/[pcre]/code/tags/pcre-6.4/doc/Tech.Notes
ViewVC logotype

Contents of /code/tags/pcre-6.4/doc/Tech.Notes

Parent Directory Parent Directory | Revision Log Revision Log


Revision 86 - (show annotations) (download)
Sat Feb 24 21:41:15 2007 UTC (7 years, 1 month ago) by nigel
File size: 13866 byte(s)
Tag code/trunk as code/tags/pcre-6.4.

1 Technical Notes about PCRE
2 --------------------------
3
4 Historical note 1
5 -----------------
6
7 Many years ago I implemented some regular expression functions to an algorithm
8 suggested by Martin Richards. These were not Unix-like in form, and were quite
9 restricted in what they could do by comparison with Perl. The interesting part
10 about the algorithm was that the amount of space required to hold the compiled
11 form of an expression was known in advance. The code to apply an expression did
12 not operate by backtracking, as the original Henry Spencer code and current
13 Perl code does, but instead checked all possibilities simultaneously by keeping
14 a list of current states and checking all of them as it advanced through the
15 subject string. In the terminology of Jeffrey Friedl's book, it was a "DFA
16 algorithm". When the pattern was all used up, all remaining states were
17 possible matches, and the one matching the longest subset of the subject string
18 was chosen. This did not necessarily maximize the individual wild portions of
19 the pattern, as is expected in Unix and Perl-style regular expressions.
20
21 Historical note 2
22 -----------------
23
24 By contrast, the code originally written by Henry Spencer and subsequently
25 heavily modified for Perl actually compiles the expression twice: once in a
26 dummy mode in order to find out how much store will be needed, and then for
27 real. The execution function operates by backtracking and maximizing (or,
28 optionally, minimizing in Perl) the amount of the subject that matches
29 individual wild portions of the pattern. This is an "NFA algorithm" in Friedl's
30 terminology.
31
32 OK, here's the real stuff
33 -------------------------
34
35 For the set of functions that form the "basic" PCRE library (which are
36 unrelated to those mentioned above), I tried at first to invent an algorithm
37 that used an amount of store bounded by a multiple of the number of characters
38 in the pattern, to save on compiling time. However, because of the greater
39 complexity in Perl regular expressions, I couldn't do this. In any case, a
40 first pass through the pattern is needed, for a number of reasons. PCRE works
41 by running a very degenerate first pass to calculate a maximum store size, and
42 then a second pass to do the real compile - which may use a bit less than the
43 predicted amount of store. The idea is that this is going to turn out faster
44 because the first pass is degenerate and the second pass can just store stuff
45 straight into the vector, which it knows is big enough. It does make the
46 compiling functions bigger, of course, but they have got quite big anyway to
47 handle all the Perl stuff.
48
49 Traditional matching function
50 -----------------------------
51
52 The "traditional", and original, matching function is called pcre_exec(), and
53 it implements an NFA algorithm, similar to the original Henry Spencer algorithm
54 and the way that Perl works. Not surprising, since it is intended to be as
55 compatible with Perl as possible. This is the function most users of PCRE will
56 use most of the time.
57
58 Supplementary matching function
59 -------------------------------
60
61 From PCRE 6.0, there is also a supplementary matching function called
62 pcre_dfa_exec(). This implements a DFA matching algorithm that searches
63 simultaneously for all possible matches that start at one point in the subject
64 string. (Going back to my roots: see Historical Note 1 above.) This function
65 intreprets the same compiled pattern data as pcre_exec(); however, not all the
66 facilities are available, and those that are don't always work in quite the
67 same way. See the user documentation for details.
68
69 Format of compiled patterns
70 ---------------------------
71
72 The compiled form of a pattern is a vector of bytes, containing items of
73 variable length. The first byte in an item is an opcode, and the length of the
74 item is either implicit in the opcode or contained in the data bytes that
75 follow it.
76
77 In many cases below "two-byte" data values are specified. This is in fact just
78 a default. PCRE can be compiled to use 3-byte or 4-byte values (impairing the
79 performance). This is necessary only when patterns whose compiled length is
