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revision 434 by ph10, Mon Aug 31 17:10:26 2009 UTC revision 435 by ph10, Sat Sep 5 10:20:44 2009 UTC
# Line 26  is very long and is not all available at Line 26  is very long and is not all available at
26  .P  .P
27  PCRE supports partial matching by means of the PCRE_PARTIAL_SOFT and  PCRE supports partial matching by means of the PCRE_PARTIAL_SOFT and
28  PCRE_PARTIAL_HARD options, which can be set when calling \fBpcre_exec()\fP or  PCRE_PARTIAL_HARD options, which can be set when calling \fBpcre_exec()\fP or
29  \fBpcre_dfa_exec()\fP. For backwards compatibility, PCRE_PARTIAL is a synonym  \fBpcre_dfa_exec()\fP. For backwards compatibility, PCRE_PARTIAL is a synonym
30  for PCRE_PARTIAL_SOFT. The essential difference between the two options is  for PCRE_PARTIAL_SOFT. The essential difference between the two options is
31  whether or not a partial match is preferred to an alternative complete match,  whether or not a partial match is preferred to an alternative complete match,
32  though the details differ between the two matching functions. If both options  though the details differ between the two matching functions. If both options
33  are set, PCRE_PARTIAL_HARD takes precedence.  are set, PCRE_PARTIAL_HARD takes precedence.
34  .P  .P
35  Setting a partial matching option disables one of PCRE's optimizations. PCRE  Setting a partial matching option disables one of PCRE's optimizations. PCRE
# Line 49  been matched. (In other words, a partial Line 49  been matched. (In other words, a partial
49  If PCRE_PARTIAL_SOFT is set, the partial match is remembered, but matching  If PCRE_PARTIAL_SOFT is set, the partial match is remembered, but matching
50  continues as normal, and other alternatives in the pattern are tried. If no  continues as normal, and other alternatives in the pattern are tried. If no
51  complete match can be found, \fBpcre_exec()\fP returns PCRE_ERROR_PARTIAL  complete match can be found, \fBpcre_exec()\fP returns PCRE_ERROR_PARTIAL
52  instead of PCRE_ERROR_NOMATCH, and if there are at least two slots in the  instead of PCRE_ERROR_NOMATCH. If there are at least two slots in the offsets
53  offsets vector, they are filled in with the offsets of the longest string that  vector, the first of them is set to the offset of the earliest character that
54  partially matched. Consider this pattern:  was inspected when the partial match was found. For convenience, the second
55    offset points to the end of the string so that a substring can easily be
56    extracted.
57    .P
58    For the majority of patterns, the first offset identifies the start of the
59    partially matched string. However, for patterns that contain lookbehind
60    assertions, or \eK, or begin with \eb or \eB, earlier characters have been
61    inspected while carrying out the match. For example:
62    .sp
63      /(?<=abc)123/
64    .sp
65    This pattern matches "123", but only if it is preceded by "abc". If the subject
66    string is "xyzabc12", the offsets after a partial match are for the substring
67    "abc12", because all these characters are needed if another match is tried
68    with extra characters added.
69    .P
70    If there is more than one partial match, the first one that was found provides
71    the data that is returned. Consider this pattern:
72  .sp  .sp
73    /123\ew+X|dogY/    /123\ew+X|dogY/
74  .sp  .sp
75  If this is matched against the subject string "abc123dog", both  If this is matched against the subject string "abc123dog", both
76  alternatives fail to match, but the end of the subject is reached during  alternatives fail to match, but the end of the subject is reached during
77  matching, so PCRE_ERROR_PARTIAL is returned instead of PCRE_ERROR_NOMATCH. The  matching, so PCRE_ERROR_PARTIAL is returned instead of PCRE_ERROR_NOMATCH. The
78  offsets are set to 3 and 9, identifying "123dog" as the longest partial match  offsets are set to 3 and 9, identifying "123dog" as the first partial match
79  that was found. (In this example, there are two partial matches, because "dog"  that was found. (In this example, there are two partial matches, because "dog"
80  on its own partially matches the second alternative.)  on its own partially matches the second alternative.)
