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docs: Put correction version in which the 32-bit library appeared
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 Support for 16-bit and 32-bit data strings
53 -------------------------------------------
54
55 From release 8.30, PCRE supports 16-bit as well as 8-bit data strings; and from
56 release 8.32, PCRE supports 32-bit data strings. The library can be compiled
57 in any combination of 8-bit, 16-bit or 32-bit modes, creating different
58 libraries. In the description that follows, the word "short" is
59 used for a 16-bit data quantity, and the word "unit" is used for a quantity
60 that is a byte in 8-bit mode, a short in 16-bit mode and a 32-bit unsigned
61 integer in 32-bit mode. However, so as not to over-complicate the text, the
62 names of PCRE functions are given in 8-bit form only.
63
64
65 Computing the memory requirement: how it was
66 --------------------------------------------
67
68 Up to and including release 6.7, PCRE worked by running a very degenerate first
69 pass to calculate a maximum store size, and then a second pass to do the real
70 compile - which might use a bit less than the predicted amount of memory. The
71 idea was that this would turn out faster than the Henry Spencer code because
72 the first pass is degenerate and the second pass can just store stuff straight
73 into the vector, which it knows is big enough.
74
75
76 Computing the memory requirement: how it is
77 -------------------------------------------
78
79 By the time I was working on a potential 6.8 release, the degenerate first pass
80 had become very complicated and hard to maintain. Indeed one of the early
81 things I did for 6.8 was to fix Yet Another Bug in the memory computation. Then
82 I had a flash of inspiration as to how I could run the real compile function in
83 a "fake" mode that enables it to compute how much memory it would need, while
84 actually only ever using a few hundred bytes of working memory, and without too
85 many tests of the mode that might slow it down. So I refactored the compiling
86 functions to work this way. This got rid of about 600 lines of source. It
87 should make future maintenance and development easier. As this was such a major
88 change, I never released 6.8, instead upping the number to 7.0 (other quite
89 major changes were also present in the 7.0 release).
90
91 A side effect of this work was that the previous limit of 200 on the nesting
92 depth of parentheses was removed. However, there is a downside: pcre_compile()
93 runs more slowly than before (30% or more, depending on the pattern) because it
94 is doing a full analysis of the pattern. My hope was that this would not be a
95 big issue, and in the event, nobody has commented on it.
96
97
98 Traditional matching function
99 -----------------------------
100
101 The "traditional", and original, matching function is called pcre_exec(), and
102 it implements an NFA algorithm, similar to the original Henry Spencer algorithm
103 and the way that Perl works. This is not surprising, since it is intended to be
104 as compatible with Perl as possible. This is the function most users of PCRE
105 will use most of the time. From release 8.20, if PCRE is compiled with
106 just-in-time (JIT) support, and studying a compiled pattern with JIT is
107 successful, the JIT code is run instead of the normal pcre_exec() code, but the
108 result is the same.
109
110
111 Supplementary matching function
112 -------------------------------
113
114 From PCRE 6.0, there is also a supplementary matching function called
115 pcre_dfa_exec(). This implements a DFA matching algorithm that searches
116 simultaneously for all possible matches that start at one point in the subject
117 string. (Going back to my roots: see Historical Note 1 above.) This function
118 intreprets the same compiled pattern data as pcre_exec(); however, not all the
119 facilities are available, and those that are do not always work in quite the
120 same way. See the user documentation for details.
121
122 The algorithm that is used for pcre_dfa_exec() is not a traditional FSM,
123 because it may have a number of states active at one time. More work would be
124 needed at compile time to produce a traditional FSM where only one state is
125 ever active at once. I believe some other regex matchers work this way.
126
127
128 Changeable options
129 ------------------
130
131 The /i, /m, or /s options (PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL) may
132 change in the middle of patterns. From PCRE 8.13, their processing is handled
133 entirely at compile time by generating different opcodes for the different
134 settings. The runtime functions do not need to keep track of an options state
135 any more.
136
137
138 Format of compiled patterns
139 ---------------------------
140
141 The compiled form of a pattern is a vector of units (bytes in 8-bit mode, or
142 shorts in 16-bit mode, 32-bit unsigned integers in 32-bit mode), containing
143 items of variable length. The first unit in an item contains an opcode, and
144 the length of the item is either implicit in the opcode or contained in the
145 data that follows it.
146
147 In many cases listed below, LINK_SIZE data values are specified for offsets
148 within the compiled pattern. LINK_SIZE always specifies a number of bytes. The
149 default value for LINK_SIZE is 2, but PCRE can be compiled to use 3-byte or
150 4-byte values for these offsets, although this impairs the performance. (3-byte
151 LINK_SIZE values are available only in 8-bit mode.) Specifing a LINK_SIZE
152 larger than 2 is necessary only when patterns whose compiled length is greater
153 than 64K are going to be processed. In this description, we assume the "normal"
154 compilation options. Data values that are counts (e.g. for quantifiers) are
155 always just two bytes long (one short in 16-bit mode).
