Annotation of embedaddon/pcre/doc/pcrepartial.3, revision 1.1.1.1
1.1 misho 1: .TH PCREPARTIAL 3
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
8: \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP matches as far as it goes, but is
9: too short to match the entire pattern, PCRE_ERROR_NOMATCH is returned. There
10: are circumstances where it might be helpful to distinguish this case from other
11: cases in which there is no 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 \fBpcre_exec()\fP or
29: \fBpcre_dfa_exec()\fP. For backwards compatibility, PCRE_PARTIAL is a synonym
30: for PCRE_PARTIAL_SOFT. The essential difference between the two options is
31: whether or not a partial match is preferred to an alternative complete match,
32: though the details differ between the two matching functions. If both options
33: are set, PCRE_PARTIAL_HARD takes precedence.
34: .P
35: Setting a partial matching option for \fBpcre_exec()\fP disables the use of any
36: just-in-time code that was set up by calling \fBpcre_study()\fP with the
37: PCRE_STUDY_JIT_COMPILE option. It also disables two of PCRE's standard
38: optimizations. PCRE remembers the last literal byte in a pattern, and abandons
39: matching immediately if such a byte is not present in the subject string. This
40: optimization cannot be used for a subject string that might match only
41: partially. If the pattern was studied, PCRE knows the minimum length of a
42: matching string, and does not bother to run the matching function on shorter
43: strings. This optimization is also disabled for partial matching.
44: .
45: .
46: .SH "PARTIAL MATCHING USING pcre_exec()"
47: .rs
48: .sp
49: A partial match occurs during a call to \fBpcre_exec()\fP when the end of the
50: subject string is reached successfully, but matching cannot continue because
51: more characters are needed. However, at least one character in the subject must
52: have been inspected. This character need not form part of the final matched
53: string; lookbehind assertions and the \eK escape sequence provide ways of
54: inspecting characters before the start of a matched substring. The requirement
55: for inspecting at least one character exists because an empty string can always
56: be matched; without such a restriction there would always be a partial match of
57: an empty string at the end of the subject.
58: .P
59: If there are at least two slots in the offsets vector when \fBpcre_exec()\fP
60: returns with a partial match, the first slot is set to the offset of the
61: earliest character that was inspected when the partial match was found. For
62: convenience, the second offset points to the end of the subject so that a
63: substring can easily be identified.
64: .P
65: For the majority of patterns, the first offset identifies the start of the
66: partially matched string. However, for patterns that contain lookbehind
67: assertions, or \eK, or begin with \eb or \eB, earlier characters have been
68: inspected while carrying out the match. For example:
69: .sp
70: /(?<=abc)123/
71: .sp
72: This pattern matches "123", but only if it is preceded by "abc". If the subject
73: string is "xyzabc12", the offsets after a partial match are for the substring
74: "abc12", because all these characters are needed if another match is tried
75: with extra characters added to the subject.
76: .P
77: What happens when a partial match is identified depends on which of the two
78: partial matching options are set.
79: .
80: .
81: .SS "PCRE_PARTIAL_SOFT with pcre_exec()"
82: .rs
83: .sp
84: If PCRE_PARTIAL_SOFT is set when \fBpcre_exec()\fP identifies a partial match,
85: the partial match is remembered, but matching continues as normal, and other
86: alternatives in the pattern are tried. If no complete match can be found,
87: \fBpcre_exec()\fP returns PCRE_ERROR_PARTIAL instead of PCRE_ERROR_NOMATCH.
88: .P
89: This option is "soft" because it prefers a complete match over a partial match.
90: All the various matching items in a pattern behave as if the subject string is
91: potentially complete. For example, \ez, \eZ, and $ match at the end of the
92: subject, as normal, and for \eb and \eB the end of the subject is treated as a
93: non-alphanumeric.
94: .P
95: If there is more than one partial match, the first one that was found provides
96: the data that is returned. Consider this pattern:
97: .sp
98: /123\ew+X|dogY/
99: .sp
100: If this is matched against the subject string "abc123dog", both
101: alternatives fail to match, but the end of the subject is reached during
102: matching, so PCRE_ERROR_PARTIAL is returned. The offsets are set to 3 and 9,
103: identifying "123dog" as the first partial match that was found. (In this
104: example, there are two partial matches, because "dog" on its own partially
105: matches the second alternative.)
106: .
107: .
108: .SS "PCRE_PARTIAL_HARD with pcre_exec()"
109: .rs
110: .sp
111: If PCRE_PARTIAL_HARD is set for \fBpcre_exec()\fP, it returns
112: PCRE_ERROR_PARTIAL as soon as a partial match is found, without continuing to
113: search for possible complete matches. This option is "hard" because it prefers
114: an earlier partial match over a later complete match. For this reason, the
115: assumption is made that the end of the supplied subject string may not be the
116: true end of the available data, and so, if \ez, \eZ, \eb, \eB, or $ are
117: encountered at the end of the subject, the result is PCRE_ERROR_PARTIAL.
