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Load pcre-6.7 into code/trunk.

1 Technical Notes about PCRE
2 --------------------------
3
4 These are very rough technical notes that record potentially useful information
5 about PCRE internals.
6
7 Historical note 1
8 -----------------
9
10 Many years ago I implemented some regular expression functions to an algorithm
11 suggested by Martin Richards. These were not Unix-like in form, and were quite
12 restricted in what they could do by comparison with Perl. The interesting part
13 about the algorithm was that the amount of space required to hold the compiled
14 form of an expression was known in advance. The code to apply an expression did
15 not operate by backtracking, as the original Henry Spencer code and current
16 Perl code does, but instead checked all possibilities simultaneously by keeping
17 a list of current states and checking all of them as it advanced through the
18 subject string. In the terminology of Jeffrey Friedl's book, it was a "DFA
19 algorithm". When the pattern was all used up, all remaining states were
20 possible matches, and the one matching the longest subset of the subject string
21 was chosen. This did not necessarily maximize the individual wild portions of
22 the pattern, as is expected in Unix and Perl-style regular expressions.
23
24 Historical note 2
25 -----------------
26
27 By contrast, the code originally written by Henry Spencer (which was
28 subsequently heavily modified for Perl) compiles the expression twice: once in
29 a dummy mode in order to find out how much store will be needed, and then for
30 real. (The Perl version probably doesn't do this any more; I'm talking about
31 the original library.) The execution function operates by backtracking and
32 maximizing (or, optionally, minimizing in Perl) the amount of the subject that
33 matches individual wild portions of the pattern. This is an "NFA algorithm" in
34 Friedl's terminology.
35
36 OK, here's the real stuff
37 -------------------------
38
39 For the set of functions that form the "basic" PCRE library (which are
40 unrelated to those mentioned above), I tried at first to invent an algorithm
41 that used an amount of store bounded by a multiple of the number of characters
42 in the pattern, to save on compiling time. However, because of the greater
43 complexity in Perl regular expressions, I couldn't do this. In any case, a
44 first pass through the pattern is needed, for a number of reasons. PCRE works
45 by running a very degenerate first pass to calculate a maximum store size, and
46 then a second pass to do the real compile - which may use a bit less than the
47 predicted amount of store. The idea is that this is going to turn out faster
48 because the first pass is degenerate and the second pass can just store stuff
49 straight into the vector, which it knows is big enough. It does make the
50 compiling functions bigger, of course, but they have become quite big anyway to
51 handle all the Perl stuff.
52
53 Traditional matching function
54 -----------------------------
55
56 The "traditional", and original, matching function is called pcre_exec(), and
57 it implements an NFA algorithm, similar to the original Henry Spencer algorithm
58 and the way that Perl works. Not surprising, since it is intended to be as
59 compatible with Perl as possible. This is the function most users of PCRE will
60 use most of the time.
61
62 Supplementary matching function
63 -------------------------------
64
65 From PCRE 6.0, there is also a supplementary matching function called
66 pcre_dfa_exec(). This implements a DFA matching algorithm that searches
67 simultaneously for all possible matches that start at one point in the subject
68 string. (Going back to my roots: see Historical Note 1 above.) This function
69 intreprets the same compiled pattern data as pcre_exec(); however, not all the
70 facilities are available, and those that are do not always work in quite the
71 same way. See the user documentation for details.
72
73 Format of compiled patterns
74 ---------------------------
75
76 The compiled form of a pattern is a vector of bytes, containing items of
77 variable length. The first byte in an item is an opcode, and the length of the
78 item is either implicit in the opcode or contained in the data bytes that
79 follow it.
80
81 In many cases below "two-byte" data values are specified. This is in fact just
82 a default. PCRE can be compiled to use 3-byte or 4-byte values (impairing the
83 performance). This is necessary only when patterns whose compiled length is
84 greater than 64K are going to be processed. In this description, we assume the
85 "normal" compilation options.
86
87 A list of all the opcodes follows:
88
89 Opcodes with no following data
90 ------------------------------
91
92 These items are all just one byte long
93
94 OP_END end of pattern
95 OP_ANY match any character
96 OP_ANYBYTE match any single byte, even in UTF-8 mode
97 OP_SOD match start of data: \A
98 OP_SOM, start of match (subject + offset): \G
99 OP_CIRC ^ (start of data, or after \n in multiline)
100 OP_NOT_WORD_BOUNDARY \W
101 OP_WORD_BOUNDARY \w
102 OP_NOT_DIGIT \D
103 OP_DIGIT \d
104 OP_NOT_WHITESPACE \S
105 OP_WHITESPACE \s
106 OP_NOT_WORDCHAR \W
107 OP_WORDCHAR \w
108 OP_EODN match end of data or \n at end: \Z
109 OP_EOD match end of data: \z
110 OP_DOLL $ (end of data, or before \n in multiline)
111 OP_EXTUNI match an extended Unicode character
112
113
114 Repeating single characters
115 ---------------------------
116
117 The common repeats (*, +, ?) when applied to a single character use the
118 following opcodes:
