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Revision 716 - (show annotations) (download)
Tue Oct 4 16:38:05 2011 UTC (3 years, 2 months ago) by ph10
File size: 19827 byte(s)
Make (*THEN) work as in Perl in subpatterns that do not contain | alternatives.

1 Technical Notes about PCRE
2 --------------------------
3
4 These are very rough technical notes that record potentially useful information
5 about PCRE internals. For information about testing PCRE, see the pcretest
6 documentation and the comment at the head of the RunTest file.
7
8
9 Historical note 1
10 -----------------
11
12 Many years ago I implemented some regular expression functions to an algorithm
13 suggested by Martin Richards. These were not Unix-like in form, and were quite
14 restricted in what they could do by comparison with Perl. The interesting part
15 about the algorithm was that the amount of space required to hold the compiled
16 form of an expression was known in advance. The code to apply an expression did
17 not operate by backtracking, as the original Henry Spencer code and current
18 Perl code does, but instead checked all possibilities simultaneously by keeping
19 a list of current states and checking all of them as it advanced through the
20 subject string. In the terminology of Jeffrey Friedl's book, it was a "DFA
21 algorithm", though it was not a traditional Finite State Machine (FSM). When
22 the pattern was all used up, all remaining states were possible matches, and
23 the one matching the longest subset of the subject string was chosen. This did
24 not necessarily maximize the individual wild portions of the pattern, as is
25 expected in Unix and Perl-style regular expressions.
26
27
28 Historical note 2
29 -----------------
30
31 By contrast, the code originally written by Henry Spencer (which was
32 subsequently heavily modified for Perl) compiles the expression twice: once in
33 a dummy mode in order to find out how much store will be needed, and then for
34 real. (The Perl version probably doesn't do this any more; I'm talking about
35 the original library.) The execution function operates by backtracking and
36 maximizing (or, optionally, minimizing in Perl) the amount of the subject that
37 matches individual wild portions of the pattern. This is an "NFA algorithm" in
38 Friedl's terminology.
39
40
41 OK, here's the real stuff
42 -------------------------
43
44 For the set of functions that form the "basic" PCRE library (which are
45 unrelated to those mentioned above), I tried at first to invent an algorithm
46 that used an amount of store bounded by a multiple of the number of characters
47 in the pattern, to save on compiling time. However, because of the greater
48 complexity in Perl regular expressions, I couldn't do this. In any case, a
49 first pass through the pattern is helpful for other reasons.
50
51
52 Computing the memory requirement: how it was
53 --------------------------------------------
54
55 Up to and including release 6.7, PCRE worked by running a very degenerate first
56 pass to calculate a maximum store size, and then a second pass to do the real
57 compile - which might use a bit less than the predicted amount of memory. The
58 idea was that this would turn out faster than the Henry Spencer code because
59 the first pass is degenerate and the second pass can just store stuff straight
60 into the vector, which it knows is big enough.
61
62
63 Computing the memory requirement: how it is
64 -------------------------------------------
65
66 By the time I was working on a potential 6.8 release, the degenerate first pass
67 had become very complicated and hard to maintain. Indeed one of the early
68 things I did for 6.8 was to fix Yet Another Bug in the memory computation. Then
69 I had a flash of inspiration as to how I could run the real compile function in
70 a "fake" mode that enables it to compute how much memory it would need, while
71 actually only ever using a few hundred bytes of working memory, and without too
72 many tests of the mode that might slow it down. So I refactored the compiling
73 functions to work this way. This got rid of about 600 lines of source. It
74 should make future maintenance and development easier. As this was such a major
75 change, I never released 6.8, instead upping the number to 7.0 (other quite
76 major changes were also present in the 7.0 release).
77
78 A side effect of this work was that the previous limit of 200 on the nesting
79 depth of parentheses was removed. However, there is a downside: pcre_compile()
80 runs more slowly than before (30% or more, depending on the pattern) because it
81 is doing a full analysis of the pattern. My hope was that this would not be a
82 big issue, and in the event, nobody has commented on it.
