1: .TH PCREPARTIAL 3 "24 February 2012" "PCRE 8.31"
2: .SH NAME
3: PCRE - Perl-compatible regular expressions
4: .SH "PARTIAL MATCHING IN PCRE"
5: .rs
6: .sp
7: In normal use of PCRE, if the subject string that is passed to a matching
8: function matches as far as it goes, but is too short to match the entire
9: pattern, PCRE_ERROR_NOMATCH is returned. There are circumstances where it might
10: be helpful to distinguish this case from other cases in which there is no
11: match.
12: .P
13: Consider, for example, an application where a human is required to type in data
14: for a field with specific formatting requirements. An example might be a date
15: in the form \fIddmmmyy\fP, defined by this pattern:
16: .sp
17: ^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$
18: .sp
19: 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
21: as soon as a mistake is made, by beeping and not reflecting the character that
22: 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
24: entered. Partial matching can also be useful when the subject string is very
25: long and is not all available at once.
26: .P
27: PCRE supports partial matching by means of the PCRE_PARTIAL_SOFT and
28: PCRE_PARTIAL_HARD options, which can be set when calling any of the matching
29: functions. For backwards compatibility, PCRE_PARTIAL is a synonym for
30: PCRE_PARTIAL_SOFT. The essential difference between the two options is whether
31: or not a partial match is preferred to an alternative complete match, though
32: the details differ between the two types of matching function. If both options
33: are set, PCRE_PARTIAL_HARD takes precedence.
34: .P
35: If you want to use partial matching with just-in-time optimized code, you must
36: call \fBpcre_study()\fP or \fBpcre16_study()\fP with one or both of these
37: options:
38: .sp
39: PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE
40: PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE
41: .sp
42: PCRE_STUDY_JIT_COMPILE should also be set if you are going to run non-partial
43: matches on the same pattern. If the appropriate JIT study mode has not been set
44: for a match, the interpretive matching code is used.
45: .P
46: Setting a partial matching option disables two of PCRE's standard
47: optimizations. PCRE remembers the last literal data unit in a pattern, and
48: abandons matching immediately if it is not present in the subject string. This
49: optimization cannot be used for a subject string that might match only
50: partially. If the pattern was studied, PCRE knows the minimum length of a
51: matching string, and does not bother to run the matching function on shorter
52: strings. This optimization is also disabled for partial matching.
53: .
54: .
55: .SH "PARTIAL MATCHING USING pcre_exec() OR pcre16_exec()"
56: .rs
57: .sp
58: A partial match occurs during a call to \fBpcre_exec()\fP or
59: \fBpcre16_exec()\fP when the end of the subject string is reached successfully,
60: but matching cannot continue because more characters are needed. However, at
61: least one character in the subject must have been inspected. This character
62: need not form part of the final matched string; lookbehind assertions and the
63: \eK escape sequence provide ways of inspecting characters before the start of a
64: matched substring. The requirement for inspecting at least one character exists
65: because an empty string can always be matched; without such a restriction there
66: would always be a partial match of an empty string at the end of the subject.
67: .P
68: If there are at least two slots in the offsets vector when a partial match is
69: returned, the first slot is set to the offset of the earliest character that
70: was inspected. For convenience, the second offset points to the end of the
71: subject so that a substring can easily be identified.
72: .P
73: For the majority of patterns, the first offset identifies the start of the
74: partially matched string. However, for patterns that contain lookbehind
75: assertions, or \eK, or begin with \eb or \eB, earlier characters have been
76: inspected while carrying out the match. For example:
77: .sp
78: /(?<=abc)123/
79: .sp
80: This pattern matches "123", but only if it is preceded by "abc". If the subject
81: string is "xyzabc12", the offsets after a partial match are for the substring
82: "abc12", because all these characters are needed if another match is tried
83: with extra characters added to the subject.
84: .P
85: What happens when a partial match is identified depends on which of the two
86: partial matching options are set.
87: .
88: .
