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# Line 14  man page, in case the conversion went wr Line 14  man page, in case the conversion went wr
14  <br>  <br>
15  <ul>  <ul>
16  <li><a name="TOC1" href="#SEC1">PARTIAL MATCHING IN PCRE</a>  <li><a name="TOC1" href="#SEC1">PARTIAL MATCHING IN PCRE</a>
17  <li><a name="TOC2" href="#SEC2">RESTRICTED PATTERNS FOR PCRE_PARTIAL</a>  <li><a name="TOC2" href="#SEC2">PARTIAL MATCHING USING pcre_exec() OR pcre16_exec()</a>
18  <li><a name="TOC3" href="#SEC3">EXAMPLE OF PARTIAL MATCHING USING PCRETEST</a>  <li><a name="TOC3" href="#SEC3">PARTIAL MATCHING USING pcre_dfa_exec() OR pcre16_dfa_exec()</a>
19  <li><a name="TOC4" href="#SEC4">MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()</a>  <li><a name="TOC4" href="#SEC4">PARTIAL MATCHING AND WORD BOUNDARIES</a>
20  <li><a name="TOC5" href="#SEC5">AUTHOR</a>  <li><a name="TOC5" href="#SEC5">FORMERLY RESTRICTED PATTERNS</a>
21  <li><a name="TOC6" href="#SEC6">REVISION</a>  <li><a name="TOC6" href="#SEC6">EXAMPLE OF PARTIAL MATCHING USING PCRETEST</a>
22    <li><a name="TOC7" href="#SEC7">MULTI-SEGMENT MATCHING WITH pcre_dfa_exec() OR pcre16_dfa_exec()</a>
23    <li><a name="TOC8" href="#SEC8">MULTI-SEGMENT MATCHING WITH pcre_exec() OR pcre16_exec()</a>
24    <li><a name="TOC9" href="#SEC9">ISSUES WITH MULTI-SEGMENT MATCHING</a>
25    <li><a name="TOC10" href="#SEC10">AUTHOR</a>
26    <li><a name="TOC11" href="#SEC11">REVISION</a>
27  </ul>  </ul>
28  <br><a name="SEC1" href="#TOC1">PARTIAL MATCHING IN PCRE</a><br>  <br><a name="SEC1" href="#TOC1">PARTIAL MATCHING IN PCRE</a><br>
29  <P>  <P>
30  In normal use of PCRE, if the subject string that is passed to  In normal use of PCRE, if the subject string that is passed to a matching
31  <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b> matches as far as it goes, but is  function matches as far as it goes, but is too short to match the entire
32  too short to match the entire pattern, PCRE_ERROR_NOMATCH is returned. There  pattern, PCRE_ERROR_NOMATCH is returned. There are circumstances where it might
33  are circumstances where it might be helpful to distinguish this case from other  be helpful to distinguish this case from other cases in which there is no
34  cases in which there is no match.  match.
35  </P>  </P>
36  <P>  <P>
37  Consider, for example, an application where a human is required to type in data  Consider, for example, an application where a human is required to type in data
# Line 37  in the form <i>ddmmmyy</i>, defined by t Line 42  in the form <i>ddmmmyy</i>, defined by t
42  </pre>  </pre>
43  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
44  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
45  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
46  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
47  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
48  entered.  entered. Partial matching can also be useful when the subject string is very
49    long and is not all available at once.
50  </P>  </P>
51  <P>  <P>
52  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
53  option, which can be set when calling <b>pcre_exec()</b> or  PCRE_PARTIAL_HARD options, which can be set when calling any of the matching
54  <b>pcre_dfa_exec()</b>. When this flag is set for <b>pcre_exec()</b>, the return  functions. For backwards compatibility, PCRE_PARTIAL is a synonym for
55  code PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if at any time  PCRE_PARTIAL_SOFT. The essential difference between the two options is whether
56  during the matching process the last part of the subject string matched part of  or not a partial match is preferred to an alternative complete match, though
57  the pattern. Unfortunately, for non-anchored matching, it is not possible to  the details differ between the two types of matching function. If both options
58  obtain the position of the start of the partial match. No captured data is set  are set, PCRE_PARTIAL_HARD takes precedence.
