/[pcre]/code/trunk/doc/pcrepartial.3

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-revision 148 by ph10,
Mon Apr 16 13:25:10 2007 UTC
+revision 435 by ph10,
Sat Sep  5 10:20:44 2009 UTC
 Line 18 
 in the form \fIddmmmyy\fP, defined by th
  .sp
  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
+ as soon as a mistake is made, by beeping and not reflecting the character that
- character that has been typed. This immediate feedback is likely to be a better
+ 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.
+ entered. Partial matching can also sometimes be useful when the subject string
+ is very long and is not all available at once.
  .P
- PCRE supports the concept of partial matching by means of the PCRE_PARTIAL
+ PCRE supports partial matching by means of the PCRE_PARTIAL_SOFT and
- option, which can be set when calling \fBpcre_exec()\fP or
+ PCRE_PARTIAL_HARD options, which can be set when calling \fBpcre_exec()\fP or
- \fBpcre_dfa_exec()\fP. When this flag is set for \fBpcre_exec()\fP, the return
+ \fBpcre_dfa_exec()\fP. For backwards compatibility, PCRE_PARTIAL is a synonym
- code PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if at any time
+ for PCRE_PARTIAL_SOFT. The essential difference between the two options is
- during the matching process the last part of the subject string matched part of
+ whether or not a partial match is preferred to an alternative complete match,
- the pattern. Unfortunately, for non-anchored matching, it is not possible to
+ though the details differ between the two matching functions. If both options
- obtain the position of the start of the partial match. No captured data is set
+ are set, PCRE_PARTIAL_HARD takes precedence.
- when PCRE_ERROR_PARTIAL is returned.
  .P
- When PCRE_PARTIAL is set for \fBpcre_dfa_exec()\fP, the return code
+ Setting a partial matching option disables one of PCRE's optimizations. PCRE
- PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if the end of the
+ remembers the last literal byte in a pattern, and abandons matching immediately
- subject is reached, there have been no complete matches, but there is still at
+ if such a byte is not present in the subject string. This optimization cannot
- least one matching possibility. The portion of the string that provided the
+ be used for a subject string that might match only partially.
- partial match is set as the first matching string.
- .P
- 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.
  .
  .
- .SH "RESTRICTED PATTERNS FOR PCRE_PARTIAL"
+ .SH "PARTIAL MATCHING USING pcre_exec()"
  .rs
  .sp
- Because of the way certain internal optimizations are implemented in the
+ A partial match occurs during a call to \fBpcre_exec()\fP whenever the end of
- \fBpcre_exec()\fP function, the PCRE_PARTIAL option cannot be used with all
+ the subject string is reached successfully, but matching cannot continue
- patterns. These restrictions do not apply when \fBpcre_dfa_exec()\fP is used.
+ because more characters are needed. However, at least one character must have
- For \fBpcre_exec()\fP, repeated single characters such as
+ been matched. (In other words, a partial match can never be an empty string.)
+ .P
+ If PCRE_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, \fBpcre_exec()\fP returns PCRE_ERROR_PARTIAL
+ instead of PCRE_ERROR_NOMATCH. If there are at least two slots in the offsets
+ vector, the first of them is set to the offset of the earliest character that
+ was inspected when the partial match was found. For convenience, the second
+ offset points to the end of the string so that a substring can easily be
+ extracted.
+ .P
+ For the majority of patterns, the first offset identifies the start of the
+ partially matched string. However, for patterns that contain lookbehind
+ assertions, or \eK, or begin with \eb or \eB, earlier characters have been
+ inspected while carrying out the match. For example:
+ .sp
+   /(?<=abc)123/
+ .sp
+ This pattern matches "123", but only if it is preceded by "abc". If the subject
+ string is "xyzabc12", the offsets after a partial match are for the substring
+ "abc12", because all these characters are needed if another match is tried
+ with extra characters added.
+ .P
+ If there is more than one partial match, the first one that was found provides
+ the data that is returned. Consider this pattern:
+ .sp
+   /123\ew+X|dogY/
  .sp
-   a{2,4}
+ 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 instead of PCRE_ERROR_NOMATCH. 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.)