156
157 Opcodes with no following data
158 ------------------------------
159
160 These items are all just one unit long
161
162 OP_END end of pattern
163 OP_ANY match any one character other than newline
164 OP_ALLANY match any one character, including newline
165 OP_ANYBYTE match any single byte, even in UTF-8 mode
166 OP_SOD match start of data: \A
167 OP_SOM, start of match (subject + offset): \G
168 OP_SET_SOM, set start of match (\K)
169 OP_CIRC ^ (start of data)
170 OP_CIRCM ^ multiline mode (start of data or after newline)
171 OP_NOT_WORD_BOUNDARY \W
172 OP_WORD_BOUNDARY \w
173 OP_NOT_DIGIT \D
174 OP_DIGIT \d
175 OP_NOT_HSPACE \H
176 OP_HSPACE \h
177 OP_NOT_WHITESPACE \S
178 OP_WHITESPACE \s
179 OP_NOT_VSPACE \V
180 OP_VSPACE \v
181 OP_NOT_WORDCHAR \W
182 OP_WORDCHAR \w
183 OP_EODN match end of data or \n at end: \Z
184 OP_EOD match end of data: \z
185 OP_DOLL $ (end of data, or before final newline)
186 OP_DOLLM $ multiline mode (end of data or before newline)
187 OP_EXTUNI match an extended Unicode character
188 OP_ANYNL match any Unicode newline sequence
189
190 OP_ACCEPT ) These are Perl 5.10's "backtracking control
191 OP_COMMIT ) verbs". If OP_ACCEPT is inside capturing
192 OP_FAIL ) parentheses, it may be preceded by one or more
193 OP_PRUNE ) OP_CLOSE, followed by a 2-byte number,
194 OP_SKIP ) indicating which parentheses must be closed.
195
196
197 Backtracking control verbs with (optional) data
198 -----------------------------------------------
199
200 (*THEN) without an argument generates the opcode OP_THEN and no following data.
201 OP_MARK is followed by the mark name, preceded by a one-unit length, and
202 followed by a binary zero. For (*PRUNE), (*SKIP), and (*THEN) with arguments,
203 the opcodes OP_PRUNE_ARG, OP_SKIP_ARG, and OP_THEN_ARG are used, with the name
204 following in the same format.
205
206
207 Matching literal characters
208 ---------------------------
209
210 The OP_CHAR opcode is followed by a single character that is to be matched
211 casefully. For caseless matching, OP_CHARI is used. In UTF-8 or UTF-16 modes,
212 the character may be more than one unit long. In UTF-32 mode, characters
213 are always exactly one unit long.
214
215
216 Repeating single characters
217 ---------------------------
218
219 The common repeats (*, +, ?), when applied to a single character, use the
220 following opcodes, which come in caseful and caseless versions:
221
222 Caseful Caseless
223 OP_STAR OP_STARI
224 OP_MINSTAR OP_MINSTARI
225 OP_POSSTAR OP_POSSTARI
226 OP_PLUS OP_PLUSI
227 OP_MINPLUS OP_MINPLUSI
228 OP_POSPLUS OP_POSPLUSI
229 OP_QUERY OP_QUERYI
230 OP_MINQUERY OP_MINQUERYI
231 OP_POSQUERY OP_POSQUERYI
232
233 Each opcode is followed by the character that is to be repeated. In ASCII mode,
234 these are two-unit items; in UTF-8 or UTF-16 modes, the length is variable; in
235 UTF-32 mode these are one-unit items.
236 Those with "MIN" in their names are the minimizing versions. Those with "POS"
237 in their names are possessive versions. Other repeats make use of these
238 opcodes:
239
240 Caseful Caseless
241 OP_UPTO OP_UPTOI
242 OP_MINUPTO OP_MINUPTOI
243 OP_POSUPTO OP_POSUPTOI
244 OP_EXACT OP_EXACTI
245
246 Each of these is followed by a two-byte (one short) count (most significant
247 byte first in 8-bit mode) and then the repeated character. OP_UPTO matches from
248 0 to the given number. A repeat with a non-zero minimum and a fixed maximum is
249 coded as an OP_EXACT followed by an OP_UPTO (or OP_MINUPTO or OPT_POSUPTO).