59  when PCRE_ERROR_PARTIAL is returned.  </P>
60    <P>
61    Setting a partial matching option disables the use of any just-in-time code
62    that was set up by studying the compiled pattern with the
63    PCRE_STUDY_JIT_COMPILE option. It also disables two of PCRE's standard
64    optimizations. PCRE remembers the last literal data unit in a pattern, and
65    abandons matching immediately if it is not present in the subject string. This
66    optimization cannot be used for a subject string that might match only
67    partially. If the pattern was studied, PCRE knows the minimum length of a
68    matching string, and does not bother to run the matching function on shorter
69    strings. This optimization is also disabled for partial matching.
70    </P>
71    <br><a name="SEC2" href="#TOC1">PARTIAL MATCHING USING pcre_exec() OR pcre16_exec()</a><br>
72    <P>
73    A partial match occurs during a call to <b>pcre_exec()</b> or
74    <b>pcre16_exec()</b> when the end of the subject string is reached successfully,
75    but matching cannot continue because more characters are needed. However, at
76    least one character in the subject must have been inspected. This character
77    need not form part of the final matched string; lookbehind assertions and the
78    \K escape sequence provide ways of inspecting characters before the start of a
79    matched substring. The requirement for inspecting at least one character exists
80    because an empty string can always be matched; without such a restriction there
81    would always be a partial match of an empty string at the end of the subject.
82    </P>
83    <P>
84    If there are at least two slots in the offsets vector when a partial match is
85    returned, the first slot is set to the offset of the earliest character that
86    was inspected. For convenience, the second offset points to the end of the
87    subject so that a substring can easily be identified.
88    </P>
89    <P>
90    For the majority of patterns, the first offset identifies the start of the
91    partially matched string. However, for patterns that contain lookbehind
92    assertions, or \K, or begin with \b or \B, earlier characters have been
93    inspected while carrying out the match. For example:
94    <pre>
95      /(?&#60;=abc)123/
96    </pre>
97    This pattern matches "123", but only if it is preceded by "abc". If the subject
98    string is "xyzabc12", the offsets after a partial match are for the substring
99    "abc12", because all these characters are needed if another match is tried
100    with extra characters added to the subject.
101  </P>  </P>
102  <P>  <P>
103  When PCRE_PARTIAL is set for <b>pcre_dfa_exec()</b>, the return code  What happens when a partial match is identified depends on which of the two
104  PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if the end of the  partial matching options are set.
 subject is reached, there have been no complete matches, but there is still at  
 least one matching possibility. The portion of the string that provided the  
 partial match is set as the first matching string.  
105  </P>  </P>
106    <br><b>
107    PCRE_PARTIAL_SOFT WITH pcre_exec() OR pcre16_exec()
108    </b><br>
109  <P>  <P>
110  Using PCRE_PARTIAL disables one of PCRE's optimizations. PCRE remembers the  If PCRE_PARTIAL_SOFT is set when <b>pcre_exec()</b> or <b>pcre16_exec()</b>
111  last literal byte in a pattern, and abandons matching immediately if such a  identifies a partial match, the partial match is remembered, but matching
112  byte is not present in the subject string. This optimization cannot be used  continues as normal, and other alternatives in the pattern are tried. If no
113  for a subject string that might match only partially.  complete match can be found, PCRE_ERROR_PARTIAL is returned instead of
114    PCRE_ERROR_NOMATCH.
115  </P>  </P>
 <br><a name="SEC2" href="#TOC1">RESTRICTED PATTERNS FOR PCRE_PARTIAL</a><br>  
116  <P>  <P>
117  Because of the way certain internal optimizations are implemented in the  This option is "soft" because it prefers a complete match over a partial match.
118  <b>pcre_exec()</b> function, the PCRE_PARTIAL option cannot be used with all  All the various matching items in a pattern behave as if the subject string is
119  patterns. These restrictions do not apply when <b>pcre_dfa_exec()</b> is used.  potentially complete. For example, \z, \Z, and $ match at the end of the
120  For <b>pcre_exec()</b>, repeated single characters such as  subject, as normal, and for \b and \B the end of the subject is treated as a
121    non-alphanumeric.