+ .P
+ If PCRE_PARTIAL_HARD is set for \fBpcre_exec()\fP, it returns
+ PCRE_ERROR_PARTIAL as soon as a partial match is found, without continuing to
+ search for possible complete matches. The difference between the two options
+ can be illustrated by a pattern such as:
+ .sp
+   /dog(sbody)?/
+ .sp
+ 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:
+ .sp
+   /dog(sbody)??/
+ .sp
+ In this case the result is always a complete match because \fBpcre_exec()\fP
+ finds that first, and it never continues after finding a match. It might be
+ easier to follow this explanation by thinking of the two patterns like this:
  .sp
- and repeated single metasequences such as
+   /dog(sbody)?/    is the same as  /dogsbody|dog/
+   /dog(sbody)??/   is the same as  /dog|dogsbody/
  .sp
-   \ed+
+ The second pattern will never match "dogsbody" when \fBpcre_exec()\fP is
+ used, because it will always find the shorter match first.
+ .
+ .
+ .SH "PARTIAL MATCHING USING pcre_dfa_exec()"
+ .rs
  .sp
- are not permitted if the maximum number of occurrences is greater than one.
+ The \fBpcre_dfa_exec()\fP function moves along the subject string character by
- Optional items such as \ed? (where the maximum is one) are permitted.
+ character, without backtracking, searching for all possible matches
- Quantifiers with any values are permitted after parentheses, so the invalid
+ simultaneously. If the end of the subject is reached before the end of the
- examples above can be coded thus:
+ pattern, there is the possibility of a partial match, again provided that at
+ least one character has matched.
+ .P
+ 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.
+ .P
+ Because \fBpcre_dfa_exec()\fP always searches for all possible matches, and
+ there is no difference between greedy and ungreedy repetition, its behaviour is
+ different from \fBpcre_exec\fP when PCRE_PARTIAL_HARD is set. Consider the
+ string "dog" matched against the ungreedy pattern shown above:
+ .sp
+   /dog(sbody)??/
+ .sp
+ Whereas \fBpcre_exec()\fP stops as soon as it finds the complete match for
+ "dog", \fBpcre_dfa_exec()\fP also finds the partial match for "dogsbody", and
+ so returns that when PCRE_PARTIAL_HARD is set.
+ .
+ .
+ .SH "PARTIAL MATCHING AND WORD BOUNDARIES"
+ .rs
  .sp
-   (a){2,4}
+ If a pattern ends with one of sequences \ew or \eW, which test for word
-   (\ed)+
+ boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter-intuitive
+ results. Consider this pattern:
+ .sp
+   /\ebcat\eb/
+ .sp
+ 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,
+ \fBpcre_exec()\fP carries on with normal matching, which matches \eb at the end
+ of the subject when the last character is a letter, thus finding a complete
+ match. The result, therefore, is \fInot\fP PCRE_ERROR_PARTIAL. The same thing
+ happens with \fBpcre_dfa_exec()\fP, because it also finds the complete match.
+ .P
+ Using PCRE_PARTIAL_HARD in this case does yield PCRE_ERROR_PARTIAL, because
+ then the partial match takes precedence.
+ .
+ .
+ .SH "FORMERLY RESTRICTED PATTERNS"
+ .rs
  .sp
- These constructions run more slowly, but for the kinds of application that are
+ For releases of PCRE prior to 8.00, because of the way certain internal
- envisaged for this facility, this is not felt to be a major restriction.
+ optimizations were implemented in the \fBpcre_exec()\fP 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 \fBpcre_exec()\fP can be requested for any pattern.
  .P
- If PCRE_PARTIAL is set for a pattern that does not conform to the restrictions,
+ Items that were formerly restricted were repeated single characters and
- \fBpcre_exec()\fP returns the error code PCRE_ERROR_BADPARTIAL (-13).
+ repeated metasequences. If PCRE_PARTIAL was set for a pattern that did not
+ conform to the restrictions, \fBpcre_exec()\fP returned the error code
+ PCRE_ERROR_BADPARTIAL (-13). This error code is no longer in use. The
+ PCRE_INFO_OKPARTIAL call to \fBpcre_fullinfo()\fP to find out if a compiled
+ pattern can be used for partial matching now always returns 1.
  .
  .