83
84
85 Traditional matching function
86 -----------------------------
87
88 The "traditional", and original, matching function is called pcre_exec(), and
89 it implements an NFA algorithm, similar to the original Henry Spencer algorithm
90 and the way that Perl works. This is not surprising, since it is intended to be
91 as compatible with Perl as possible. This is the function most users of PCRE
92 will use most of the time. From release 8.20, if PCRE is compiled with
93 just-in-time (JIT) support, and studying a compiled pattern with JIT is
94 successful, the JIT code is run instead of the normal pcre_exec() code, but the
95 result is the same.
96
97
98 Supplementary matching function
99 -------------------------------
100
101 From PCRE 6.0, there is also a supplementary matching function called
102 pcre_dfa_exec(). This implements a DFA matching algorithm that searches
103 simultaneously for all possible matches that start at one point in the subject
104 string. (Going back to my roots: see Historical Note 1 above.) This function
105 intreprets the same compiled pattern data as pcre_exec(); however, not all the
106 facilities are available, and those that are do not always work in quite the
107 same way. See the user documentation for details.
108
109 The algorithm that is used for pcre_dfa_exec() is not a traditional FSM,
110 because it may have a number of states active at one time. More work would be
111 needed at compile time to produce a traditional FSM where only one state is
112 ever active at once. I believe some other regex matchers work this way.
113
114
115 Changeable options
116 ------------------
117
118 The /i, /m, or /s options (PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL) may
119 change in the middle of patterns. From PCRE 8.13, their processing is handled
120 entirely at compile time by generating different opcodes for the different
121 settings. The runtime functions do not need to keep track of an options state
122 any more.
123
124
125 Format of compiled patterns
126 ---------------------------
127
128 The compiled form of a pattern is a vector of bytes, containing items of
129 variable length. The first byte in an item is an opcode, and the length of the
130 item is either implicit in the opcode or contained in the data bytes that
131 follow it.
132
133 In many cases below LINK_SIZE data values are specified for offsets within the
134 compiled pattern. The default value for LINK_SIZE is 2, but PCRE can be
135 compiled to use 3-byte or 4-byte values for these offsets (impairing the
136 performance). This is necessary only when patterns whose compiled length is
137 greater than 64K are going to be processed. In this description, we assume the
138 "normal" compilation options. Data values that are counts (e.g. for
139 quantifiers) are always just two bytes long.
140
141 Opcodes with no following data
142 ------------------------------
143
144 These items are all just one byte long
145
146 OP_END end of pattern
147 OP_ANY match any one character other than newline
148 OP_ALLANY match any one character, including newline
149 OP_ANYBYTE match any single byte, even in UTF-8 mode
150 OP_SOD match start of data: \A
151 OP_SOM, start of match (subject + offset): \G
152 OP_SET_SOM, set start of match (\K)
153 OP_CIRC ^ (start of data)
154 OP_CIRCM ^ multiline mode (start of data or after newline)
155 OP_NOT_WORD_BOUNDARY \W
156 OP_WORD_BOUNDARY \w
157 OP_NOT_DIGIT \D
158 OP_DIGIT \d
159 OP_NOT_HSPACE \H
160 OP_HSPACE \h
161 OP_NOT_WHITESPACE \S
162 OP_WHITESPACE \s
163 OP_NOT_VSPACE \V
164 OP_VSPACE \v
165 OP_NOT_WORDCHAR \W
166 OP_WORDCHAR \w
167 OP_EODN match end of data or \n at end: \Z
168 OP_EOD match end of data: \z
169 OP_DOLL $ (end of data, or before final newline)
170 OP_DOLLM $ multiline mode (end of data or before newline)
171 OP_EXTUNI match an extended Unicode character
172 OP_ANYNL match any Unicode newline sequence
173
174 OP_ACCEPT ) These are Perl 5.10's "backtracking control
175 OP_COMMIT ) verbs". If OP_ACCEPT is inside capturing
176 OP_FAIL ) parentheses, it may be preceded by one or more
177 OP_PRUNE ) OP_CLOSE, followed by a 2-byte number,
178 OP_SKIP ) indicating which parentheses must be closed.
179
180
181 Backtracking control verbs with (optional) data
182 -----------------------------------------------
183
184 (*THEN) without an argument generates the opcode OP_THEN and no following data.