122    </P>
123    <P>
124    If there is more than one partial match, the first one that was found provides
125    the data that is returned. Consider this pattern:
126  <pre>  <pre>
127    a{2,4}    /123\w+X|dogY/
128  </pre>  </pre>
129  and repeated single metasequences such as  If this is matched against the subject string "abc123dog", both
130    alternatives fail to match, but the end of the subject is reached during
131    matching, so PCRE_ERROR_PARTIAL is returned. The offsets are set to 3 and 9,
132    identifying "123dog" as the first partial match that was found. (In this
133    example, there are two partial matches, because "dog" on its own partially
134    matches the second alternative.)
135    </P>
136    <br><b>
137    PCRE_PARTIAL_HARD WITH pcre_exec() OR pcre16_exec()
138    </b><br>
139    <P>
140    If PCRE_PARTIAL_HARD is set for <b>pcre_exec()</b> or <b>pcre16_exec()</b>,
141    PCRE_ERROR_PARTIAL is returned as soon as a partial match is found, without
142    continuing to search for possible complete matches. This option is "hard"
143    because it prefers an earlier partial match over a later complete match. For
144    this reason, the assumption is made that the end of the supplied subject string
145    may not be the true end of the available data, and so, if \z, \Z, \b, \B,
146    or $ are encountered at the end of the subject, the result is
147    PCRE_ERROR_PARTIAL.
148    </P>
149    <P>
150    Setting PCRE_PARTIAL_HARD also affects the way UTF-8 and UTF-16
151    subject strings are checked for validity. Normally, an invalid sequence
152    causes the error PCRE_ERROR_BADUTF8 or PCRE_ERROR_BADUTF16. However, in the
153    special case of a truncated character at the end of the subject,
154    PCRE_ERROR_SHORTUTF8 or PCRE_ERROR_SHORTUTF16 is returned when
155    PCRE_PARTIAL_HARD is set.
156    </P>
157    <br><b>
158    Comparing hard and soft partial matching
159    </b><br>
160    <P>
161    The difference between the two partial matching options can be illustrated by a
162    pattern such as:
163  <pre>  <pre>
164    \d+    /dog(sbody)?/
165  </pre>  </pre>
166  are not permitted if the maximum number of occurrences is greater than one.  This matches either "dog" or "dogsbody", greedily (that is, it prefers the
167  Optional items such as \d? (where the maximum is one) are permitted.  longer string if possible). If it is matched against the string "dog" with
168  Quantifiers with any values are permitted after parentheses, so the invalid  PCRE_PARTIAL_SOFT, it yields a complete match for "dog". However, if
169  examples above can be coded thus:  PCRE_PARTIAL_HARD is set, the result is PCRE_ERROR_PARTIAL. On the other hand,
170    if the pattern is made ungreedy the result is different:
171  <pre>  <pre>
172    (a){2,4}    /dog(sbody)??/
   (\d)+  
173  </pre>  </pre>
174  These constructions run more slowly, but for the kinds of application that are  In this case the result is always a complete match because that is found first,
175  envisaged for this facility, this is not felt to be a major restriction.  and matching never continues after finding a complete match. It might be easier
176    to follow this explanation by thinking of the two patterns like this:
177    <pre>
178      /dog(sbody)?/    is the same as  /dogsbody|dog/
179      /dog(sbody)??/   is the same as  /dog|dogsbody/
180    </pre>
181    The second pattern will never match "dogsbody", because it will always find the
182    shorter match first.
183  </P>  </P>
184    <br><a name="SEC3" href="#TOC1">PARTIAL MATCHING USING pcre_dfa_exec() OR pcre16_dfa_exec()</a><br>
185  <P>  <P>
186  If PCRE_PARTIAL is set for a pattern that does not conform to the restrictions,  The DFA functions move along the subject string character by character, without
187  <b>pcre_exec()</b> returns the error code PCRE_ERROR_BADPARTIAL (-13).  backtracking, searching for all possible matches simultaneously. If the end of
188    the subject is reached before the end of the pattern, there is the possibility
189    of a partial match, again provided that at least one character has been
190    inspected.