250
251
252 Repeating character types
253 -------------------------
254
255 Repeats of things like \d are done exactly as for single characters, except
256 that instead of a character, the opcode for the type is stored in the data
257 unit. The opcodes are:
258
259 OP_TYPESTAR
260 OP_TYPEMINSTAR
261 OP_TYPEPOSSTAR
262 OP_TYPEPLUS
263 OP_TYPEMINPLUS
264 OP_TYPEPOSPLUS
265 OP_TYPEQUERY
266 OP_TYPEMINQUERY
267 OP_TYPEPOSQUERY
268 OP_TYPEUPTO
269 OP_TYPEMINUPTO
270 OP_TYPEPOSUPTO
271 OP_TYPEEXACT
272
273
274 Match by Unicode property
275 -------------------------
276
277 OP_PROP and OP_NOTPROP are used for positive and negative matches of a
278 character by testing its Unicode property (the \p and \P escape sequences).
279 Each is followed by two units that encode the desired property as a type and a
280 value.
281
282 Repeats of these items use the OP_TYPESTAR etc. set of opcodes, followed by
283 three units: OP_PROP or OP_NOTPROP, and then the desired property type and
284 value.
285
286
287 Character classes
288 -----------------
289
290 If there is only one character in the class, OP_CHAR or OP_CHARI is used for a
291 positive class, and OP_NOT or OP_NOTI for a negative one (that is, for
292 something like [^a]).
293
294 Another set of 13 repeating opcodes (called OP_NOTSTAR etc.) are used for
295 repeated, negated, single-character classes. The normal single-character
296 opcodes (OP_STAR, etc.) are used for repeated positive single-character
297 classes.
298
299 When there is more than one character in a class and all the characters are
300 less than 256, OP_CLASS is used for a positive class, and OP_NCLASS for a
301 negative one. In either case, the opcode is followed by a 32-byte (16-short)
302 bit map containing a 1 bit for every character that is acceptable. The bits are
303 counted from the least significant end of each unit. In caseless mode, bits for
304 both cases are set.
305
306 The reason for having both OP_CLASS and OP_NCLASS is so that, in UTF-8/16/32 mode,
307 subject characters with values greater than 255 can be handled correctly. For
308 OP_CLASS they do not match, whereas for OP_NCLASS they do.
309
310 For classes containing characters with values greater than 255, OP_XCLASS is
311 used. It optionally uses a bit map (if any characters lie within it), followed
312 by a list of pairs (for a range) and single characters. In caseless mode, both
313 cases are explicitly listed. There is a flag character than indicates whether
314 it is a positive or a negative class.
315
316
317 Back references
318 ---------------
319
320 OP_REF (caseful) or OP_REFI (caseless) is followed by two bytes (one short)
321 containing the reference number.
322
323
324 Repeating character classes and back references
325 -----------------------------------------------
326
327 Single-character classes are handled specially (see above). This section
328 applies to OP_CLASS and OP_REF[I]. In both cases, the repeat information
329 follows the base item. The matching code looks at the following opcode to see
330 if it is one of
331
332 OP_CRSTAR
333 OP_CRMINSTAR
334 OP_CRPLUS
335 OP_CRMINPLUS
336 OP_CRQUERY
337 OP_CRMINQUERY
338 OP_CRRANGE
339 OP_CRMINRANGE
340
341 All but the last two are just single-unit items. The others are followed by
342 four bytes (two shorts) of data, comprising the minimum and maximum repeat
343 counts. There are no special possessive opcodes for these repeats; a possessive
344 repeat is compiled into an atomic group.
345
346
347 Brackets and alternation
348 ------------------------
349
350 A pair of non-capturing (round) brackets is wrapped round each expression at
351 compile time, so alternation always happens in the context of brackets.
352
353 [Note for North Americans: "bracket" to some English speakers, including
354 myself, can be round, square, curly, or pointy. Hence this usage rather than
355 "parentheses".]
356
357 Non-capturing brackets use the opcode OP_BRA. Originally PCRE was limited to 99
358 capturing brackets and it used a different opcode for each one. From release
359 3.5, the limit was removed by putting the bracket number into the data for
360 higher-numbered brackets. From release 7.0 all capturing brackets are handled
361 this way, using the single opcode OP_CBRA.
362
363 A bracket opcode is followed by LINK_SIZE bytes which give the offset to the
364 next alternative OP_ALT or, if there aren't any branches, to the matching
365 OP_KET opcode. Each OP_ALT is followed by LINK_SIZE bytes giving the offset to
366 the next one, or to the OP_KET opcode. For capturing brackets, the bracket
367 number immediately follows the offset, always as a 2-byte (one short) item.
368
369 OP_KET is used for subpatterns that do not repeat indefinitely, and
370 OP_KETRMIN and OP_KETRMAX are used for indefinite repetitions, minimally or
371 maximally respectively (see below for possessive repetitions). All three are
372 followed by LINK_SIZE bytes giving (as a positive number) the offset back to
373 the matching bracket opcode.
