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revision 426 by ph10, Wed Aug 26 15:38:32 2009 UTC revision 435 by ph10, Sat Sep 5 10:20:44 2009 UTC
# Line 18  in the form \fIddmmmyy\fP, defined by th Line 18  in the form \fIddmmmyy\fP, defined by th
18  .sp  .sp
19  If the application sees the user's keystrokes one by one, and can check that  If the application sees the user's keystrokes one by one, and can check that
20  what has been typed so far is potentially valid, it is able to raise an error  what has been typed so far is potentially valid, it is able to raise an error
21  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
22  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
23  user interface than a check that is delayed until the entire string has been  user interface than a check that is delayed until the entire string has been
24  entered.  entered. Partial matching can also sometimes be useful when the subject string
25    is very long and is not all available at once.
26  .P  .P
27  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
28  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
29  \fBpcre_dfa_exec()\fP.  \fBpcre_dfa_exec()\fP. For backwards compatibility, PCRE_PARTIAL is a synonym
30  .P  for PCRE_PARTIAL_SOFT. The essential difference between the two options is
31  When PCRE_PARTIAL is set for \fBpcre_exec()\fP, the return code  whether or not a partial match is preferred to an alternative complete match,
32  PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if at any time during  though the details differ between the two matching functions. If both options
33  the matching process the last part of the subject string matched part of the  are set, PCRE_PARTIAL_HARD takes precedence.
34  pattern. If there are at least two slots in the offsets vector, they are filled  .P
35  in with the offsets of the longest found string that partially matched. No  Setting a partial matching option disables one of PCRE's optimizations. PCRE
36  other captured data is set when PCRE_ERROR_PARTIAL is returned. The second  remembers the last literal byte in a pattern, and abandons matching immediately
37  offset is always that for the end of the subject. Consider this pattern:  if such a byte is not present in the subject string. This optimization cannot
38    be used for a subject string that might match only partially.
39    .
40    .
41    .SH "PARTIAL MATCHING USING pcre_exec()"
42    .rs
43    .sp
44    A partial match occurs during a call to \fBpcre_exec()\fP whenever the end of
45    the subject string is reached successfully, but matching cannot continue
46    because more characters are needed. However, at least one character must have
47    been matched. (In other words, a partial match can never be an empty string.)
48    .P
49    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
51    complete match can be found, \fBpcre_exec()\fP returns PCRE_ERROR_PARTIAL
52    instead of PCRE_ERROR_NOMATCH. If there are at least two slots in the offsets
53    vector, the first of them is set to the offset of the earliest character that
54    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, so  alternatives fail to match, but the end of the subject is reached during
77  PCRE_ERROR_PARTIAL is returned instead of PCRE_ERROR_NOMATCH if the  matching, so PCRE_ERROR_PARTIAL is returned instead of PCRE_ERROR_NOMATCH. The
78  PCRE_PARTIAL option is set. The offsets are set to 3 and 9, identifying  offsets are set to 3 and 9, identifying "123dog" as the first partial match
79  "123dog" as the longest partial match that was found. (In this example, there  that was found. (In this example, there are two partial matches, because "dog"
80  are two partial matches, because "dog" on its own partially matches the second  on its own partially matches the second alternative.)
81  alternative.)  .P
82  .P  If PCRE_PARTIAL_HARD is set for \fBpcre_exec()\fP, it returns
83  When PCRE_PARTIAL is set for \fBpcre_dfa_exec()\fP, the return code  PCRE_ERROR_PARTIAL as soon as a partial match is found, without continuing to
84  PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if the end of the  search for possible complete matches. The difference between the two options
85  subject is reached, there have been no complete matches, but there is still at  can be illustrated by a pattern such as:
86  least one matching possibility. The portion of the string that provided the  .sp
87  longest partial match is set as the first matching string, provided there are    /dog(sbody)?/
88    .sp
89    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
91    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,
93    if the pattern is made ungreedy the result is different:
94    .sp
95      /dog(sbody)??/
96    .sp
97    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
99    easier to follow this explanation by thinking of the two patterns like this:
100    .sp
101      /dog(sbody)?/    is the same as  /dogsbody|dog/
102      /dog(sbody)??/   is the same as  /dog|dogsbody/
103    .sp
104    The second pattern will never match "dogsbody" when \fBpcre_exec()\fP is
105    used, because it will always find the shorter match first.
106    .
107    .
108    .SH "PARTIAL MATCHING USING pcre_dfa_exec()"
109    .rs
110    .sp
111    The \fBpcre_dfa_exec()\fP function moves along the subject string character by
112    character, without backtracking, searching for all possible matches
113    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
115    least one character has matched.
116    .P
117    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.
119    However, if PCRE_PARTIAL_HARD is set, a partial match takes precedence over any
120    complete matches. The portion of the string that was inspected when the longest
121    partial match was found is set as the first matching string, provided there are
122  at least two slots in the offsets vector.  at least two slots in the offsets vector.