191    </P>
192    <P>
193    When PCRE_PARTIAL_SOFT is set, PCRE_ERROR_PARTIAL is returned only if there
194    have been no complete matches. Otherwise, the complete matches are returned.
195    However, if PCRE_PARTIAL_HARD is set, a partial match takes precedence over any
196    complete matches. The portion of the string that was inspected when the longest
197    partial match was found is set as the first matching string, provided there are
198    at least two slots in the offsets vector.
199    </P>
200    <P>
201    Because the DFA functions always search for all possible matches, and there is
202    no difference between greedy and ungreedy repetition, their behaviour is
203    different from the standard functions when PCRE_PARTIAL_HARD is set. Consider
204    the string "dog" matched against the ungreedy pattern shown above:
205    <pre>
206      /dog(sbody)??/
207    </pre>
208    Whereas the standard functions stop as soon as they find the complete match for
209    "dog", the DFA functions also find the partial match for "dogsbody", and so
210    return that when PCRE_PARTIAL_HARD is set.
211    </P>
212    <br><a name="SEC4" href="#TOC1">PARTIAL MATCHING AND WORD BOUNDARIES</a><br>
213    <P>
214    If a pattern ends with one of sequences \b or \B, which test for word
215    boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter-intuitive
216    results. Consider this pattern:
217    <pre>
218      /\bcat\b/
219    </pre>
220    This matches "cat", provided there is a word boundary at either end. If the
221    subject string is "the cat", the comparison of the final "t" with a following
222    character cannot take place, so a partial match is found. However, normal
223    matching carries on, and \b matches at the end of the subject when the last
224    character is a letter, so a complete match is found. The result, therefore, is
225    <i>not</i> PCRE_ERROR_PARTIAL. Using PCRE_PARTIAL_HARD in this case does yield
226    PCRE_ERROR_PARTIAL, because then the partial match takes precedence.
227    </P>
228    <br><a name="SEC5" href="#TOC1">FORMERLY RESTRICTED PATTERNS</a><br>
229    <P>
230    For releases of PCRE prior to 8.00, because of the way certain internal
231    optimizations were implemented in the <b>pcre_exec()</b> function, the
232    PCRE_PARTIAL option (predecessor of PCRE_PARTIAL_SOFT) could not be used with
233    all patterns. From release 8.00 onwards, the restrictions no longer apply, and
234    partial matching with can be requested for any pattern.
235    </P>
236    <P>
237    Items that were formerly restricted were repeated single characters and
238    repeated metasequences. If PCRE_PARTIAL was set for a pattern that did not
239    conform to the restrictions, <b>pcre_exec()</b> returned the error code
240    PCRE_ERROR_BADPARTIAL (-13). This error code is no longer in use. The
241    PCRE_INFO_OKPARTIAL call to <b>pcre_fullinfo()</b> to find out if a compiled
242    pattern can be used for partial matching now always returns 1.
243  </P>  </P>
244  <br><a name="SEC3" href="#TOC1">EXAMPLE OF PARTIAL MATCHING USING PCRETEST</a><br>  <br><a name="SEC6" href="#TOC1">EXAMPLE OF PARTIAL MATCHING USING PCRETEST</a><br>
245  <P>  <P>
246  If the escape sequence \P is present in a <b>pcretest</b> data line, the  If the escape sequence \P is present in a <b>pcretest</b> data line, the
247  PCRE_PARTIAL flag is used for the match. Here is a run of <b>pcretest</b> that  PCRE_PARTIAL_SOFT option is used for the match. Here is a run of <b>pcretest</b>
248  uses the date example quoted above:  that uses the date example quoted above:
249  <pre>  <pre>
250      re&#62; /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/      re&#62; /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
251    data&#62; 25jun04\P    data&#62; 25jun04\P
252     0: 25jun04     0: 25jun04
253     1: jun     1: jun
254    data&#62; 25dec3\P    data&#62; 25dec3\P
255    Partial match    Partial match: 23dec3
256    data&#62; 3ju\P    data&#62; 3ju\P
257    Partial match    Partial match: 3ju
258    data&#62; 3juj\P    data&#62; 3juj\P
259    No match    No match
260    data&#62; j\P    data&#62; j\P
# Line 114  uses the date example quoted above: Line 262  uses the date example quoted above:
262  </pre>  </pre>
263  The first data string is matched completely, so <b>pcretest</b> shows the  The first data string is matched completely, so <b>pcretest</b> shows the
264  matched substrings. The remaining four strings do not match the complete  matched substrings. The remaining four strings do not match the complete
265  pattern, but the first two are partial matches. The same test, using  pattern, but the first two are partial matches. Similar output is obtained
266  <b>pcre_dfa_exec()</b> matching (by means of the \D escape sequence), produces  if DFA matching is used.