81  .P  .P
82  If PCRE_PARTIAL_HARD is set for \fBpcre_exec()\fP, it returns  If PCRE_PARTIAL_HARD is set for \fBpcre_exec()\fP, it returns
83  PCRE_ERROR_PARTIAL as soon as a partial match is found, without continuing to  PCRE_ERROR_PARTIAL as soon as a partial match is found, without continuing to
84  search for possible complete matches. The difference between the two options  search for possible complete matches. The difference between the two options
85  can be illustrated by a pattern such as:  can be illustrated by a pattern such as:
86  .sp  .sp
87    /dog(sbody)?/    /dog(sbody)?/
88  .sp  .sp
89  This matches either "dog" or "dogsbody", greedily (that is, it prefers the  This matches either "dog" or "dogsbody", greedily (that is, it prefers the
90  longer string if possible). If it is matched against the string "dog" with  longer string if possible). If it is matched against the string "dog" with
91  PCRE_PARTIAL_SOFT, it yields a complete match for "dog". However, if  PCRE_PARTIAL_SOFT, it yields a complete match for "dog". However, if
92  PCRE_PARTIAL_HARD is set, the result is PCRE_ERROR_PARTIAL. On the other hand,  PCRE_PARTIAL_HARD is set, the result is PCRE_ERROR_PARTIAL. On the other hand,
93  if the pattern is made ungreedy the result is different:  if the pattern is made ungreedy the result is different:
94  .sp  .sp
95    /dog(sbody)??/    /dog(sbody)??/
96  .sp  .sp
97  In this case the result is always a complete match because \fBpcre_exec()\fP  In this case the result is always a complete match because \fBpcre_exec()\fP
98  finds that first, and it never continues after finding a match. It might be  finds that first, and it never continues after finding a match. It might be
99  easier to follow this explanation by thinking of the two patterns like this:  easier to follow this explanation by thinking of the two patterns like this:
100  .sp  .sp
101    /dog(sbody)?/    is the same as  /dogsbody|dog/    /dog(sbody)?/    is the same as  /dogsbody|dog/
102    /dog(sbody)??/   is the same as  /dog|dogsbody/    /dog(sbody)??/   is the same as  /dog|dogsbody/
103  .sp  .sp
104  The second pattern will never match "dogsbody" when \fBpcre_exec()\fP is  The second pattern will never match "dogsbody" when \fBpcre_exec()\fP is
105  used, because it will always find the shorter match first.  used, because it will always find the shorter match first.
106  .  .
107  .  .
108  .SH "PARTIAL MATCHING USING pcre_dfa_exec()"  .SH "PARTIAL MATCHING USING pcre_dfa_exec()"
109  .rs  .rs
110  .sp  .sp
111  The \fBpcre_dfa_exec()\fP function moves along the subject string character by  The \fBpcre_dfa_exec()\fP function moves along the subject string character by
112  character, without backtracking, searching for all possible matches  character, without backtracking, searching for all possible matches
113  simultaneously. If the end of the subject is reached before the end of the  simultaneously. If the end of the subject is reached before the end of the
114  pattern, there is the possibility of a partial match, again provided that at  pattern, there is the possibility of a partial match, again provided that at
115  least one character has matched.  least one character has matched.
116  .P  .P
117  When PCRE_PARTIAL_SOFT is set, PCRE_ERROR_PARTIAL is returned only if there  When PCRE_PARTIAL_SOFT is set, PCRE_ERROR_PARTIAL is returned only if there
118  have been no complete matches. Otherwise, the complete matches are returned.  have been no complete matches. Otherwise, the complete matches are returned.