89: .SS "PCRE_PARTIAL_SOFT WITH pcre_exec() OR pcre16_exec()"
90: .rs
91: .sp
92: If PCRE_PARTIAL_SOFT is set when \fBpcre_exec()\fP or \fBpcre16_exec()\fP
93: identifies a partial match, the partial match is remembered, but matching
94: continues as normal, and other alternatives in the pattern are tried. If no
95: complete match can be found, PCRE_ERROR_PARTIAL is returned instead of
96: PCRE_ERROR_NOMATCH.
97: .P
98: This option is "soft" because it prefers a complete match over a partial match.
99: All the various matching items in a pattern behave as if the subject string is
100: potentially complete. For example, \ez, \eZ, and $ match at the end of the
101: subject, as normal, and for \eb and \eB the end of the subject is treated as a
102: non-alphanumeric.
103: .P
104: If there is more than one partial match, the first one that was found provides
105: the data that is returned. Consider this pattern:
106: .sp
107: /123\ew+X|dogY/
108: .sp
109: If this is matched against the subject string "abc123dog", both
110: alternatives fail to match, but the end of the subject is reached during
111: matching, so PCRE_ERROR_PARTIAL is returned. The offsets are set to 3 and 9,
112: identifying "123dog" as the first partial match that was found. (In this
113: example, there are two partial matches, because "dog" on its own partially
114: matches the second alternative.)
115: .
116: .
117: .SS "PCRE_PARTIAL_HARD WITH pcre_exec() OR pcre16_exec()"
118: .rs
119: .sp
120: If PCRE_PARTIAL_HARD is set for \fBpcre_exec()\fP or \fBpcre16_exec()\fP,
121: PCRE_ERROR_PARTIAL is returned as soon as a partial match is found, without
122: continuing to search for possible complete matches. This option is "hard"
123: because it prefers an earlier partial match over a later complete match. For
124: this reason, the assumption is made that the end of the supplied subject string
125: may not be the true end of the available data, and so, if \ez, \eZ, \eb, \eB,
126: or $ are encountered at the end of the subject, the result is
127: PCRE_ERROR_PARTIAL, provided that at least one character in the subject has
128: been inspected.
129: .P
130: Setting PCRE_PARTIAL_HARD also affects the way UTF-8 and UTF-16
131: subject strings are checked for validity. Normally, an invalid sequence
132: causes the error PCRE_ERROR_BADUTF8 or PCRE_ERROR_BADUTF16. However, in the
133: special case of a truncated character at the end of the subject,
134: PCRE_ERROR_SHORTUTF8 or PCRE_ERROR_SHORTUTF16 is returned when
135: PCRE_PARTIAL_HARD is set.
136: .
137: .
138: .SS "Comparing hard and soft partial matching"
139: .rs
140: .sp
141: The difference between the two partial matching options can be illustrated by a
142: pattern such as:
143: .sp
144: /dog(sbody)?/
145: .sp
146: This matches either "dog" or "dogsbody", greedily (that is, it prefers the
147: longer string if possible). If it is matched against the string "dog" with
148: PCRE_PARTIAL_SOFT, it yields a complete match for "dog". However, if
149: PCRE_PARTIAL_HARD is set, the result is PCRE_ERROR_PARTIAL. On the other hand,
150: if the pattern is made ungreedy the result is different:
151: .sp
152: /dog(sbody)??/
153: .sp
154: In this case the result is always a complete match because that is found first,
155: and matching never continues after finding a complete match. It might be easier
156: to follow this explanation by thinking of the two patterns like this:
157: .sp
158: /dog(sbody)?/ is the same as /dogsbody|dog/
159: /dog(sbody)??/ is the same as /dog|dogsbody/
160: .sp
161: The second pattern will never match "dogsbody", because it will always find the
162: shorter match first.
163: .
164: .
165: .SH "PARTIAL MATCHING USING pcre_dfa_exec() OR pcre16_dfa_exec()"
166: .rs
167: .sp
168: The DFA functions move along the subject string character by character, without
169: backtracking, searching for all possible matches simultaneously. If the end of
170: the subject is reached before the end of the pattern, there is the possibility
171: of a partial match, again provided that at least one character has been
172: inspected.