123  .P  .P
124  Using PCRE_PARTIAL disables one of PCRE's optimizations. PCRE remembers the  Because \fBpcre_dfa_exec()\fP always searches for all possible matches, and
125  last literal byte in a pattern, and abandons matching immediately if such a  there is no difference between greedy and ungreedy repetition, its behaviour is
126  byte is not present in the subject string. This optimization cannot be used  different from \fBpcre_exec\fP when PCRE_PARTIAL_HARD is set. Consider the
127  for a subject string that might match only partially.  string "dog" matched against the ungreedy pattern shown above:
128    .sp
129      /dog(sbody)??/
130    .sp
131    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
133    so returns that when PCRE_PARTIAL_HARD is set.
134  .  .
135  .  .
136  .SH "FORMERLY RESTRICTED PATTERNS FOR PCRE_PARTIAL"  .SH "PARTIAL MATCHING AND WORD BOUNDARIES"
137    .rs
138    .sp
139    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
141    results. Consider this pattern:
142    .sp
143      /\ebcat\eb/
144    .sp
145    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
147    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
149    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
151    happens with \fBpcre_dfa_exec()\fP, because it also finds the complete match.
152    .P
153    Using PCRE_PARTIAL_HARD in this case does yield PCRE_ERROR_PARTIAL, because
154    then the partial match takes precedence.
155    .
156    .
157    .SH "FORMERLY RESTRICTED PATTERNS"
158  .rs  .rs
159  .sp  .sp
160  For releases of PCRE prior to 8.00, because of the way certain internal  For releases of PCRE prior to 8.00, because of the way certain internal
161  optimizations were implemented in the \fBpcre_exec()\fP function, the  optimizations were implemented in the \fBpcre_exec()\fP function, the
162  PCRE_PARTIAL option could not be used with all patterns. From release 8.00  PCRE_PARTIAL option (predecessor of PCRE_PARTIAL_SOFT) could not be used with
163  onwards, the restrictions no longer apply, and partial matching can be  all patterns. From release 8.00 onwards, the restrictions no longer apply, and
164  requested for any pattern.  partial matching with \fBpcre_exec()\fP can be requested for any pattern.
165  .P  .P
166  Items that were formerly restricted were repeated single characters and  Items that were formerly restricted were repeated single characters and
167  repeated metasequences. If PCRE_PARTIAL was set for a pattern that did not  repeated metasequences. If PCRE_PARTIAL was set for a pattern that did not
# Line 79  pattern can be used for partial matching Line 175  pattern can be used for partial matching
175  .rs  .rs
176  .sp  .sp
177  If the escape sequence \eP is present in a \fBpcretest\fP data line, the  If the escape sequence \eP is present in a \fBpcretest\fP data line, the
178  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
179  uses the date example quoted above:  that uses the date example quoted above:
180  .sp  .sp
181      re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/      re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/
182    data> 25jun04\eP    data> 25jun04\eP
# Line 99  The first data string is matched complet Line 195  The first data string is matched complet
195  matched substrings. The remaining four strings do not match the complete  matched substrings. The remaining four strings do not match the complete
196  pattern, but the first two are partial matches. Similar output is obtained  pattern, but the first two are partial matches. Similar output is obtained
197  when \fBpcre_dfa_exec()\fP is used.  when \fBpcre_dfa_exec()\fP is used.
198  .  .P
199  .  If the escape sequence \eP is present more than once in a \fBpcretest\fP data
200  .SH "ISSUES WITH PARTIAL MATCHING"  line, the PCRE_PARTIAL_HARD option is set for the match.
 .rs  
 .sp  
 Certain types of pattern may behave in unintuitive ways when partial matching  
 is requested, whichever matching function is used. For example, matching a  
 pattern that ends with (*FAIL), or any other assertion that causes a match to  
 fail without inspecting any data, yields PCRE_ERROR_PARTIAL rather than  
 PCRE_ERROR_NOMATCH:  
 .sp  
     re> /a+(*FAIL)/  
   data> aaa\eP  
   Partial match: aaa  
 .sp  
 Although (*FAIL) itself could possibly be made a special case, there are other  
 assertions, for example (?!), which behave in the same way, and it is not  
 possible to catch all cases. For consistency, therefore, there are no  
 exceptions to the rule that PCRE_ERROR_PARTIAL is returned instead of  
 PCRE_ERROR_NOMATCH if at any time during the match the end of the subject  
 string was reached.  
201  .  .
202  .  .