119  However, if PCRE_PARTIAL_HARD is set, a partial match takes precedence over any  However, if PCRE_PARTIAL_HARD is set, a partial match takes precedence over any
120  complete matches. The portion of the string that provided the longest partial  complete matches. The portion of the string that was inspected when the longest
121  match is set as the first matching string, provided there are at least two  partial match was found is set as the first matching string, provided there are
122  slots in the offsets vector.  at least two slots in the offsets vector.
123  .P  .P
124  Because \fBpcre_dfa_exec()\fP always searches for all possible matches, and  Because \fBpcre_dfa_exec()\fP always searches for all possible matches, and
125  there is no difference between greedy and ungreedy repetition, its behaviour is  there is no difference between greedy and ungreedy repetition, its behaviour is
126  different from \fBpcre_exec\fP when PCRE_PARTIAL_HARD is set. Consider the  different from \fBpcre_exec\fP when PCRE_PARTIAL_HARD is set. Consider the
127  string "dog" matched against the ungreedy pattern shown above:  string "dog" matched against the ungreedy pattern shown above:
128  .sp  .sp
129    /dog(sbody)??/    /dog(sbody)??/
130  .sp  .sp
131  Whereas \fBpcre_exec()\fP stops as soon as it finds the complete match for  Whereas \fBpcre_exec()\fP stops as soon as it finds the complete match for
132  "dog", \fBpcre_dfa_exec()\fP also finds the partial match for "dogsbody", and  "dog", \fBpcre_dfa_exec()\fP also finds the partial match for "dogsbody", and
133  so returns that when PCRE_PARTIAL_HARD is set.  so returns that when PCRE_PARTIAL_HARD is set.
134  .  .
# Line 119  so returns that when PCRE_PARTIAL_HARD i Line 136  so returns that when PCRE_PARTIAL_HARD i
136  .SH "PARTIAL MATCHING AND WORD BOUNDARIES"  .SH "PARTIAL MATCHING AND WORD BOUNDARIES"
137  .rs  .rs
138  .sp  .sp
139  If a pattern ends with one of sequences \ew or \eW, which test for word  If a pattern ends with one of sequences \ew or \eW, which test for word
140  boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter-intuitive  boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter-intuitive
141  results. Consider this pattern:  results. Consider this pattern:
142  .sp  .sp
143    /\ebcat\eb/    /\ebcat\eb/
144  .sp  .sp
145  This matches "cat", provided there is a word boundary at either end. If the  This matches "cat", provided there is a word boundary at either end. If the
146  subject string is "the cat", the comparison of the final "t" with a following  subject string is "the cat", the comparison of the final "t" with a following
147  character cannot take place, so a partial match is found. However,  character cannot take place, so a partial match is found. However,
148  \fBpcre_exec()\fP carries on with normal matching, which matches \eb at the end  \fBpcre_exec()\fP carries on with normal matching, which matches \eb at the end
149  of the subject when the last character is a letter, thus finding a complete  of the subject when the last character is a letter, thus finding a complete
150  match. The result, therefore, is \fInot\fP PCRE_ERROR_PARTIAL. The same thing  match. The result, therefore, is \fInot\fP PCRE_ERROR_PARTIAL. The same thing
151  happens with \fBpcre_dfa_exec()\fP, because it also finds the complete match.  happens with \fBpcre_dfa_exec()\fP, because it also finds the complete match.
152  .P  .P
153  Using PCRE_PARTIAL_HARD in this case does yield PCRE_ERROR_PARTIAL, because  Using PCRE_PARTIAL_HARD in this case does yield PCRE_ERROR_PARTIAL, because
154  then the partial match takes precedence.  then the partial match takes precedence.
155  .  .
156  .  .
# Line 182  when \fBpcre_dfa_exec()\fP is used. Line 199  when \fBpcre_dfa_exec()\fP is used.
199  If the escape sequence \eP is present more than once in a \fBpcretest\fP data  If the escape sequence \eP is present more than once in a \fBpcretest\fP data
200  line, the PCRE_PARTIAL_HARD option is set for the match.  line, the PCRE_PARTIAL_HARD option is set for the match.