119
120 OP_STAR
121 OP_MINSTAR
122 OP_PLUS
123 OP_MINPLUS
124 OP_QUERY
125 OP_MINQUERY
126
127 In ASCII mode, these are two-byte items; in UTF-8 mode, the length is variable.
128 Those with "MIN" in their name are the minimizing versions. Each is followed by
129 the character that is to be repeated. Other repeats make use of
130
131 OP_UPTO
132 OP_MINUPTO
133 OP_EXACT
134
135 which are followed by a two-byte count (most significant first) and the
136 repeated character. OP_UPTO matches from 0 to the given number. A repeat with a
137 non-zero minimum and a fixed maximum is coded as an OP_EXACT followed by an
138 OP_UPTO (or OP_MINUPTO).
139
140
141 Repeating character types
142 -------------------------
143
144 Repeats of things like \d are done exactly as for single characters, except
145 that instead of a character, the opcode for the type is stored in the data
146 byte. The opcodes are:
147
148 OP_TYPESTAR
149 OP_TYPEMINSTAR
150 OP_TYPEPLUS
151 OP_TYPEMINPLUS
152 OP_TYPEQUERY
153 OP_TYPEMINQUERY
154 OP_TYPEUPTO
155 OP_TYPEMINUPTO
156 OP_TYPEEXACT
157
158
159 Match by Unicode property
160 -------------------------
161
162 OP_PROP and OP_NOTPROP are used for positive and negative matches of a
163 character by testing its Unicode property (the \p and \P escape sequences).
164 Each is followed by two bytes that encode the desired property as a type and a
165 value.
166
167 Repeats of these items use the OP_TYPESTAR etc. set of opcodes, followed by
168 three bytes: OP_PROP or OP_NOTPROP and then the desired property type and
169 value.
170
171
172 Matching literal characters
173 ---------------------------
174
175 The OP_CHAR opcode is followed by a single character that is to be matched
176 casefully. For caseless matching, OP_CHARNC is used. In UTF-8 mode, the
177 character may be more than one byte long. (Earlier versions of PCRE used
178 multi-character strings, but this was changed to allow some new features to be
179 added.)
180
181
182 Character classes
183 -----------------
184
185 If there is only one character, OP_CHAR or OP_CHARNC is used for a positive
186 class, and OP_NOT for a negative one (that is, for something like [^a]).
187 However, in UTF-8 mode, the use of OP_NOT applies only to characters with
188 values < 128, because OP_NOT is confined to single bytes.
189
190 Another set of repeating opcodes (OP_NOTSTAR etc.) are used for a repeated,
191 negated, single-character class. The normal ones (OP_STAR etc.) are used for a
192 repeated positive single-character class.
193
194 When there's more than one character in a class and all the characters are less
195 than 256, OP_CLASS is used for a positive class, and OP_NCLASS for a negative
196 one. In either case, the opcode is followed by a 32-byte bit map containing a 1
197 bit for every character that is acceptable. The bits are counted from the least
198 significant end of each byte.
199
200 The reason for having both OP_CLASS and OP_NCLASS is so that, in UTF-8 mode,
201 subject characters with values greater than 256 can be handled correctly. For
202 OP_CLASS they don't match, whereas for OP_NCLASS they do.
203
204 For classes containing characters with values > 255, OP_XCLASS is used. It
205 optionally uses a bit map (if any characters lie within it), followed by a list
206 of pairs and single characters. There is a flag character than indicates
207 whether it's a positive or a negative class.
208
209
210 Back references
211 ---------------
212
213 OP_REF is followed by two bytes containing the reference number.
214
215
216 Repeating character classes and back references
217 -----------------------------------------------
218
219 Single-character classes are handled specially (see above). This applies to
220 OP_CLASS and OP_REF. In both cases, the repeat information follows the base
221 item. The matching code looks at the following opcode to see if it is one of
222
223 OP_CRSTAR
224 OP_CRMINSTAR
225 OP_CRPLUS
226 OP_CRMINPLUS
227 OP_CRQUERY
228 OP_CRMINQUERY
229 OP_CRRANGE
230 OP_CRMINRANGE
231
232 All but the last two are just single-byte items. The others are followed by
233 four bytes of data, comprising the minimum and maximum repeat counts.
234
235
236 Brackets and alternation
237 ------------------------
238
239 A pair of non-capturing (round) brackets is wrapped round each expression at
240 compile time, so alternation always happens in the context of brackets.