  .SH "EXAMPLE OF PARTIAL MATCHING USING PCRETEST"
  .rs
  .sp
  If the escape sequence \eP is present in a \fBpcretest\fP data line, the
- PCRE_PARTIAL flag is used for the match. Here is a run of \fBpcretest\fP that
+ PCRE_PARTIAL_SOFT option is used for the match. Here is a run of \fBpcretest\fP
- uses the date example quoted above:
+ that uses the date example quoted above:
  .sp
      re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/
    data> 25jun04\eP
 : 25jun04
 : jun
    data> 25dec3\eP
-   Partial match
+   Partial match: 23dec3
    data> 3ju\eP
-   Partial match
+   Partial match: 3ju
    data> 3juj\eP
    No match
    data> j\eP
-Line 95 
 uses the date example quoted above:
+Line 193 
 uses the date example quoted above:
  .sp
  The first data string is matched completely, so \fBpcretest\fP shows the
  matched substrings. The remaining four strings do not match the complete
- pattern, but the first two are partial matches. The same test, using
+ pattern, but the first two are partial matches. Similar output is obtained
- \fBpcre_dfa_exec()\fP matching (by means of the \eD escape sequence), produces
+ when \fBpcre_dfa_exec()\fP is used.
- the following output:
+ .P
- .sp
+ If the escape sequence \eP is present more than once in a \fBpcretest\fP data
-     re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
+ line, the PCRE_PARTIAL_HARD option is set for the match.
-   data> 25jun04\eP\eD
-: 25jun04
-   data> 23dec3\eP\eD
-   Partial match: 23dec3
-   data> 3ju\eP\eD
-   Partial match: 3ju
-   data> 3juj\eP\eD
-   No match
-   data> j\eP\eD
-   No match
- .sp
- Notice that in this case the portion of the string that was matched is made
- available.
  .
  .
  .SH "MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()"
-Line 121 
 available.
+Line 206 
 available.
  When a partial match has been found using \fBpcre_dfa_exec()\fP, it is possible
  to continue the match by providing additional subject data and calling
  \fBpcre_dfa_exec()\fP again with the same compiled regular expression, this
- time setting the PCRE_DFA_RESTART option. You must also pass the same working
+ 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 \fBpcretest\fP, using the \eR escape
- sequence to set the PCRE_DFA_RESTART option (\eP and \eD are as above):
+ sequence to set the PCRE_DFA_RESTART option (\eD specifies the use of
+ \fBpcre_dfa_exec()\fP):
  .sp
-     re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
+     re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/
    data> 23ja\eP\eD
    Partial match: 23ja
    data> n05\eR\eD
-Line 138 
 Notice that when the match is complete,
+Line 224 
 Notice that when the match is complete,
  not retain the previously partially-matched string. It is up to the calling
  program to do that if it needs to.
  .P
- You can set PCRE_PARTIAL with PCRE_DFA_RESTART to continue partial matching
+ You can set the PCRE_PARTIAL_SOFT or PCRE_PARTIAL_HARD options with
- over multiple segments. This facility can be used to pass very long subject
+ PCRE_DFA_RESTART to continue partial matching over multiple segments. This
- strings to \fBpcre_dfa_exec()\fP. However, some care is needed for certain
+ facility can be used to pass very long subject strings to
- types of pattern.
+ \fBpcre_dfa_exec()\fP.
+ .
+ .
+ .SH "MULTI-SEGMENT MATCHING WITH pcre_exec()"
+ .rs
+ .sp
+ From release 8.00, \fBpcre_exec()\fP can also be used to do multi-segment
+ matching. Unlike \fBpcre_dfa_exec()\fP, 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.
+ Consider an unanchored pattern that matches dates:
+ .sp
+     re> /\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed/
+   data> The date is 23ja\eP
+   Partial match: 23ja
+ .sp
+ The this stage, an application could discard the text preceding "23ja", add on
+ text from the next segment, and call \fBpcre_exec()\fP again. Unlike
+ \fBpcre_dfa_exec()\fP, 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
+ \fBNote:\fP If the pattern contains lookbehind assertions, or \eK, or starts
+ with \eb or \eB, the string that is returned for a partial match will include
+ characters that precede the partially matched string itself, because these must
+ be retained when adding on more characters for a subsequent matching attempt.
+ .
+ .
+ .SH "ISSUES WITH MULTI-SEGMENT MATCHING"
+ .rs
+ .sp
+ Certain types of pattern may give problems with multi-segment matching,
+ whichever matching function is used.
  .P
 . If the pattern contains tests for the beginning or end of a line, you need
  to pass the PCRE_NOTBOL or PCRE_NOTEOL options, as appropriate, when the
  subject string for any call does not contain the beginning or end of a line.
