/[pcre]/code/trunk/doc/html/pcrepartial.html
ViewVC logotype

Diff of /code/trunk/doc/html/pcrepartial.html

Parent Directory Parent Directory | Revision Log Revision Log | View Patch Patch

revision 429 by ph10, Tue Sep 1 16:10:16 2009 UTC revision 453 by ph10, Fri Sep 18 19:12:35 2009 UTC
# Line 51  is very long and is not all available at Line 51  is very long and is not all available at
51  <P>  <P>
52  PCRE supports partial matching by means of the PCRE_PARTIAL_SOFT and  PCRE supports partial matching by means of the PCRE_PARTIAL_SOFT and
53  PCRE_PARTIAL_HARD options, which can be set when calling <b>pcre_exec()</b> or  PCRE_PARTIAL_HARD options, which can be set when calling <b>pcre_exec()</b> or
54  <b>pcre_dfa_exec()</b>. For backwards compatibility, PCRE_PARTIAL is a synonym  <b>pcre_dfa_exec()</b>. For backwards compatibility, PCRE_PARTIAL is a synonym
55  for PCRE_PARTIAL_SOFT. The essential difference between the two options is  for PCRE_PARTIAL_SOFT. The essential difference between the two options is
56  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,
57  though the details differ between the two matching functions. If both options  though the details differ between the two matching functions. If both options
58  are set, PCRE_PARTIAL_HARD takes precedence.  are set, PCRE_PARTIAL_HARD takes precedence.
59  </P>  </P>
60  <P>  <P>
# Line 74  been matched. (In other words, a partial Line 74  been matched. (In other words, a partial
74  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
75  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
76  complete match can be found, <b>pcre_exec()</b> returns PCRE_ERROR_PARTIAL  complete match can be found, <b>pcre_exec()</b> returns PCRE_ERROR_PARTIAL
77  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
78  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
79  partially matched. Consider this pattern:  was inspected when the partial match was found. For convenience, the second
80    offset points to the end of the string so that a substring can easily be
81    extracted.
82    </P>
83    <P>
84    For the majority of patterns, the first offset identifies the start of the
85    partially matched string. However, for patterns that contain lookbehind
86    assertions, or \K, or begin with \b or \B, earlier characters have been
87    inspected while carrying out the match. For example:
88    <pre>
89      /(?&#60;=abc)123/
90    </pre>
91    This pattern matches "123", but only if it is preceded by "abc". If the subject
92    string is "xyzabc12", the offsets after a partial match are for the substring
93    "abc12", because all these characters are needed if another match is tried
94    with extra characters added.
95    </P>
96    <P>
97    If there is more than one partial match, the first one that was found provides
98    the data that is returned. Consider this pattern:
99  <pre>  <pre>
100    /123\w+X|dogY/    /123\w+X|dogY/
101  </pre>  </pre>
102  If this is matched against the subject string "abc123dog", both  If this is matched against the subject string "abc123dog", both
103  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
104  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
105  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
106  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"
107  on its own partially matches the second alternative.)  on its own partially matches the second alternative.)
108  </P>  </P>
109  <P>  <P>
110  If PCRE_PARTIAL_HARD is set for <b>pcre_exec()</b>, it returns  If PCRE_PARTIAL_HARD is set for <b>pcre_exec()</b>, it returns
111  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
112  search for possible complete matches. The difference between the two options  search for possible complete matches. The difference between the two options
113  can be illustrated by a pattern such as:  can be illustrated by a pattern such as:
114  <pre>  <pre>
115    /dog(sbody)?/    /dog(sbody)?/
116  </pre>  </pre>
117  This matches either "dog" or "dogsbody", greedily (that is, it prefers the  This matches either "dog" or "dogsbody", greedily (that is, it prefers the
118  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
119  PCRE_PARTIAL_SOFT, it yields a complete match for "dog". However, if  PCRE_PARTIAL_SOFT, it yields a complete match for "dog". However, if
120  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,
121  if the pattern is made ungreedy the result is different:  if the pattern is made ungreedy the result is different:
122  <pre>  <pre>
123    /dog(sbody)??/    /dog(sbody)??/
124  </pre>  </pre>
125  In this case the result is always a complete match because <b>pcre_exec()</b>  In this case the result is always a complete match because <b>pcre_exec()</b>
126  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
127  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:
128  <pre>  <pre>
129    /dog(sbody)?/    is the same as  /dogsbody|dog/    /dog(sbody)?/    is the same as  /dogsbody|dog/
130    /dog(sbody)??/   is the same as  /dog|dogsbody/    /dog(sbody)??/   is the same as  /dog|dogsbody/
131  </pre>  </pre>
132  The second pattern will never match "dogsbody" when <b>pcre_exec()</b> is  The second pattern will never match "dogsbody" when <b>pcre_exec()</b> is