173: .P
174: When PCRE_PARTIAL_SOFT is set, PCRE_ERROR_PARTIAL is returned only if there
175: have been no complete matches. Otherwise, the complete matches are returned.
176: However, if PCRE_PARTIAL_HARD is set, a partial match takes precedence over any
177: complete matches. The portion of the string that was inspected when the longest
178: partial match was found is set as the first matching string, provided there are
179: at least two slots in the offsets vector.
180: .P
181: Because the DFA functions always search for all possible matches, and there is
182: no difference between greedy and ungreedy repetition, their behaviour is
183: different from the standard functions when PCRE_PARTIAL_HARD is set. Consider
184: the string "dog" matched against the ungreedy pattern shown above:
185: .sp
186: /dog(sbody)??/
187: .sp
188: Whereas the standard functions stop as soon as they find the complete match for
189: "dog", the DFA functions also find the partial match for "dogsbody", and so
190: return that when PCRE_PARTIAL_HARD is set.
191: .
192: .
193: .SH "PARTIAL MATCHING AND WORD BOUNDARIES"
194: .rs
195: .sp
196: If a pattern ends with one of sequences \eb or \eB, which test for word
197: boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter-intuitive
198: results. Consider this pattern:
199: .sp
200: /\ebcat\eb/
201: .sp
202: This matches "cat", provided there is a word boundary at either end. If the
203: subject string is "the cat", the comparison of the final "t" with a following
204: character cannot take place, so a partial match is found. However, normal
205: matching carries on, and \eb matches at the end of the subject when the last
206: character is a letter, so a complete match is found. The result, therefore, is
207: \fInot\fP PCRE_ERROR_PARTIAL. Using PCRE_PARTIAL_HARD in this case does yield
208: PCRE_ERROR_PARTIAL, because then the partial match takes precedence.
209: .
210: .
211: .SH "FORMERLY RESTRICTED PATTERNS"
212: .rs
213: .sp
214: For releases of PCRE prior to 8.00, because of the way certain internal
215: optimizations were implemented in the \fBpcre_exec()\fP function, the
216: PCRE_PARTIAL option (predecessor of PCRE_PARTIAL_SOFT) could not be used with
217: all patterns. From release 8.00 onwards, the restrictions no longer apply, and
218: partial matching with can be requested for any pattern.
219: .P
220: Items that were formerly restricted were repeated single characters and
221: repeated metasequences. If PCRE_PARTIAL was set for a pattern that did not
222: conform to the restrictions, \fBpcre_exec()\fP returned the error code
223: PCRE_ERROR_BADPARTIAL (-13). This error code is no longer in use. The
224: PCRE_INFO_OKPARTIAL call to \fBpcre_fullinfo()\fP to find out if a compiled
225: pattern can be used for partial matching now always returns 1.
226: .
227: .
228: .SH "EXAMPLE OF PARTIAL MATCHING USING PCRETEST"
229: .rs
230: .sp
231: If the escape sequence \eP is present in a \fBpcretest\fP data line, the
232: PCRE_PARTIAL_SOFT option is used for the match. Here is a run of \fBpcretest\fP
233: that uses the date example quoted above:
234: .sp
235: re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/
236: data> 25jun04\eP
237: 0: 25jun04
238: 1: jun
239: data> 25dec3\eP
240: Partial match: 23dec3
241: data> 3ju\eP
242: Partial match: 3ju
243: data> 3juj\eP
244: No match
245: data> j\eP
246: No match
247: .sp
248: The first data string is matched completely, so \fBpcretest\fP shows the
249: matched substrings. The remaining four strings do not match the complete
250: pattern, but the first two are partial matches. Similar output is obtained
251: if DFA matching is used.
252: .P
253: If the escape sequence \eP is present more than once in a \fBpcretest\fP data
254: line, the PCRE_PARTIAL_HARD option is set for the match.
255: .
256: .
