table of contents
- NAME
- PARTIAL MATCHING IN PCRE
- PARTIAL MATCHING USING pcre_exec() OR pcre16_exec()
- PARTIAL MATCHING USING pcre_dfa_exec() OR pcre16_dfa_exec()
- PARTIAL MATCHING AND WORD BOUNDARIES
- FORMERLY RESTRICTED PATTERNS
- EXAMPLE OF PARTIAL MATCHING USING PCRETEST
- MULTI-SEGMENT MATCHING WITH pcre_dfa_exec() OR pcre16_dfa_exec()
- MULTI-SEGMENT MATCHING WITH pcre_exec() OR pcre16_exec()
- ISSUES WITH MULTI-SEGMENT MATCHING
- AUTHOR
- REVISION
PCREPARTIAL(3) | Library Functions Manual | PCREPARTIAL(3) |
NAME¶
PCRE - Perl-compatible regular expressionsPARTIAL MATCHING IN PCRE¶
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$
PARTIAL MATCHING USING pcre_exec() OR pcre16_exec()¶
A partial match occurs during a call to pcre_exec() or pcre16_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. For the majority of patterns, the first offset identifies the start of the partially matched string. However, for patterns that contain lookbehind assertions, or \K, or begin with \b or \B, earlier characters have been inspected while carrying out the match. For example:/(?<=abc)123/
PCRE_PARTIAL_SOFT WITH pcre_exec() OR pcre16_exec()¶
If PCRE_PARTIAL_SOFT is set when pcre_exec() or pcre16_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/
PCRE_PARTIAL_HARD WITH pcre_exec() OR pcre16_exec()¶
If PCRE_PARTIAL_HARD is set for pcre_exec() or pcre16_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)?/
/dog(sbody)??/
/dog(sbody)?/ is the same as /dogsbody|dog/
/dog(sbody)??/ is the same as /dog|dogsbody/
PARTIAL MATCHING USING pcre_dfa_exec() OR pcre16_dfa_exec()¶
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)??/
PARTIAL MATCHING AND WORD BOUNDARIES¶
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/
FORMERLY RESTRICTED PATTERNS¶
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¶
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$/
data> 25jun04\P
0: 25jun04
1: jun
data> 25dec3\P
Partial match: 23dec3
data> 3ju\P
Partial match: 3ju
data> 3juj\P
No match
data> j\P
No match
MULTI-SEGMENT MATCHING WITH pcre_dfa_exec() OR pcre16_dfa_exec()¶
When a partial match has been found using a DFA matching function, it is possible to continue the match by providing additional subject data and calling the function again with the same compiled regular expression, this time setting the PCRE_DFA_RESTART option. You must pass the same working space as before, because this is where details of the previous partial match are stored. Here is an example using pcretest, using the \R escape sequence to set the PCRE_DFA_RESTART option (\D specifies the use of the DFA matching function):re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
data> 23ja\P\D
Partial match: 23ja
data> n05\R\D
0: n05
MULTI-SEGMENT MATCHING WITH pcre_exec() OR pcre16_exec()¶
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
ISSUES WITH MULTI-SEGMENT MATCHING¶
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 at the start of a pattern are catered for in the offsets that are returned for a partial match. However, in theory, a lookbehind assertion later in the pattern could require even earlier characters to be inspected, and it might not have been reached when a partial match occurs. This is probably an extremely unlikely case; you could guard against it to a certain extent by always including extra characters at the start. 3. 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
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
1234|3789
1234|ABCD
re> /1234|3789/
data> ABC123\P\P
Partial match: 123
data> 1237890
0: 3789
AUTHOR¶
Philip Hazel University Computing Service Cambridge CB2 3QH, England.
REVISION¶
Last updated: 21 January 2012 Copyright (c) 1997-2012 University of Cambridge.