118: .P
119: Setting PCRE_PARTIAL_HARD also affects the way \fBpcre_exec()\fP checks UTF-8
120: subject strings for validity. Normally, an invalid UTF-8 sequence causes the
121: error PCRE_ERROR_BADUTF8. However, in the special case of a truncated UTF-8
122: character at the end of the subject, PCRE_ERROR_SHORTUTF8 is returned when
123: PCRE_PARTIAL_HARD is set.
124: .
125: .
126: .SS "Comparing hard and soft partial matching"
127: .rs
128: .sp
129: The difference between the two partial matching options can be illustrated by a
130: pattern such as:
131: .sp
132: /dog(sbody)?/
133: .sp
134: This matches either "dog" or "dogsbody", greedily (that is, it prefers the
135: longer string if possible). If it is matched against the string "dog" with
136: PCRE_PARTIAL_SOFT, it yields a complete match for "dog". However, if
137: PCRE_PARTIAL_HARD is set, the result is PCRE_ERROR_PARTIAL. On the other hand,
138: if the pattern is made ungreedy the result is different:
139: .sp
140: /dog(sbody)??/
141: .sp
142: In this case the result is always a complete match because \fBpcre_exec()\fP
143: finds that first, and it never continues after finding a match. It might be
144: easier to follow this explanation by thinking of the two patterns like this:
145: .sp
146: /dog(sbody)?/ is the same as /dogsbody|dog/
147: /dog(sbody)??/ is the same as /dog|dogsbody/
148: .sp
149: The second pattern will never match "dogsbody" when \fBpcre_exec()\fP is
150: used, because it will always find the shorter match first.
151: .
152: .
153: .SH "PARTIAL MATCHING USING pcre_dfa_exec()"
154: .rs
155: .sp
156: The \fBpcre_dfa_exec()\fP function moves along the subject string character by
157: character, without backtracking, searching for all possible matches
158: simultaneously. If the end of the subject is reached before the end of the
159: pattern, there is the possibility of a partial match, again provided that at
160: least one character has been inspected.
161: .P
162: When PCRE_PARTIAL_SOFT is set, PCRE_ERROR_PARTIAL is returned only if there
163: have been no complete matches. Otherwise, the complete matches are returned.
164: However, if PCRE_PARTIAL_HARD is set, a partial match takes precedence over any
165: complete matches. The portion of the string that was inspected when the longest
166: partial match was found is set as the first matching string, provided there are
167: at least two slots in the offsets vector.
168: .P
169: Because \fBpcre_dfa_exec()\fP always searches for all possible matches, and
170: there is no difference between greedy and ungreedy repetition, its behaviour is
171: different from \fBpcre_exec\fP when PCRE_PARTIAL_HARD is set. Consider the
172: string "dog" matched against the ungreedy pattern shown above:
173: .sp
174: /dog(sbody)??/
175: .sp
176: Whereas \fBpcre_exec()\fP stops as soon as it finds the complete match for
177: "dog", \fBpcre_dfa_exec()\fP also finds the partial match for "dogsbody", and
178: so returns that when PCRE_PARTIAL_HARD is set.
179: .
180: .
181: .SH "PARTIAL MATCHING AND WORD BOUNDARIES"
182: .rs
183: .sp
184: If a pattern ends with one of sequences \eb or \eB, which test for word
185: boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter-intuitive
186: results. Consider this pattern:
187: .sp
188: /\ebcat\eb/
189: .sp
190: This matches "cat", provided there is a word boundary at either end. If the
191: subject string is "the cat", the comparison of the final "t" with a following
192: character cannot take place, so a partial match is found. However,
193: \fBpcre_exec()\fP carries on with normal matching, which matches \eb at the end
194: of the subject when the last character is a letter, thus finding a complete
195: match. The result, therefore, is \fInot\fP PCRE_ERROR_PARTIAL. The same thing
196: happens with \fBpcre_dfa_exec()\fP, because it also finds the complete match.
197: .P
198: Using PCRE_PARTIAL_HARD in this case does yield PCRE_ERROR_PARTIAL, because
199: then the partial match takes precedence.
200: .
201: .
202: .SH "FORMERLY RESTRICTED PATTERNS"
203: .rs
204: .sp
205: For releases of PCRE prior to 8.00, because of the way certain internal
206: optimizations were implemented in the \fBpcre_exec()\fP function, the
207: PCRE_PARTIAL option (predecessor of PCRE_PARTIAL_SOFT) could not be used with
208: all patterns. From release 8.00 onwards, the restrictions no longer apply, and
209: partial matching with \fBpcre_exec()\fP can be requested for any pattern.