374
375 If a subpattern is quantified such that it is permitted to match zero times, it
376 is preceded by one of OP_BRAZERO, OP_BRAMINZERO, or OP_SKIPZERO. These are
377 single-unit opcodes that tell the matcher that skipping the following
378 subpattern entirely is a valid branch. In the case of the first two, not
379 skipping the pattern is also valid (greedy and non-greedy). The third is used
380 when a pattern has the quantifier {0,0}. It cannot be entirely discarded,
381 because it may be called as a subroutine from elsewhere in the regex.
382
383 A subpattern with an indefinite maximum repetition is replicated in the
384 compiled data its minimum number of times (or once with OP_BRAZERO if the
385 minimum is zero), with the final copy terminating with OP_KETRMIN or OP_KETRMAX
386 as appropriate.
387
388 A subpattern with a bounded maximum repetition is replicated in a nested
389 fashion up to the maximum number of times, with OP_BRAZERO or OP_BRAMINZERO
390 before each replication after the minimum, so that, for example, (abc){2,5} is
391 compiled as (abc)(abc)((abc)((abc)(abc)?)?)?, except that each bracketed group
392 has the same number.
393
394 When a repeated subpattern has an unbounded upper limit, it is checked to see
395 whether it could match an empty string. If this is the case, the opcode in the
396 final replication is changed to OP_SBRA or OP_SCBRA. This tells the matcher
397 that it needs to check for matching an empty string when it hits OP_KETRMIN or
398 OP_KETRMAX, and if so, to break the loop.
399
400 Possessive brackets
401 -------------------
402
403 When a repeated group (capturing or non-capturing) is marked as possessive by
404 the "+" notation, e.g. (abc)++, different opcodes are used. Their names all
405 have POS on the end, e.g. OP_BRAPOS instead of OP_BRA and OP_SCPBRPOS instead
406 of OP_SCBRA. The end of such a group is marked by OP_KETRPOS. If the minimum
407 repetition is zero, the group is preceded by OP_BRAPOSZERO.
408
409
410 Assertions
411 ----------
412
413 Forward assertions are just like other subpatterns, but starting with one of
414 the opcodes OP_ASSERT or OP_ASSERT_NOT. Backward assertions use the opcodes
415 OP_ASSERTBACK and OP_ASSERTBACK_NOT, and the first opcode inside the assertion
416 is OP_REVERSE, followed by a two byte (one short) count of the number of
417 characters to move back the pointer in the subject string. In ASCII mode, the
418 count is a number of units, but in UTF-8/16 mode each character may occupy more
419 than one unit; in UTF-32 mode each character occupies exactly one unit.
420 A separate count is present in each alternative of a lookbehind
421 assertion, allowing them to have different fixed lengths.
422
423
424 Once-only (atomic) subpatterns
425 ------------------------------
426
427 These are also just like other subpatterns, but they start with the opcode
428 OP_ONCE. The check for matching an empty string in an unbounded repeat is
429 handled entirely at runtime, so there is just this one opcode.
430
431
432 Conditional subpatterns
433 -----------------------
434
435 These are like other subpatterns, but they start with the opcode OP_COND, or
436 OP_SCOND for one that might match an empty string in an unbounded repeat. If
437 the condition is a back reference, this is stored at the start of the
438 subpattern using the opcode OP_CREF followed by two bytes (one short)
439 containing the reference number. OP_NCREF is used instead if the reference was
440 generated by name (so that the runtime code knows to check for duplicate
441 names).
442
443 If the condition is "in recursion" (coded as "(?(R)"), or "in recursion of
444 group x" (coded as "(?(Rx)"), the group number is stored at the start of the
445 subpattern using the opcode OP_RREF or OP_NRREF (cf OP_NCREF), and a value of
446 zero for "the whole pattern". For a DEFINE condition, just the single unit
447 OP_DEF is used (it has no associated data). Otherwise, a conditional subpattern
448 always starts with one of the assertions.
449
450
451 Recursion
452 ---------
453
454 Recursion either matches the current regex, or some subexpression. The opcode
455 OP_RECURSE is followed by an value which is the offset to the starting bracket
456 from the start of the whole pattern. From release 6.5, OP_RECURSE is
457 automatically wrapped inside OP_ONCE brackets (because otherwise some patterns
458 broke it). OP_RECURSE is also used for "subroutine" calls, even though they
459 are not strictly a recursion.
460
461
462 Callout
463 -------
464
465 OP_CALLOUT is followed by one unit of data that holds a callout number in the
466 range 0 to 254 for manual callouts, or 255 for an automatic callout. In both
467 cases there follows a two-byte (one short) value giving the offset in the
468 pattern to the start of the following item, and another two-byte (one short)
469 item giving the length of the next item.
470
471
472 Philip Hazel
473 February 2012

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