203  .SH "MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()"  .SH "MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()"
# Line 128  string was reached. Line 206  string was reached.
206  When a partial match has been found using \fBpcre_dfa_exec()\fP, it is possible  When a partial match has been found using \fBpcre_dfa_exec()\fP, it is possible
207  to continue the match by providing additional subject data and calling  to continue the match by providing additional subject data and calling
208  \fBpcre_dfa_exec()\fP again with the same compiled regular expression, this  \fBpcre_dfa_exec()\fP again with the same compiled regular expression, this
209  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
210  space as before, because this is where details of the previous partial match  space as before, because this is where details of the previous partial match
211  are stored. Here is an example using \fBpcretest\fP, using the \eR escape  are stored. Here is an example using \fBpcretest\fP, using the \eR escape
212  sequence to set the PCRE_DFA_RESTART option (\eP sets the PCRE_PARTIAL option,  sequence to set the PCRE_DFA_RESTART option (\eD specifies the use of
213  and \eD specifies the use of \fBpcre_dfa_exec()\fP):  \fBpcre_dfa_exec()\fP):
214  .sp  .sp
215      re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/      re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/
216    data> 23ja\eP\eD    data> 23ja\eP\eD
# Line 146  Notice that when the match is complete, Line 224  Notice that when the match is complete,
224  not retain the previously partially-matched string. It is up to the calling  not retain the previously partially-matched string. It is up to the calling
225  program to do that if it needs to.  program to do that if it needs to.
226  .P  .P
227  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
228  over multiple segments. This facility can be used to pass very long subject  PCRE_DFA_RESTART to continue partial matching over multiple segments. This
229  strings to \fBpcre_dfa_exec()\fP.  facility can be used to pass very long subject strings to
230    \fBpcre_dfa_exec()\fP.
231  .  .
232  .  .
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 165  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 this. For  offsets that are returned for a partial match. However, in theory, a lookbehind
271  example, using \fBpcre_dfa_exec()\fP, you could pass the subject in chunks that  assertion later in the pattern could require even earlier characters to be
272  are 500 bytes long, but in a buffer of 700 bytes, with the starting offset set  inspected, and it might not have been reached when a partial match occurs. This
273  to 200 and the previous 200 bytes at the start of the buffer.  is probably an extremely unlikely case; you could guard against it to a certain
274  .P  extent by always including extra characters at the start.
275  3. Matching a subject string that is split into multiple segments does not  .P
276  always produce exactly the same result as matching over one single long string.  3. Matching a subject string that is split into multiple segments may not
277  The difference arises when there are multiple matching possibilities, because a  always produce exactly the same result as matching over one single long string,
278  partial match result is given only when there are no completed matches. This  especially when PCRE_PARTIAL_SOFT is used. The section "Partial Matching and
279  means that as soon as the shortest match has been found, continuation to a new  Word Boundaries" above describes an issue that arises if the pattern ends with
280  subject segment is no longer possible. Consider this \fBpcretest\fP example:  \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
282    are no completed matches. This means that as soon as the shortest match has
283    been found, continuation to a new subject segment is no longer possible.
284    Consider again this \fBpcretest\fP example:
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 206  subject segment is no longer possible. C Line 294  subject segment is no longer possible. C
294     0: dogsbody     0: dogsbody
295     1: dog     1: dog
296  .sp  .sp
297  The pattern matches "dog" or "dogsbody". The first data line passes the string  The first data line passes the string "dogsb" to \fBpcre_exec()\fP, setting the
298  "dogsb" to \fBpcre_exec()\fP, setting the PCRE_PARTIAL option. Although the  PCRE_PARTIAL_SOFT option. Although the string is a partial match for
299  string is a partial match for "dogsbody", the result is not PCRE_ERROR_PARTIAL,  "dogsbody", the result is not PCRE_ERROR_PARTIAL, because the shorter string
300  because the shorter string "dog" is a complete match. Similarly, when the  "dog" is a complete match. Similarly, when the subject is presented to
301  subject is presented to \fBpcre_dfa_exec()\fP in several parts ("do" and "gsb"  \fBpcre_dfa_exec()\fP in several parts ("do" and "gsb" being the first two) the
302  being the first two) the match stops when "dog" has been found, and it is not  match stops when "dog" has been found, and it is not possible to continue. On
303  possible to continue. On the other hand, if "dogsbody" is presented as a single  the other hand, if "dogsbody" is presented as a single string,
304  string, \fBpcre_dfa_exec()\fP finds both matches.  \fBpcre_dfa_exec()\fP finds both matches.
305  .P  .P
306  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
307  that is going to be matched in this way with a variable repeat.  matching multi-segment data. The example above then behaves differently:
308    .sp
309        re> /dog(sbody)?/
310      data> dogsb\eP\eP
311      Partial match: dogsb
312      data> do\eP\eD
313      Partial match: do
314      data> gsb\eR\eP\eP\eD
315      Partial match: gsb
316    .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 236  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 244  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 261  Cambridge CB2 3QH, England. Line 358  Cambridge CB2 3QH, England.
358  .rs  .rs
359  .sp  .sp
360  .nf  .nf
361  Last updated: 26 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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