201  .  .
202  .  .
203  .SH "MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()"  .SH "MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()"
204  .rs  .rs
205  .sp  .sp
# Line 216  facility can be used to pass very long s Line 233  facility can be used to pass very long s
233  .SH "MULTI-SEGMENT MATCHING WITH pcre_exec()"  .SH "MULTI-SEGMENT MATCHING WITH pcre_exec()"
234  .rs  .rs
235  .sp  .sp
236  From release 8.00, \fBpcre_exec()\fP can also be used to do multi-segment  From release 8.00, \fBpcre_exec()\fP can also be used to do multi-segment
237  matching. Unlike \fBpcre_dfa_exec()\fP, it is not possible to restart the  matching. Unlike \fBpcre_dfa_exec()\fP, it is not possible to restart the
238  previous match with a new segment of data. Instead, new data must be added to  previous match with a new segment of data. Instead, new data must be added to
239  the previous subject string, and the entire match re-run, starting from the  the previous subject string, and the entire match re-run, starting from the
240  point where the partial match occurred. Earlier data can be discarded.  point where the partial match occurred. Earlier data can be discarded.
241  Consider an unanchored pattern that matches dates:  Consider an unanchored pattern that matches dates:
242  .sp  .sp
# Line 227  Consider an unanchored pattern that matc Line 244  Consider an unanchored pattern that matc
244    data> The date is 23ja\eP    data> The date is 23ja\eP
245    Partial match: 23ja    Partial match: 23ja
246  .sp  .sp
247  The this stage, an application could discard the text preceding "23ja", add on  The this stage, an application could discard the text preceding "23ja", add on
248  text from the next segment, and call \fBpcre_exec()\fP again. Unlike  text from the next segment, and call \fBpcre_exec()\fP again. Unlike
249  \fBpcre_dfa_exec()\fP, the entire matching string must always be available, and  \fBpcre_dfa_exec()\fP, the entire matching string must always be available, and
250  the complete matching process occurs for each call, so more memory and more  the complete matching process occurs for each call, so more memory and more
251  processing time is needed.  processing time is needed.
252    .P
253    \fBNote:\fP If the pattern contains lookbehind assertions, or \eK, or starts
254    with \eb or \eB, the string that is returned for a partial match will include
255    characters that precede the partially matched string itself, because these must
256    be retained when adding on more characters for a subsequent matching attempt.
257    .
258  .  .
 .  
259  .SH "ISSUES WITH MULTI-SEGMENT MATCHING"  .SH "ISSUES WITH MULTI-SEGMENT MATCHING"
260  .rs  .rs
261  .sp  .sp
262  Certain types of pattern may give problems with multi-segment matching,  Certain types of pattern may give problems with multi-segment matching,
263  whichever matching function is used.  whichever matching function is used.
264  .P  .P
265  1. If the pattern contains tests for the beginning or end of a line, you need  1. If the pattern contains tests for the beginning or end of a line, you need
266  to pass the PCRE_NOTBOL or PCRE_NOTEOL options, as appropriate, when the  to pass the PCRE_NOTBOL or PCRE_NOTEOL options, as appropriate, when the
267  subject string for any call does not contain the beginning or end of a line.  subject string for any call does not contain the beginning or end of a line.
268  .P  .P
269  2. If the pattern contains backward assertions (including \eb or \eB), you need  2. Lookbehind assertions at the start of a pattern are catered for in the
270  to arrange for some overlap in the subject strings to allow for them to be  offsets that are returned for a partial match. However, in theory, a lookbehind
271  correctly tested at the start of each substring. For example, using  assertion later in the pattern could require even earlier characters to be
272  \fBpcre_dfa_exec()\fP, you could pass the subject in chunks that are 500 bytes  inspected, and it might not have been reached when a partial match occurs. This
273  long, but in a buffer of 700 bytes, with the starting offset set to 200 and the  is probably an extremely unlikely case; you could guard against it to a certain
274  previous 200 bytes at the start of the buffer.  extent by always including extra characters at the start.