267  the following output:  </P>
268  <pre>  <P>
269      re&#62; /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/  If the escape sequence \P is present more than once in a <b>pcretest</b> data
270    data&#62; 25jun04\P\D  line, the PCRE_PARTIAL_HARD option is set for the match.
    0: 25jun04  
   data&#62; 23dec3\P\D  
   Partial match: 23dec3  
   data&#62; 3ju\P\D  
   Partial match: 3ju  
   data&#62; 3juj\P\D  
   No match  
   data&#62; j\P\D  
   No match  
 </pre>  
 Notice that in this case the portion of the string that was matched is made  
 available.  
271  </P>  </P>
272  <br><a name="SEC4" href="#TOC1">MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()</a><br>  <br><a name="SEC7" href="#TOC1">MULTI-SEGMENT MATCHING WITH pcre_dfa_exec() OR pcre16_dfa_exec()</a><br>
273  <P>  <P>
274  When a partial match has been found using <b>pcre_dfa_exec()</b>, it is possible  When a partial match has been found using a DFA matching function, it is
275  to continue the match by providing additional subject data and calling  possible to continue the match by providing additional subject data and calling
276  <b>pcre_dfa_exec()</b> again with the same compiled regular expression, this  the function again with the same compiled regular expression, this time setting
277  time setting the PCRE_DFA_RESTART option. You must also pass the same working  the PCRE_DFA_RESTART option. You must pass the same working space as before,
278  space as before, because this is where details of the previous partial match  because this is where details of the previous partial match are stored. Here is
279  are stored. Here is an example using <b>pcretest</b>, using the \R escape  an example using <b>pcretest</b>, using the \R escape sequence to set the
280  sequence to set the PCRE_DFA_RESTART option (\P and \D are as above):  PCRE_DFA_RESTART option (\D specifies the use of the DFA matching function):
281  <pre>  <pre>
282      re&#62; /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/      re&#62; /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
283    data&#62; 23ja\P\D    data&#62; 23ja\P\D
# Line 156  not retain the previously partially-matc Line 292  not retain the previously partially-matc
292  program to do that if it needs to.  program to do that if it needs to.
293  </P>  </P>
294  <P>  <P>
295  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
296  over multiple segments. This facility can be used to pass very long subject  PCRE_DFA_RESTART to continue partial matching over multiple segments. This
297  strings to <b>pcre_dfa_exec()</b>. However, some care is needed for certain  facility can be used to pass very long subject strings to the DFA matching
298  types of pattern.  functions.
299  </P>  </P>
300  <P>  <br><a name="SEC8" href="#TOC1">MULTI-SEGMENT MATCHING WITH pcre_exec() OR pcre16_exec()</a><br>
301  1. If the pattern contains tests for the beginning or end of a line, you need  <P>
302  to pass the PCRE_NOTBOL or PCRE_NOTEOL options, as appropriate, when the  From release 8.00, the standard matching functions can also be used to do
303  subject string for any call does not contain the beginning or end of a line.  multi-segment matching. Unlike the DFA functions, it is not possible to
304  </P>  restart the previous match with a new segment of data. Instead, new data must
305  <P>  be added to the previous subject string, and the entire match re-run, starting
306  2. If the pattern contains backward assertions (including \b or \B), you need  from the point where the partial match occurred. Earlier data can be discarded.