257: .SH "MULTI-SEGMENT MATCHING WITH pcre_dfa_exec() OR pcre16_dfa_exec()"
258: .rs
259: .sp
260: When a partial match has been found using a DFA matching function, it is
261: possible to continue the match by providing additional subject data and calling
262: the function again with the same compiled regular expression, this time setting
263: the PCRE_DFA_RESTART option. You must pass the same working space as before,
264: because this is where details of the previous partial match are stored. Here is
265: an example using \fBpcretest\fP, using the \eR escape sequence to set the
266: PCRE_DFA_RESTART option (\eD specifies the use of the DFA matching function):
267: .sp
268: re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/
269: data> 23ja\eP\eD
270: Partial match: 23ja
271: data> n05\eR\eD
272: 0: n05
273: .sp
274: The first call has "23ja" as the subject, and requests partial matching; the
275: second call has "n05" as the subject for the continued (restarted) match.
276: Notice that when the match is complete, only the last part is shown; PCRE does
277: not retain the previously partially-matched string. It is up to the calling
278: program to do that if it needs to.
279: .P
280: You can set the PCRE_PARTIAL_SOFT or PCRE_PARTIAL_HARD options with
281: PCRE_DFA_RESTART to continue partial matching over multiple segments. This
282: facility can be used to pass very long subject strings to the DFA matching
283: functions.
284: .
285: .
286: .SH "MULTI-SEGMENT MATCHING WITH pcre_exec() OR pcre16_exec()"
287: .rs
288: .sp
289: From release 8.00, the standard matching functions can also be used to do
290: multi-segment matching. Unlike the DFA functions, it is not possible to
291: restart the previous match with a new segment of data. Instead, new data must
292: be added to the previous subject string, and the entire match re-run, starting
293: from the point where the partial match occurred. Earlier data can be discarded.
294: .P
295: It is best to use PCRE_PARTIAL_HARD in this situation, because it does not
296: treat the end of a segment as the end of the subject when matching \ez, \eZ,
297: \eb, \eB, and $. Consider an unanchored pattern that matches dates:
298: .sp
299: re> /\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed/
300: data> The date is 23ja\eP\eP
301: Partial match: 23ja
302: .sp
303: At this stage, an application could discard the text preceding "23ja", add on
304: text from the next segment, and call the matching function again. Unlike the
305: DFA matching functions, the entire matching string must always be available,
306: and the complete matching process occurs for each call, so more memory and more
307: processing time is needed.
308: .P
309: \fBNote:\fP If the pattern contains lookbehind assertions, or \eK, or starts
310: with \eb or \eB, the string that is returned for a partial match includes
311: characters that precede the partially matched string itself, because these must
312: be retained when adding on more characters for a subsequent matching attempt.
313: However, in some cases you may need to retain even earlier characters, as
314: discussed in the next section.
315: .
316: .
317: .SH "ISSUES WITH MULTI-SEGMENT MATCHING"
318: .rs
319: .sp
320: Certain types of pattern may give problems with multi-segment matching,
321: whichever matching function is used.
322: .P
323: 1. If the pattern contains a test for the beginning of a line, you need to pass
324: the PCRE_NOTBOL option when the subject string for any call does start at the
325: beginning of a line. There is also a PCRE_NOTEOL option, but in practice when
326: doing multi-segment matching you should be using PCRE_PARTIAL_HARD, which
327: includes the effect of PCRE_NOTEOL.
328: .P
329: 2. Lookbehind assertions that have already been obeyed are catered for in the
330: offsets that are returned for a partial match. However a lookbehind assertion
331: later in the pattern could require even earlier characters to be inspected. You
332: can handle this case by using the PCRE_INFO_MAXLOOKBEHIND option of the
333: \fBpcre_fullinfo()\fP or \fBpcre16_fullinfo()\fP functions to obtain the length
334: of the largest lookbehind in the pattern. This length is given in characters,
335: not bytes. If you always retain at least that many characters before the
336: partially matched string, all should be well. (Of course, near the start of the
337: subject, fewer characters may be present; in that case all characters should be
338: retained.)