275  .P  .P
276  3. Matching a subject string that is split into multiple segments may not  3. Matching a subject string that is split into multiple segments may not
277  always produce exactly the same result as matching over one single long string,  always produce exactly the same result as matching over one single long string,
278  especially when PCRE_PARTIAL_SOFT is used. The section "Partial Matching and  especially when PCRE_PARTIAL_SOFT is used. The section "Partial Matching and
279  Word Boundaries" above describes an issue that arises if the pattern ends with  Word Boundaries" above describes an issue that arises if the pattern ends with
280  \eb or \eB. Another kind of difference may occur when there are multiple  \eb or \eB. Another kind of difference may occur when there are multiple
281  matching possibilities, because a partial match result is given only when there  matching possibilities, because a partial match result is given only when there
282  are no completed matches. This means that as soon as the shortest match has  are no completed matches. This means that as soon as the shortest match has
# Line 263  Consider again this \fBpcretest\fP examp Line 285  Consider again this \fBpcretest\fP examp
285  .sp  .sp
286      re> /dog(sbody)?/      re> /dog(sbody)?/
287    data> dogsb\eP    data> dogsb\eP
288     0: dog     0: dog
289    data> do\eP\eD    data> do\eP\eD
290    Partial match: do    Partial match: do
291    data> gsb\eR\eP\eD    data> gsb\eR\eP\eD
# Line 286  matching multi-segment data. The example Line 308  matching multi-segment data. The example
308  .sp  .sp
309      re> /dog(sbody)?/      re> /dog(sbody)?/
310    data> dogsb\eP\eP    data> dogsb\eP\eP
311    Partial match: dogsb    Partial match: dogsb
312    data> do\eP\eD    data> do\eP\eD
313    Partial match: do    Partial match: do
314    data> gsb\eR\eP\eP\eD    data> gsb\eR\eP\eP\eD
315    Partial match: gsb    Partial match: gsb
316  .sp  .sp
317  .P  .P
318  4. Patterns that contain alternatives at the top level which do not all  4. Patterns that contain alternatives at the top level which do not all
319  start with the same pattern item may not work as expected when  start with the same pattern item may not work as expected when
320  \fBpcre_dfa_exec()\fP is used. For example, consider this pattern:  \fBpcre_dfa_exec()\fP is used. For example, consider this pattern:
321  .sp  .sp
322    1234|3789    1234|3789
# Line 311  patterns or patterns such as: Line 333  patterns or patterns such as:
333    1234|ABCD    1234|ABCD
334  .sp  .sp
335  where no string can be a partial match for both alternatives. This is not a  where no string can be a partial match for both alternatives. This is not a
336  problem if \fPpcre_exec()\fP is used, because the entire match has to be rerun  problem if \fPpcre_exec()\fP is used, because the entire match has to be rerun
337  each time:  each time:
338  .sp  .sp
339      re> /1234|3789/      re> /1234|3789/
# Line 319  each time: Line 341  each time:
341    Partial match: 123    Partial match: 123
342    data> 1237890    data> 1237890
343     0: 3789     0: 3789
344  .sp  .sp
345  .  .
346  .  .
347  .SH AUTHOR  .SH AUTHOR
# Line 336  Cambridge CB2 3QH, England. Line 358  Cambridge CB2 3QH, England.
358  .rs  .rs
359  .sp  .sp
360  .nf  .nf
361  Last updated: 31 August 2009  Last updated: 05 September 2009
362  Copyright (c) 1997-2009 University of Cambridge.  Copyright (c) 1997-2009 University of Cambridge.
363  .fi  .fi

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