133  used, because it will always find the shorter match first.  used, because it will always find the shorter match first.
134  </P>  </P>
135  <br><a name="SEC3" href="#TOC1">PARTIAL MATCHING USING pcre_dfa_exec()</a><br>  <br><a name="SEC3" href="#TOC1">PARTIAL MATCHING USING pcre_dfa_exec()</a><br>
136  <P>  <P>
137  The <b>pcre_dfa_exec()</b> function moves along the subject string character by  The <b>pcre_dfa_exec()</b> function moves along the subject string character by
138  character, without backtracking, searching for all possible matches  character, without backtracking, searching for all possible matches
139  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
140  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
141  least one character has matched.  least one character has matched.
142  </P>  </P>
# Line 125  least one character has matched. Line 144  least one character has matched.
144  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
145  have been no complete matches. Otherwise, the complete matches are returned.  have been no complete matches. Otherwise, the complete matches are returned.
146  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
147  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
148  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
149  slots in the offsets vector.  at least two slots in the offsets vector.
150  </P>  </P>
151  <P>  <P>
152  Because <b>pcre_dfa_exec()</b> always searches for all possible matches, and  Because <b>pcre_dfa_exec()</b> always searches for all possible matches, and
153  there is no difference between greedy and ungreedy repetition, its behaviour is  there is no difference between greedy and ungreedy repetition, its behaviour is
154  different from <b>pcre_exec</b> when PCRE_PARTIAL_HARD is set. Consider the  different from <b>pcre_exec</b> when PCRE_PARTIAL_HARD is set. Consider the
155  string "dog" matched against the ungreedy pattern shown above:  string "dog" matched against the ungreedy pattern shown above:
156  <pre>  <pre>
157    /dog(sbody)??/    /dog(sbody)??/
158  </pre>  </pre>
159  Whereas <b>pcre_exec()</b> stops as soon as it finds the complete match for  Whereas <b>pcre_exec()</b> stops as soon as it finds the complete match for
160  "dog", <b>pcre_dfa_exec()</b> also finds the partial match for "dogsbody", and  "dog", <b>pcre_dfa_exec()</b> also finds the partial match for "dogsbody", and
161  so returns that when PCRE_PARTIAL_HARD is set.  so returns that when PCRE_PARTIAL_HARD is set.
162  </P>  </P>
163  <br><a name="SEC4" href="#TOC1">PARTIAL MATCHING AND WORD BOUNDARIES</a><br>  <br><a name="SEC4" href="#TOC1">PARTIAL MATCHING AND WORD BOUNDARIES</a><br>
164  <P>  <P>
165  If a pattern ends with one of sequences \w or \W, which test for word  If a pattern ends with one of sequences \w or \W, which test for word
166  boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter-intuitive  boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter-intuitive
167  results. Consider this pattern:  results. Consider this pattern:
168  <pre>  <pre>
169    /\bcat\b/    /\bcat\b/
170  </pre>  </pre>
171  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
172  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
173  character cannot take place, so a partial match is found. However,  character cannot take place, so a partial match is found. However,
174  <b>pcre_exec()</b> carries on with normal matching, which matches \b at the end  <b>pcre_exec()</b> carries on with normal matching, which matches \b at the end
175  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
176  match. The result, therefore, is <i>not</i> PCRE_ERROR_PARTIAL. The same thing  match. The result, therefore, is <i>not</i> PCRE_ERROR_PARTIAL. The same thing
177  happens with <b>pcre_dfa_exec()</b>, because it also finds the complete match.  happens with <b>pcre_dfa_exec()</b>, because it also finds the complete match.
178  </P>  </P>
179  <P>  <P>
180  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
181  then the partial match takes precedence.  then the partial match takes precedence.
182  </P>  </P>
183  <br><a name="SEC5" href="#TOC1">FORMERLY RESTRICTED PATTERNS</a><br>  <br><a name="SEC5" href="#TOC1">FORMERLY RESTRICTED PATTERNS</a><br>
# Line 236  facility can be used to pass very long s Line 255  facility can be used to pass very long s
255  </P>  </P>
256  <br><a name="SEC8" href="#TOC1">MULTI-SEGMENT MATCHING WITH pcre_exec()</a><br>  <br><a name="SEC8" href="#TOC1">MULTI-SEGMENT MATCHING WITH pcre_exec()</a><br>
257  <P>  <P>
258  From release 8.00, <b>pcre_exec()</b> can also be used to do multi-segment  From release 8.00, <b>pcre_exec()</b> can also be used to do multi-segment
259  matching. Unlike <b>pcre_dfa_exec()</b>, it is not possible to restart the  matching. Unlike <b>pcre_dfa_exec()</b>, it is not possible to restart the
260  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
261  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
262  point where the partial match occurred. Earlier data can be discarded.  point where the partial match occurred. Earlier data can be discarded.
263  Consider an unanchored pattern that matches dates:  Consider an unanchored pattern that matches dates:
264  <pre>  <pre>
# Line 247  Consider an unanchored pattern that matc Line 266  Consider an unanchored pattern that matc
266    data&#62; The date is 23ja\P    data&#62; The date is 23ja\P
267    Partial match: 23ja    Partial match: 23ja
268  </pre>  </pre>
269  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
270  text from the next segment, and call <b>pcre_exec()</b> again. Unlike  text from the next segment, and call <b>pcre_exec()</b> again. Unlike
271  <b>pcre_dfa_exec()</b>, the entire matching string must always be available, and  <b>pcre_dfa_exec()</b>, the entire matching string must always be available, and
272  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
273  processing time is needed.  processing time is needed.
274  </P>  </P>
275    <P>
276    <b>Note:</b> If the pattern contains lookbehind assertions, or \K, or starts
277    with \b or \B, the string that is returned for a partial match will include
278    characters that precede the partially matched string itself, because these must
279    be retained when adding on more characters for a subsequent matching attempt.
280    </P>
281  <br><a name="SEC9" href="#TOC1">ISSUES WITH MULTI-SEGMENT MATCHING</a><br>  <br><a name="SEC9" href="#TOC1">ISSUES WITH MULTI-SEGMENT MATCHING</a><br>
282  <P>  <P>
283  Certain types of pattern may give problems with multi-segment matching,  Certain types of pattern may give problems with multi-segment matching,
284  whichever matching function is used.  whichever matching function is used.
285  </P>  </P>
286  <P>  <P>
# Line 264  to pass the PCRE_NOTBOL or PCRE_NOTEOL o Line 289  to pass the PCRE_NOTBOL or PCRE_NOTEOL o
289  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.
290  </P>  </P>
291  <P>  <P>
292  2. If the pattern contains backward assertions (including \b or \B), you need  2. Lookbehind assertions at the start of a pattern are catered for in the
293  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
294  correctly tested at the start of each substring. For example, using  assertion later in the pattern could require even earlier characters to be
295  <b>pcre_dfa_exec()</b>, 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
296  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
297  previous 200 bytes at the start of the buffer.  extent by always including extra characters at the start.
298  </P>  </P>
299  <P>  <P>
300  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
301  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,
302  especially when PCRE_PARTIAL_SOFT is used. The section "Partial Matching and  especially when PCRE_PARTIAL_SOFT is used. The section "Partial Matching and
303  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
304  \b or \B. Another kind of difference may occur when there are multiple  \b or \B. Another kind of difference may occur when there are multiple
305  matching possibilities, because a partial match result is given only when there  matching possibilities, because a partial match result is given only when there
306  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 284  Consider again this <b>pcretest</b> exam Line 309  Consider again this <b>pcretest</b> exam
309  <pre>  <pre>
310      re&#62; /dog(sbody)?/      re&#62; /dog(sbody)?/
311    data&#62; dogsb\P    data&#62; dogsb\P
312     0: dog     0: dog
313    data&#62; do\P\D    data&#62; do\P\D
314    Partial match: do    Partial match: do
315    data&#62; gsb\R\P\D    data&#62; gsb\R\P\D
# Line 308  matching multi-segment data. The example Line 333  matching multi-segment data. The example
333  <pre>  <pre>
334      re&#62; /dog(sbody)?/      re&#62; /dog(sbody)?/
335    data&#62; dogsb\P\P    data&#62; dogsb\P\P
336    Partial match: dogsb    Partial match: dogsb
337    data&#62; do\P\D    data&#62; do\P\D
338    Partial match: do    Partial match: do
339    data&#62; gsb\R\P\P\D    data&#62; gsb\R\P\P\D
340    Partial match: gsb    Partial match: gsb
341    
342  </PRE>  </PRE>
343  </P>  </P>
344  <P>  <P>
345  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
346  start with the same pattern item may not work as expected when  start with the same pattern item may not work as expected when
347  <b>pcre_dfa_exec()</b> is used. For example, consider this pattern:  <b>pcre_dfa_exec()</b> is used. For example, consider this pattern:
348  <pre>  <pre>
349    1234|3789    1234|3789
# Line 335  patterns or patterns such as: Line 360  patterns or patterns such as:
360    1234|ABCD    1234|ABCD
361  </pre>  </pre>
362  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
363  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
364  each time:  each time:
365  <pre>  <pre>
366      re&#62; /1234|3789/      re&#62; /1234|3789/
# Line 357  Cambridge CB2 3QH, England. Line 382  Cambridge CB2 3QH, England.
382  </P>  </P>
383  <br><a name="SEC11" href="#TOC1">REVISION</a><br>  <br><a name="SEC11" href="#TOC1">REVISION</a><br>
384  <P>  <P>
385  Last updated: 31 August 2009  Last updated: 05 September 2009
386  <br>  <br>
387  Copyright &copy; 1997-2009 University of Cambridge.  Copyright &copy; 1997-2009 University of Cambridge.
388  <br>  <br>

Legend:
Removed from v.429  
changed lines
  Added in v.453

  ViewVC Help
Powered by ViewVC 1.1.5