339: .P
340: 3. Because a partial match must always contain at least one character, what
341: might be considered a partial match of an empty string actually gives a "no
342: match" result. For example:
343: .sp
344: re> /c(?<=abc)x/
345: data> ab\eP
346: No match
347: .sp
348: If the next segment begins "cx", a match should be found, but this will only
349: happen if characters from the previous segment are retained. For this reason, a
350: "no match" result should be interpreted as "partial match of an empty string"
351: when the pattern contains lookbehinds.
352: .P
353: 4. Matching a subject string that is split into multiple segments may not
354: always produce exactly the same result as matching over one single long string,
355: especially when PCRE_PARTIAL_SOFT is used. The section "Partial Matching and
356: Word Boundaries" above describes an issue that arises if the pattern ends with
357: \eb or \eB. Another kind of difference may occur when there are multiple
358: matching possibilities, because (for PCRE_PARTIAL_SOFT) a partial match result
359: is given only when there are no completed matches. This means that as soon as
360: the shortest match has been found, continuation to a new subject segment is no
361: longer possible. Consider again this \fBpcretest\fP example:
362: .sp
363: re> /dog(sbody)?/
364: data> dogsb\eP
365: 0: dog
366: data> do\eP\eD
367: Partial match: do
368: data> gsb\eR\eP\eD
369: 0: g
370: data> dogsbody\eD
371: 0: dogsbody
372: 1: dog
373: .sp
374: The first data line passes the string "dogsb" to a standard matching function,
375: setting the PCRE_PARTIAL_SOFT option. Although the string is a partial match
376: for "dogsbody", the result is not PCRE_ERROR_PARTIAL, because the shorter
377: string "dog" is a complete match. Similarly, when the subject is presented to
378: a DFA matching function in several parts ("do" and "gsb" being the first two)
379: the match stops when "dog" has been found, and it is not possible to continue.
380: On the other hand, if "dogsbody" is presented as a single string, a DFA
381: matching function finds both matches.
382: .P
383: Because of these problems, it is best to use PCRE_PARTIAL_HARD when matching
384: multi-segment data. The example above then behaves differently:
385: .sp
386: re> /dog(sbody)?/
387: data> dogsb\eP\eP
388: Partial match: dogsb
389: data> do\eP\eD
390: Partial match: do
391: data> gsb\eR\eP\eP\eD
392: Partial match: gsb
393: .sp
394: 5. Patterns that contain alternatives at the top level which do not all start
395: with the same pattern item may not work as expected when PCRE_DFA_RESTART is
396: used. For example, consider this pattern:
397: .sp
398: 1234|3789
399: .sp
400: If the first part of the subject is "ABC123", a partial match of the first
401: alternative is found at offset 3. There is no partial match for the second
402: alternative, because such a match does not start at the same point in the
403: subject string. Attempting to continue with the string "7890" does not yield a
404: match because only those alternatives that match at one point in the subject
405: are remembered. The problem arises because the start of the second alternative
406: matches within the first alternative. There is no problem with anchored
407: patterns or patterns such as:
408: .sp
409: 1234|ABCD
410: .sp
411: where no string can be a partial match for both alternatives. This is not a
412: problem if a standard matching function is used, because the entire match has
413: to be rerun each time:
414: .sp
415: re> /1234|3789/
416: data> ABC123\eP\eP
417: Partial match: 123
418: data> 1237890
419: 0: 3789
420: .sp
421: Of course, instead of using PCRE_DFA_RESTART, the same technique of re-running
422: the entire match can also be used with the DFA matching functions. Another
423: possibility is to work with two buffers. If a partial match at offset \fIn\fP
424: in the first buffer is followed by "no match" when PCRE_DFA_RESTART is used on
425: the second buffer, you can then try a new match starting at offset \fIn+1\fP in
426: the first buffer.
427: .
428: .
429: .SH AUTHOR
430: .rs
431: .sp
432: .nf
433: Philip Hazel
434: University Computing Service
435: Cambridge CB2 3QH, England.
436: .fi
437: .
438: .
439: .SH REVISION
440: .rs
441: .sp
442: .nf
443: Last updated: 24 February 2012
444: Copyright (c) 1997-2012 University of Cambridge.
445: .fi
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