210: .P
211: Items that were formerly restricted were repeated single characters and
212: repeated metasequences. If PCRE_PARTIAL was set for a pattern that did not
213: conform to the restrictions, \fBpcre_exec()\fP returned the error code
214: PCRE_ERROR_BADPARTIAL (-13). This error code is no longer in use. The
215: PCRE_INFO_OKPARTIAL call to \fBpcre_fullinfo()\fP to find out if a compiled
216: pattern can be used for partial matching now always returns 1.
217: .
218: .
219: .SH "EXAMPLE OF PARTIAL MATCHING USING PCRETEST"
220: .rs
221: .sp
222: If the escape sequence \eP is present in a \fBpcretest\fP data line, the
223: PCRE_PARTIAL_SOFT option is used for the match. Here is a run of \fBpcretest\fP
224: that uses the date example quoted above:
225: .sp
226: re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/
227: data> 25jun04\eP
228: 0: 25jun04
229: 1: jun
230: data> 25dec3\eP
231: Partial match: 23dec3
232: data> 3ju\eP
233: Partial match: 3ju
234: data> 3juj\eP
235: No match
236: data> j\eP
237: No match
238: .sp
239: The first data string is matched completely, so \fBpcretest\fP shows the
240: matched substrings. The remaining four strings do not match the complete
241: pattern, but the first two are partial matches. Similar output is obtained
242: when \fBpcre_dfa_exec()\fP is used.
243: .P
244: If the escape sequence \eP is present more than once in a \fBpcretest\fP data
245: line, the PCRE_PARTIAL_HARD option is set for the match.
246: .
247: .
248: .SH "MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()"
249: .rs
250: .sp
251: When a partial match has been found using \fBpcre_dfa_exec()\fP, it is possible
252: to continue the match by providing additional subject data and calling
253: \fBpcre_dfa_exec()\fP again with the same compiled regular expression, this
254: time setting the PCRE_DFA_RESTART option. You must pass the same working
255: space as before, because this is where details of the previous partial match
256: are stored. Here is an example using \fBpcretest\fP, using the \eR escape
257: sequence to set the PCRE_DFA_RESTART option (\eD specifies the use of
258: \fBpcre_dfa_exec()\fP):
259: .sp
260: re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/
261: data> 23ja\eP\eD
262: Partial match: 23ja
263: data> n05\eR\eD
264: 0: n05
265: .sp
266: The first call has "23ja" as the subject, and requests partial matching; the
267: second call has "n05" as the subject for the continued (restarted) match.
268: Notice that when the match is complete, only the last part is shown; PCRE does
269: not retain the previously partially-matched string. It is up to the calling
270: program to do that if it needs to.
271: .P
272: You can set the PCRE_PARTIAL_SOFT or PCRE_PARTIAL_HARD options with
273: PCRE_DFA_RESTART to continue partial matching over multiple segments. This
274: facility can be used to pass very long subject strings to
275: \fBpcre_dfa_exec()\fP.
276: .
277: .
278: .SH "MULTI-SEGMENT MATCHING WITH pcre_exec()"
279: .rs
280: .sp
281: From release 8.00, \fBpcre_exec()\fP can also be used to do multi-segment
282: matching. Unlike \fBpcre_dfa_exec()\fP, it is not possible to restart the
283: previous match with a new segment of data. Instead, new data must be added to
284: the previous subject string, and the entire match re-run, starting from the
285: point where the partial match occurred. Earlier data can be discarded. It is
286: best to use PCRE_PARTIAL_HARD in this situation, because it does not treat the
287: end of a segment as the end of the subject when matching \ez, \eZ, \eb, \eB,
288: and $. Consider an unanchored pattern that matches dates:
289: .sp
290: re> /\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed/
291: data> The date is 23ja\eP\eP
292: Partial match: 23ja
293: .sp
294: At this stage, an application could discard the text preceding "23ja", add on
295: text from the next segment, and call \fBpcre_exec()\fP again. Unlike
296: \fBpcre_dfa_exec()\fP, the entire matching string must always be available, and
297: the complete matching process occurs for each call, so more memory and more
298: processing time is needed.
299: .P
300: \fBNote:\fP If the pattern contains lookbehind assertions, or \eK, or starts
301: with \eb or \eB, the string that is returned for a partial match will include
302: characters that precede the partially matched string itself, because these must
303: be retained when adding on more characters for a subsequent matching attempt.
304: .
305: .
306: .SH "ISSUES WITH MULTI-SEGMENT MATCHING"
307: .rs
308: .sp
309: Certain types of pattern may give problems with multi-segment matching,
310: whichever matching function is used.