185 OP_MARK is followed by the mark name, preceded by a one-byte length, and
186 followed by a binary zero. For (*PRUNE), (*SKIP), and (*THEN) with arguments,
187 the opcodes OP_PRUNE_ARG, OP_SKIP_ARG, and OP_THEN_ARG are used, with the name
188 following in the same format.
189
190
191 Matching literal characters
192 ---------------------------
193
194 The OP_CHAR opcode is followed by a single character that is to be matched
195 casefully. For caseless matching, OP_CHARI is used. In UTF-8 mode, the
196 character may be more than one byte long. (Earlier versions of PCRE used
197 multi-character strings, but this was changed to allow some new features to be
198 added.)
199
200
201 Repeating single characters
202 ---------------------------
203
204 The common repeats (*, +, ?) when applied to a single character use the
205 following opcodes, which come in caseful and caseless versions:
206
207 Caseful Caseless
208 OP_STAR OP_STARI
209 OP_MINSTAR OP_MINSTARI
210 OP_POSSTAR OP_POSSTARI
211 OP_PLUS OP_PLUSI
212 OP_MINPLUS OP_MINPLUSI
213 OP_POSPLUS OP_POSPLUSI
214 OP_QUERY OP_QUERYI
215 OP_MINQUERY OP_MINQUERYI
216 OP_POSQUERY OP_POSQUERYI
217
218 In ASCII mode, these are two-byte items; in UTF-8 mode, the length is variable.
219 Those with "MIN" in their name are the minimizing versions. Those with "POS" in
220 their names are possessive versions. Each is followed by the character that is
221 to be repeated. Other repeats make use of these opcodes:
222
223 Caseful Caseless
224 OP_UPTO OP_UPTOI
225 OP_MINUPTO OP_MINUPTOI
226 OP_POSUPTO OP_POSUPTOI
227 OP_EXACT OP_EXACTI
228
229 Each of these is followed by a two-byte count (most significant first) and the
230 repeated character. OP_UPTO matches from 0 to the given number. A repeat with a
231 non-zero minimum and a fixed maximum is coded as an OP_EXACT followed by an
232 OP_UPTO (or OP_MINUPTO or OPT_POSUPTO).
233
234
235 Repeating character types
236 -------------------------
237
238 Repeats of things like \d are done exactly as for single characters, except
239 that instead of a character, the opcode for the type is stored in the data
240 byte. The opcodes are:
241
242 OP_TYPESTAR
243 OP_TYPEMINSTAR
244 OP_TYPEPOSSTAR
245 OP_TYPEPLUS
246 OP_TYPEMINPLUS
247 OP_TYPEPOSPLUS
248 OP_TYPEQUERY
249 OP_TYPEMINQUERY
250 OP_TYPEPOSQUERY
251 OP_TYPEUPTO
252 OP_TYPEMINUPTO
253 OP_TYPEPOSUPTO
254 OP_TYPEEXACT
255
256
257 Match by Unicode property
258 -------------------------
259
260 OP_PROP and OP_NOTPROP are used for positive and negative matches of a
261 character by testing its Unicode property (the \p and \P escape sequences).
262 Each is followed by two bytes that encode the desired property as a type and a
263 value.
264
265 Repeats of these items use the OP_TYPESTAR etc. set of opcodes, followed by
266 three bytes: OP_PROP or OP_NOTPROP and then the desired property type and
267 value.
268
269
270 Character classes
271 -----------------
272
273 If there is only one character, OP_CHAR or OP_CHARI is used for a positive
274 class, and OP_NOT or OP_NOTI for a negative one (that is, for something like
275 [^a]). However, in UTF-8 mode, the use of OP_NOT[I] applies only to characters
276 with values < 128, because OP_NOT[I] is confined to single bytes.
