|
|
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 (<i>offsets[0]</i>), the match start position
|
|
|
|
|
(<i>offsets[2]</i>) should be used, and the next match attempt started at the
|
|
|
|
|
<i>offsets[2]</i> character by setting the <i>startoffset</i> argument of
|
|
|
|
|
<b>pcre_exec()</b> or <b>pcre_dfa_exec()</b>.
|
|
|
|
|
</P>
|
|
|
|
|
<P>
|
|
|
|
|
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, <b>pcretest</b> shows it explicitly:
|
|
|
|
|
<pre>
|
|
|
|
|
re> "(?<=123)abc"
|
|
|
|
|
data> xx123a\P\P
|
|
|
|
|
Partial match at offset 5: 123a
|
|
|
|
|
</PRE>
|
2014-07-04 22:28:24 +00:00
|
|
|
</P>
|
|
|
|
|
<P>
|
|
|
|
|
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:
|
|
|
|
|
<pre>
|
|
|
|
|
re> /c(?<=abc)x/
|
|
|
|
|
data> ab\P
|
|
|
|
|
No match
|
|
|
|
|
</pre>
|
|
|
|
|
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.
|
|
|
|
|
</P>
|
|
|
|
|
<P>
|
|
|
|
|
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 <b>pcretest</b> example:
|
|
|
|
|
<pre>
|
|
|
|
|
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
|
|
|
|
|
</pre>
|
|
|
|
|
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.
|
|
|
|
|
</P>
|
|
|
|
|
<P>
|
|
|
|
|
Because of these problems, it is best to use PCRE_PARTIAL_HARD when matching
|
|
|
|
|
multi-segment data. The example above then behaves differently:
|
|
|
|
|
<pre>
|
|
|
|
|
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
|
|
|
|
|
</pre>
|
|
|
|
|
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:
|
|
|
|
|
<pre>
|
|
|
|
|
1234|3789
|
|
|
|
|
</pre>
|
|
|
|
|
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:
|
|
|
|
|
<pre>
|
|
|
|
|
1234|ABCD
|
|
|
|
|
</pre>
|
|
|
|
|
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:
|
|
|
|
|
<pre>
|
|
|
|
|
re> /1234|3789/
|
|
|
|
|
data> ABC123\P\P
|
|
|
|
|
Partial match: 123
|
|
|
|
|
data> 1237890
|
|
|
|
|
0: 3789
|
|
|
|
|
</pre>
|
|
|
|
|
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 <i>n</i>
|
|
|
|
|
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 <i>n+1</i> in
|
|
|
|
|
the first buffer.
|
|
|
|
|
</P>
|
|
|
|
|
<br><a name="SEC10" href="#TOC1">AUTHOR</a><br>
|
|
|
|
|
<P>
|
|
|
|
|
Philip Hazel
|
|
|
|
|
<br>
|
|
|
|
|
University Computing Service
|
|
|
|
|
<br>
|
|
|
|
|
Cambridge CB2 3QH, England.
|
|
|
|
|
<br>
|
|
|
|
|
</P>
|
|
|
|
|
<br><a name="SEC11" href="#TOC1">REVISION</a><br>
|
|
|
|
|
<P>
|
2014-07-05 11:53:30 +00:00
|
|
|
Last updated: 02 July 2013
|
2014-07-04 22:28:24 +00:00
|
|
|
<br>
|
2014-07-05 11:53:30 +00:00
|
|
|
Copyright © 1997-2013 University of Cambridge.
|
2014-07-04 22:28:24 +00:00
|
|
|
<br>
|
|
|
|
|
<p>
|
|
|
|
|
Return to the <a href="index.html">PCRE index page</a>.
|
|
|
|
|
</p>
|