307  to arrange for some overlap in the subject strings to allow for this. For  </P>
308  example, you could pass the subject in chunks that are 500 bytes long, but in  <P>
309  a buffer of 700 bytes, with the starting offset set to 200 and the previous 200  It is best to use PCRE_PARTIAL_HARD in this situation, because it does not
310  bytes at the start of the buffer.  treat the end of a segment as the end of the subject when matching \z, \Z,
311  </P>  \b, \B, and $. Consider an unanchored pattern that matches dates:
312  <P>  <pre>
313  3. Matching a subject string that is split into multiple segments does not      re&#62; /\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d/
314  always produce exactly the same result as matching over one single long string.    data&#62; The date is 23ja\P\P
315  The difference arises when there are multiple matching possibilities, because a    Partial match: 23ja
316  partial match result is given only when there are no completed matches in a  </pre>
317  call to fBpcre_dfa_exec()\fP. This means that as soon as the shortest match has  At this stage, an application could discard the text preceding "23ja", add on
318  been found, continuation to a new subject segment is no longer possible.  text from the next segment, and call the matching function again. Unlike the
319  Consider this <b>pcretest</b> example:  DFA matching functions the entire matching string must always be available, and
320    the complete matching process occurs for each call, so more memory and more
321    processing time is needed.
322    </P>
323    <P>
324    <b>Note:</b> If the pattern contains lookbehind assertions, or \K, or starts
325    with \b or \B, the string that is returned for a partial match includes
326    characters that precede the partially matched string itself, because these must
327    be retained when adding on more characters for a subsequent matching attempt.
328    </P>
329    <br><a name="SEC9" href="#TOC1">ISSUES WITH MULTI-SEGMENT MATCHING</a><br>
330    <P>
331    Certain types of pattern may give problems with multi-segment matching,
332    whichever matching function is used.
333    </P>
334    <P>
335    1. If the pattern contains a test for the beginning of a line, you need to pass
336    the PCRE_NOTBOL option when the subject string for any call does start at the
337    beginning of a line. There is also a PCRE_NOTEOL option, but in practice when
338    doing multi-segment matching you should be using PCRE_PARTIAL_HARD, which
339    includes the effect of PCRE_NOTEOL.
340    </P>
341    <P>
342    2. Lookbehind assertions at the start of a pattern are catered for in the
343    offsets that are returned for a partial match. However, in theory, a lookbehind
344    assertion later in the pattern could require even earlier characters to be
345    inspected, and it might not have been reached when a partial match occurs. This
346    is probably an extremely unlikely case; you could guard against it to a certain
347    extent by always including extra characters at the start.
348    </P>
349    <P>
350    3. Matching a subject string that is split into multiple segments may not
351    always produce exactly the same result as matching over one single long string,
352    especially when PCRE_PARTIAL_SOFT is used. The section "Partial Matching and
353    Word Boundaries" above describes an issue that arises if the pattern ends with
354    \b or \B. Another kind of difference may occur when there are multiple
355    matching possibilities, because (for PCRE_PARTIAL_SOFT) a partial match result
356    is given only when there are no completed matches. This means that as soon as
357    the shortest match has been found, continuation to a new subject segment is no
358    longer possible. Consider again this <b>pcretest</b> example:
359  <pre>  <pre>
360      re&#62; /dog(sbody)?/      re&#62; /dog(sbody)?/
361      data&#62; dogsb\P
362       0: dog
363    data&#62; do\P\D    data&#62; do\P\D
364    Partial match: do    Partial match: do
365    data&#62; gsb\R\P\D    data&#62; gsb\R\P\D
# Line 191  Consider this <b>pcretest</b> example: Line 368  Consider this <b>pcretest</b> example:
368     0: dogsbody     0: dogsbody
369     1: dog     1: dog
370  </pre>  </pre>
371  The pattern matches the words "dog" or "dogsbody". When the subject is  The first data line passes the string "dogsb" to a standard matching function,
372  presented in several parts ("do" and "gsb" being the first two) the match stops  setting the PCRE_PARTIAL_SOFT option. Although the string is a partial match
373  when "dog" has been found, and it is not possible to continue. On the other  for "dogsbody", the result is not PCRE_ERROR_PARTIAL, because the shorter
374  hand, if "dogsbody" is presented as a single string, both matches are found.  string "dog" is a complete match. Similarly, when the subject is presented to
375    a DFA matching function in several parts ("do" and "gsb" being the first two)
376    the match stops when "dog" has been found, and it is not possible to continue.