277
278 Another set of 13 repeating opcodes (called OP_NOTSTAR etc.) are used for a
279 repeated, negated, single-character class. The normal single-character opcodes
280 (OP_STAR, etc.) are used for a repeated positive single-character class.
281
282 When there is more than one character in a class and all the characters are
283 less than 256, OP_CLASS is used for a positive class, and OP_NCLASS for a
284 negative one. In either case, the opcode is followed by a 32-byte bit map
285 containing a 1 bit for every character that is acceptable. The bits are counted
286 from the least significant end of each byte. In caseless mode, bits for both
287 cases are set.
288
289 The reason for having both OP_CLASS and OP_NCLASS is so that, in UTF-8 mode,
290 subject characters with values greater than 256 can be handled correctly. For
291 OP_CLASS they do not match, whereas for OP_NCLASS they do.
292
293 For classes containing characters with values > 255, OP_XCLASS is used. It
294 optionally uses a bit map (if any characters lie within it), followed by a list
295 of pairs (for a range) and single characters. In caseless mode, both cases are
296 explicitly listed. There is a flag character than indicates whether it is a
297 positive or a negative class.
298
299
300 Back references
301 ---------------
302
303 OP_REF (caseful) or OP_REFI (caseless) is followed by two bytes containing the
304 reference number.
305
306
307 Repeating character classes and back references
308 -----------------------------------------------
309
310 Single-character classes are handled specially (see above). This section
311 applies to OP_CLASS and OP_REF[I]. In both cases, the repeat information
312 follows the base item. The matching code looks at the following opcode to see
313 if it is one of
314
315 OP_CRSTAR
316 OP_CRMINSTAR
317 OP_CRPLUS
318 OP_CRMINPLUS
319 OP_CRQUERY
320 OP_CRMINQUERY
321 OP_CRRANGE
322 OP_CRMINRANGE
323
324 All but the last two are just single-byte items. The others are followed by
325 four bytes of data, comprising the minimum and maximum repeat counts. There are
326 no special possessive opcodes for these repeats; a possessive repeat is
327 compiled into an atomic group.
328
329
330 Brackets and alternation
331 ------------------------
332
333 A pair of non-capturing (round) brackets is wrapped round each expression at
334 compile time, so alternation always happens in the context of brackets.
335
336 [Note for North Americans: "bracket" to some English speakers, including
337 myself, can be round, square, curly, or pointy. Hence this usage.]
338
339 Non-capturing brackets use the opcode OP_BRA. Originally PCRE was limited to 99
340 capturing brackets and it used a different opcode for each one. From release
341 3.5, the limit was removed by putting the bracket number into the data for
342 higher-numbered brackets. From release 7.0 all capturing brackets are handled
343 this way, using the single opcode OP_CBRA.
344
345 A bracket opcode is followed by LINK_SIZE bytes which give the offset to the
346 next alternative OP_ALT or, if there aren't any branches, to the matching
347 OP_KET opcode. Each OP_ALT is followed by LINK_SIZE bytes giving the offset to
348 the next one, or to the OP_KET opcode. For capturing brackets, the bracket
349 number immediately follows the offset, always as a 2-byte item.
350
351 OP_KET is used for subpatterns that do not repeat indefinitely, while
352 OP_KETRMIN and OP_KETRMAX are used for indefinite repetitions, minimally or
353 maximally respectively (see below for possessive repetitions). All three are
354 followed by LINK_SIZE bytes giving (as a positive number) the offset back to
355 the matching bracket opcode.
356
357 If a subpattern is quantified such that it is permitted to match zero times, it
358 is preceded by one of OP_BRAZERO, OP_BRAMINZERO, or OP_SKIPZERO. These are
359 single-byte opcodes that tell the matcher that skipping the following
360 subpattern entirely is a valid branch. In the case of the first two, not
361 skipping the pattern is also valid (greedy and non-greedy). The third is used
362 when a pattern has the quantifier {0,0}. It cannot be entirely discarded,
363 because it may be called as a subroutine from elsewhere in the regex.