  .P
-. If the pattern contains backward assertions (including \eb or \eB), you need
+. Lookbehind assertions at the start of a pattern are catered for in the
- to arrange for some overlap in the subject strings to allow for this. For
+ offsets that are returned for a partial match. However, in theory, a lookbehind
- example, you could pass the subject in chunks that are 500 bytes long, but in
+ assertion later in the pattern could require even earlier characters to be
- a buffer of 700 bytes, with the starting offset set to 200 and the previous 200
+ inspected, and it might not have been reached when a partial match occurs. This
- bytes at the start of the buffer.
+ is probably an extremely unlikely case; you could guard against it to a certain
- .P
+ extent by always including extra characters at the start.
-. Matching a subject string that is split into multiple segments does not
+ .P
- always produce exactly the same result as matching over one single long string.
+. Matching a subject string that is split into multiple segments may not
- The difference arises when there are multiple matching possibilities, because a
+ always produce exactly the same result as matching over one single long string,
- partial match result is given only when there are no completed matches in a
+ especially when PCRE_PARTIAL_SOFT is used. The section "Partial Matching and
- call to \fBpcre_dfa_exec()\fP. This means that as soon as the shortest match has
+ Word Boundaries" above describes an issue that arises if the pattern ends with
+ \eb or \eB. Another kind of difference may occur when there are multiple
+ matching possibilities, because 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 this \fBpcretest\fP example:
+ Consider again this \fBpcretest\fP example:
  .sp
      re> /dog(sbody)?/
+   data> dogsb\eP
+: dog
    data> do\eP\eD
    Partial match: do
    data> gsb\eR\eP\eD
-Line 170 
 Consider this \fBpcretest\fP example:
+Line 294 
 Consider this \fBpcretest\fP example:
 : dogsbody
 : dog
  .sp
- The pattern matches the words "dog" or "dogsbody". When the subject is
+ The first data line passes the string "dogsb" to \fBpcre_exec()\fP, setting the
- presented in several parts ("do" and "gsb" being the first two) the match stops
+ PCRE_PARTIAL_SOFT option. Although the string is a partial match for
- when "dog" has been found, and it is not possible to continue. On the other
+ "dogsbody", the result is not PCRE_ERROR_PARTIAL, because the shorter string
- hand, if "dogsbody" is presented as a single string, both matches are found.
+ "dog" is a complete match. Similarly, when the subject is presented to
+ \fBpcre_dfa_exec()\fP 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,
+ \fBpcre_dfa_exec()\fP finds both matches.
  .P
- Because of this phenomenon, it does not usually make sense to end a pattern
+ Because of these problems, it is probably best to use PCRE_PARTIAL_HARD when
- that is going to be matched in this way with a variable repeat.
+ matching multi-segment data. The example above then behaves differently:
+ .sp
+     re> /dog(sbody)?/
+   data> dogsb\eP\eP
+   Partial match: dogsb
+   data> do\eP\eD
+   Partial match: do
+   data> gsb\eR\eP\eP\eD
+   Partial match: gsb
+ .sp
  .P
 . Patterns that contain alternatives at the top level which do not all
- start with the same pattern item may not work as expected. For example,
+ start with the same pattern item may not work as expected when
- consider this pattern:
+ \fBpcre_dfa_exec()\fP is used. For example, consider this pattern:
  .sp
 |3789
  .sp
  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 "789" does not yield a
+ 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
-Line 195 
 patterns or patterns such as:
+Line 332 
 patterns or patterns such as:
  .sp
 |ABCD
  .sp
- where no string can be a partial match for both alternatives.
+ where no string can be a partial match for both alternatives. This is not a
+ problem if \fPpcre_exec()\fP is used, because the entire match has to be rerun
+ each time:
+ .sp
+     re> /1234|3789/
+   data> ABC123\eP
+   Partial match: 123
+   data> 1237890
+: 3789
+ .sp
  .
  .
  .SH AUTHOR
-Line 212 
 Cambridge CB2 3QH, England.
+Line 358 
 Cambridge CB2 3QH, England.
  .rs
  .sp
  .nf
- Last updated: 06 March 2007
+ Last updated: 05 September 2009
- Copyright (c) 1997-2007 University of Cambridge.
+ Copyright (c) 1997-2009 University of Cambridge.
  .fi

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