377    On the other hand, if "dogsbody" is presented as a single string, a DFA
378    matching function finds both matches.
379  </P>  </P>
380  <P>  <P>
381  Because of this phenomenon, it does not usually make sense to end a pattern  Because of these problems, it is best to use PCRE_PARTIAL_HARD when matching
382  that is going to be matched in this way with a variable repeat.  multi-segment data. The example above then behaves differently:
383  </P>  <pre>
384  <P>      re&#62; /dog(sbody)?/
385  4. Patterns that contain alternatives at the top level which do not all    data&#62; dogsb\P\P
386  start with the same pattern item may not work as expected. For example,    Partial match: dogsb
387  consider this pattern:    data&#62; do\P\D
388      Partial match: do
389      data&#62; gsb\R\P\P\D
390      Partial match: gsb
391    </pre>
392    4. Patterns that contain alternatives at the top level which do not all start
393    with the same pattern item may not work as expected when PCRE_DFA_RESTART is
394    used. For example, consider this pattern:
395  <pre>  <pre>
396    1234|3789    1234|3789
397  </pre>  </pre>
398  If the first part of the subject is "ABC123", a partial match of the first  If the first part of the subject is "ABC123", a partial match of the first
399  alternative is found at offset 3. There is no partial match for the second  alternative is found at offset 3. There is no partial match for the second
400  alternative, because such a match does not start at the same point in the  alternative, because such a match does not start at the same point in the
401  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
402  match because only those alternatives that match at one point in the subject  match because only those alternatives that match at one point in the subject
403  are remembered. The problem arises because the start of the second alternative  are remembered. The problem arises because the start of the second alternative
404  matches within the first alternative. There is no problem with anchored  matches within the first alternative. There is no problem with anchored
# Line 218  patterns or patterns such as: Line 406  patterns or patterns such as:
406  <pre>  <pre>
407    1234|ABCD    1234|ABCD
408  </pre>  </pre>
409  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
410    problem if a standard matching function is used, because the entire match has
411    to be rerun each time:
412    <pre>
413        re&#62; /1234|3789/
414      data&#62; ABC123\P\P
415      Partial match: 123
416      data&#62; 1237890
417       0: 3789
418    </pre>
419    Of course, instead of using PCRE_DFA_RESTART, the same technique of re-running
420    the entire match can also be used with the DFA matching functions. Another
421    possibility is to work with two buffers. If a partial match at offset <i>n</i>
422    in the first buffer is followed by "no match" when PCRE_DFA_RESTART is used on
423    the second buffer, you can then try a new match starting at offset <i>n+1</i> in
424    the first buffer.
425  </P>  </P>
426  <br><a name="SEC5" href="#TOC1">AUTHOR</a><br>  <br><a name="SEC10" href="#TOC1">AUTHOR</a><br>
427  <P>  <P>
428  Philip Hazel  Philip Hazel
429  <br>  <br>
# Line 229  University Computing Service Line 432  University Computing Service
432  Cambridge CB2 3QH, England.  Cambridge CB2 3QH, England.
433  <br>  <br>
434  </P>  </P>
435  <br><a name="SEC6" href="#TOC1">REVISION</a><br>  <br><a name="SEC11" href="#TOC1">REVISION</a><br>
436  <P>  <P>
437  Last updated: 06 March 2007  Last updated: 08 January 2012
438  <br>  <br>
439  Copyright &copy; 1997-2007 University of Cambridge.  Copyright &copy; 1997-2012 University of Cambridge.
440  <br>  <br>
441  <p>  <p>
442  Return to the <a href="index.html">PCRE index page</a>.  Return to the <a href="index.html">PCRE index page</a>.

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