364
365 A subpattern with an indefinite maximum repetition is replicated in the
366 compiled data its minimum number of times (or once with OP_BRAZERO if the
367 minimum is zero), with the final copy terminating with OP_KETRMIN or OP_KETRMAX
368 as appropriate.
369
370 A subpattern with a bounded maximum repetition is replicated in a nested
371 fashion up to the maximum number of times, with OP_BRAZERO or OP_BRAMINZERO
372 before each replication after the minimum, so that, for example, (abc){2,5} is
373 compiled as (abc)(abc)((abc)((abc)(abc)?)?)?, except that each bracketed group
374 has the same number.
375
376 When a repeated subpattern has an unbounded upper limit, it is checked to see
377 whether it could match an empty string. If this is the case, the opcode in the
378 final replication is changed to OP_SBRA or OP_SCBRA. This tells the matcher
379 that it needs to check for matching an empty string when it hits OP_KETRMIN or
380 OP_KETRMAX, and if so, to break the loop.
381
382 Possessive brackets
383 -------------------
384
385 When a repeated group (capturing or non-capturing) is marked as possessive by
386 the "+" notation, e.g. (abc)++, different opcodes are used. Their names all
387 have POS on the end, e.g. OP_BRAPOS instead of OP_BRA and OP_SCPBRPOS instead
388 of OP_SCBRA. The end of such a group is marked by OP_KETRPOS. If the minimum
389 repetition is zero, the group is preceded by OP_BRAPOSZERO.
390
391
392 Assertions
393 ----------
394
395 Forward assertions are just like other subpatterns, but starting with one of
396 the opcodes OP_ASSERT or OP_ASSERT_NOT. Backward assertions use the opcodes
397 OP_ASSERTBACK and OP_ASSERTBACK_NOT, and the first opcode inside the assertion
398 is OP_REVERSE, followed by a two byte count of the number of characters to move
399 back the pointer in the subject string. When operating in UTF-8 mode, the count
400 is a character count rather than a byte count. A separate count is present in
401 each alternative of a lookbehind assertion, allowing them to have different
402 fixed lengths.
403
404
405 Once-only (atomic) subpatterns
406 ------------------------------
407
408 These are also just like other subpatterns, but they start with the opcode
409 OP_ONCE. The check for matching an empty string in an unbounded repeat is
410 handled entirely at runtime, so there is just this one opcode.
411
412
413 Conditional subpatterns
414 -----------------------
415
416 These are like other subpatterns, but they start with the opcode OP_COND, or
417 OP_SCOND for one that might match an empty string in an unbounded repeat. If
418 the condition is a back reference, this is stored at the start of the
419 subpattern using the opcode OP_CREF followed by two bytes containing the
420 reference number. OP_NCREF is used instead if the reference was generated by
421 name (so that the runtime code knows to check for duplicate names).
422
423 If the condition is "in recursion" (coded as "(?(R)"), or "in recursion of
424 group x" (coded as "(?(Rx)"), the group number is stored at the start of the
425 subpattern using the opcode OP_RREF or OP_NRREF (cf OP_NCREF), and a value of
426 zero for "the whole pattern". For a DEFINE condition, just the single byte
427 OP_DEF is used (it has no associated data). Otherwise, a conditional subpattern
428 always starts with one of the assertions.
429
430
431 Recursion
432 ---------
433
434 Recursion either matches the current regex, or some subexpression. The opcode
435 OP_RECURSE is followed by an value which is the offset to the starting bracket
436 from the start of the whole pattern. From release 6.5, OP_RECURSE is
437 automatically wrapped inside OP_ONCE brackets (because otherwise some patterns
438 broke it). OP_RECURSE is also used for "subroutine" calls, even though they
439 are not strictly a recursion.
440
441
442 Callout
443 -------
444
445 OP_CALLOUT is followed by one byte of data that holds a callout number in the
446 range 0 to 254 for manual callouts, or 255 for an automatic callout. In both
447 cases there follows a two-byte value giving the offset in the pattern to the
448 start of the following item, and another two-byte item giving the length of the
449 next item.
450
451
452 Philip Hazel
453 October 2011

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