241
242 Non-capturing brackets use the opcode OP_BRA, while capturing brackets use
243 OP_BRA+1, OP_BRA+2, etc. [Note for North Americans: "bracket" to some English
244 speakers, including myself, can be round, square, curly, or pointy. Hence this
245 usage.]
246
247 Originally PCRE was limited to 99 capturing brackets (so as not to use up all
248 the opcodes). From release 3.5, there is no limit. What happens is that the
249 first ones, up to EXTRACT_BASIC_MAX are handled with separate opcodes, as
250 above. If there are more, the opcode is set to EXTRACT_BASIC_MAX+1, and the
251 first operation in the bracket is OP_BRANUMBER, followed by a 2-byte bracket
252 number. This opcode is ignored while matching, but is fished out when handling
253 the bracket itself. (They could have all been done like this, but I was making
254 minimal changes.)
255
256 A bracket opcode is followed by LINK_SIZE bytes which give the offset to the
257 next alternative OP_ALT or, if there aren't any branches, to the matching
258 OP_KET opcode. Each OP_ALT is followed by LINK_SIZE bytes giving the offset to
259 the next one, or to the OP_KET opcode.
260
261 OP_KET is used for subpatterns that do not repeat indefinitely, while
262 OP_KETRMIN and OP_KETRMAX are used for indefinite repetitions, minimally or
263 maximally respectively. All three are followed by LINK_SIZE bytes giving (as a
264 positive number) the offset back to the matching OP_BRA opcode.
265
266 If a subpattern is quantified such that it is permitted to match zero times, it
267 is preceded by one of OP_BRAZERO or OP_BRAMINZERO. These are single-byte
268 opcodes which tell the matcher that skipping this subpattern entirely is a
269 valid branch.
270
271 A subpattern with an indefinite maximum repetition is replicated in the
272 compiled data its minimum number of times (or once with OP_BRAZERO if the
273 minimum is zero), with the final copy terminating with OP_KETRMIN or OP_KETRMAX
274 as appropriate.
275
276 A subpattern with a bounded maximum repetition is replicated in a nested
277 fashion up to the maximum number of times, with OP_BRAZERO or OP_BRAMINZERO
278 before each replication after the minimum, so that, for example, (abc){2,5} is
279 compiled as (abc)(abc)((abc)((abc)(abc)?)?)?.
280
281
282 Assertions
283 ----------
284
285 Forward assertions are just like other subpatterns, but starting with one of
286 the opcodes OP_ASSERT or OP_ASSERT_NOT. Backward assertions use the opcodes
287 OP_ASSERTBACK and OP_ASSERTBACK_NOT, and the first opcode inside the assertion
288 is OP_REVERSE, followed by a two byte count of the number of characters to move
289 back the pointer in the subject string. When operating in UTF-8 mode, the count
290 is a character count rather than a byte count. A separate count is present in
291 each alternative of a lookbehind assertion, allowing them to have different
292 fixed lengths.
293
294
295 Once-only subpatterns
296 ---------------------
297
298 These are also just like other subpatterns, but they start with the opcode
299 OP_ONCE.
300
301
302 Conditional subpatterns
303 -----------------------
304
305 These are like other subpatterns, but they start with the opcode OP_COND. If
306 the condition is a back reference, this is stored at the start of the
307 subpattern using the opcode OP_CREF followed by two bytes containing the
308 reference number. If the condition is "in recursion" (coded as "(?(R)"), the
309 same scheme is used, with a "reference number" of 0xffff. Otherwise, a
310 conditional subpattern always starts with one of the assertions.
311
312
313 Recursion
314 ---------
315
316 Recursion either matches the current regex, or some subexpression. The opcode
317 OP_RECURSE is followed by an value which is the offset to the starting bracket
318 from the start of the whole pattern. From release 6.5, OP_RECURSE is
319 automatically wrapped inside OP_ONCE brackets (because otherwise some patterns
320 broke it). OP_RECURSE is also used for "subroutine" calls, even though they
321 are not strictly a recursion.
322
323
324 Callout
325 -------
326
327 OP_CALLOUT is followed by one byte of data that holds a callout number in the
328 range 0 to 254 for manual callouts, or 255 for an automatic callout. In both
329 cases there follows a two-byte value giving the offset in the pattern to the
330 start of the following item, and another two-byte item giving the length of the
331 next item.
332
333
334 Changing options
335 ----------------
336
337 If any of the /i, /m, or /s options are changed within a pattern, an OP_OPT
338 opcode is compiled, followed by one byte containing the new settings of these
339 flags. If there are several alternatives, there is an occurrence of OP_OPT at
340 the start of all those following the first options change, to set appropriate
341 options for the start of the alternative. Immediately after the end of the
342 group there is another such item to reset the flags to their previous values. A
343 change of flag right at the very start of the pattern can be handled entirely
344 at compile time, and so does not cause anything to be put into the compiled
345 data.
346
347 Philip Hazel
348 June 2006

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