311: .P
312: 1. If the pattern contains a test for the beginning of a line, you need to pass
313: the PCRE_NOTBOL option when the subject string for any call does start at the
314: beginning of a line. There is also a PCRE_NOTEOL option, but in practice when
315: doing multi-segment matching you should be using PCRE_PARTIAL_HARD, which
316: includes the effect of PCRE_NOTEOL.
317: .P
318: 2. Lookbehind assertions at the start of a pattern are catered for in the
319: offsets that are returned for a partial match. However, in theory, a lookbehind
320: assertion later in the pattern could require even earlier characters to be
321: inspected, and it might not have been reached when a partial match occurs. This
322: is probably an extremely unlikely case; you could guard against it to a certain
323: extent by always including extra characters at the start.
324: .P
325: 3. Matching a subject string that is split into multiple segments may not
326: always produce exactly the same result as matching over one single long string,
327: especially when PCRE_PARTIAL_SOFT is used. The section "Partial Matching and
328: Word Boundaries" above describes an issue that arises if the pattern ends with
329: \eb or \eB. Another kind of difference may occur when there are multiple
330: matching possibilities, because (for PCRE_PARTIAL_SOFT) a partial match result
331: is given only when there are no completed matches. This means that as soon as
332: the shortest match has been found, continuation to a new subject segment is no
333: longer possible. Consider again this \fBpcretest\fP example:
334: .sp
335: re> /dog(sbody)?/
336: data> dogsb\eP
337: 0: dog
338: data> do\eP\eD
339: Partial match: do
340: data> gsb\eR\eP\eD
341: 0: g
342: data> dogsbody\eD
343: 0: dogsbody
344: 1: dog
345: .sp
346: The first data line passes the string "dogsb" to \fBpcre_exec()\fP, setting the
347: PCRE_PARTIAL_SOFT option. Although the string is a partial match for
348: "dogsbody", the result is not PCRE_ERROR_PARTIAL, because the shorter string
349: "dog" is a complete match. Similarly, when the subject is presented to
350: \fBpcre_dfa_exec()\fP in several parts ("do" and "gsb" being the first two) the
351: match stops when "dog" has been found, and it is not possible to continue. On
352: the other hand, if "dogsbody" is presented as a single string,
353: \fBpcre_dfa_exec()\fP finds both matches.
354: .P
355: Because of these problems, it is best to use PCRE_PARTIAL_HARD when matching
356: multi-segment data. The example above then behaves differently:
357: .sp
358: re> /dog(sbody)?/
359: data> dogsb\eP\eP
360: Partial match: dogsb
361: data> do\eP\eD
362: Partial match: do
363: data> gsb\eR\eP\eP\eD
364: Partial match: gsb
365: .sp
366: 4. Patterns that contain alternatives at the top level which do not all
367: start with the same pattern item may not work as expected when
368: PCRE_DFA_RESTART is used with \fBpcre_dfa_exec()\fP. For example, consider this
369: pattern:
370: .sp
371: 1234|3789
372: .sp
373: If the first part of the subject is "ABC123", a partial match of the first
374: alternative is found at offset 3. There is no partial match for the second
375: alternative, because such a match does not start at the same point in the
376: subject string. Attempting to continue with the string "7890" does not yield a
377: match because only those alternatives that match at one point in the subject
378: are remembered. The problem arises because the start of the second alternative
379: matches within the first alternative. There is no problem with anchored
380: patterns or patterns such as:
381: .sp
382: 1234|ABCD
383: .sp
384: where no string can be a partial match for both alternatives. This is not a
385: problem if \fBpcre_exec()\fP is used, because the entire match has to be rerun
386: each time:
387: .sp
388: re> /1234|3789/
389: data> ABC123\eP\eP
390: Partial match: 123
391: data> 1237890
392: 0: 3789
393: .sp
394: Of course, instead of using PCRE_DFA_RESTART, the same technique of re-running
395: the entire match can also be used with \fBpcre_dfa_exec()\fP. Another
396: possibility is to work with two buffers. If a partial match at offset \fIn\fP
397: in the first buffer is followed by "no match" when PCRE_DFA_RESTART is used on
398: the second buffer, you can then try a new match starting at offset \fIn+1\fP in
399: the first buffer.
400: .
401: .
402: .SH AUTHOR
403: .rs
404: .sp
405: .nf
406: Philip Hazel
407: University Computing Service
408: Cambridge CB2 3QH, England.
409: .fi
410: .
411: .
412: .SH REVISION
413: .rs
414: .sp
415: .nf
416: Last updated: 26 August 2011
417: Copyright (c) 1997-2011 University of Cambridge.
418: .fi
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