80 greater than 64K are going to be processed. In this description, we assume the
81 "normal" compilation options.
82
83 A list of all the opcodes follows:
84
85 Opcodes with no following data
86 ------------------------------
87
88 These items are all just one byte long
89
90 OP_END end of pattern
91 OP_ANY match any character
92 OP_ANYBYTE match any single byte, even in UTF-8 mode
93 OP_SOD match start of data: \A
94 OP_SOM, start of match (subject + offset): \G
95 OP_CIRC ^ (start of data, or after \n in multiline)
96 OP_NOT_WORD_BOUNDARY \W
97 OP_WORD_BOUNDARY \w
98 OP_NOT_DIGIT \D
99 OP_DIGIT \d
100 OP_NOT_WHITESPACE \S
101 OP_WHITESPACE \s
102 OP_NOT_WORDCHAR \W
103 OP_WORDCHAR \w
104 OP_EODN match end of data or \n at end: \Z
105 OP_EOD match end of data: \z
106 OP_DOLL $ (end of data, or before \n in multiline)
107 OP_EXTUNI match an extended Unicode character
108
109
110 Repeating single characters
111 ---------------------------
112
113 The common repeats (*, +, ?) when applied to a single character use the
114 following opcodes:
115
116 OP_STAR
117 OP_MINSTAR
118 OP_PLUS
119 OP_MINPLUS
120 OP_QUERY
121 OP_MINQUERY
122
123 In ASCII mode, these are two-byte items; in UTF-8 mode, the length is variable.
124 Those with "MIN" in their name are the minimizing versions. Each is followed by
125 the character that is to be repeated. Other repeats make use of
126
127 OP_UPTO
128 OP_MINUPTO
129 OP_EXACT
130
131 which are followed by a two-byte count (most significant first) and the
132 repeated character. OP_UPTO matches from 0 to the given number. A repeat with a
133 non-zero minimum and a fixed maximum is coded as an OP_EXACT followed by an
134 OP_UPTO (or OP_MINUPTO).
135
136
137 Repeating character types
138 -------------------------
139
140 Repeats of things like \d are done exactly as for single characters, except
141 that instead of a character, the opcode for the type is stored in the data
142 byte. The opcodes are:
143
144 OP_TYPESTAR
145 OP_TYPEMINSTAR
146 OP_TYPEPLUS
147 OP_TYPEMINPLUS
148 OP_TYPEQUERY
149 OP_TYPEMINQUERY
150 OP_TYPEUPTO
151 OP_TYPEMINUPTO
152 OP_TYPEEXACT
153
154
155 Match by Unicode property
156 -------------------------
157
158 OP_PROP and OP_NOTPROP are used for positive and negative matches of a
159 character by testing its Unicode property (the \p and \P escape sequences).
160 Each is followed by a single byte that encodes the desired property value.
161
162 Repeats of these items use the OP_TYPESTAR etc. set of opcodes, followed by two
163 bytes: OP_PROP or OP_NOTPROP and then the desired property value.
164
165
166 Matching literal characters
167 ---------------------------
168
169 The OP_CHAR opcode is followed by a single character that is to be matched
170 casefully. For caseless matching, OP_CHARNC is used. In UTF-8 mode, the
171 character may be more than one byte long. (Earlier versions of PCRE used
172 multi-character strings, but this was changed to allow some new features to be
173 added.)
174
175
176 Character classes
177 -----------------
178
179 If there is only one character, OP_CHAR or OP_CHARNC is used for a positive
180 class, and OP_NOT for a negative one (that is, for something like [^a]).
181 However, in UTF-8 mode, the use of OP_NOT applies only to characters with
182 values < 128, because OP_NOT is confined to single bytes.
183
184 Another set of repeating opcodes (OP_NOTSTAR etc.) are used for a repeated,
185 negated, single-character class. The normal ones (OP_STAR etc.) are used for a
186 repeated positive single-character class.
187
188 When there's more than one character in a class and all the characters are less
189 than 256, OP_CLASS is used for a positive class, and OP_NCLASS for a negative
190 one. In either case, the opcode is followed by a 32-byte bit map containing a 1
191 bit for every character that is acceptable. The bits are counted from the least
192 significant end of each byte.
193
194 The reason for having both OP_CLASS and OP_NCLASS is so that, in UTF-8 mode,
195 subject characters with values greater than 256 can be handled correctly. For
196 OP_CLASS they don't match, whereas for OP_NCLASS they do.
197
198 For classes containing characters with values > 255, OP_XCLASS is used. It
199 optionally uses a bit map (if any characters lie within it), followed by a list
200 of pairs and single characters. There is a flag character than indicates
201 whether it's a positive or a negative class.
202
203
204 Back references
205 ---------------
206
207 OP_REF is followed by two bytes containing the reference number.
208
209
210 Repeating character classes and back references
211 -----------------------------------------------
212
213 Single-character classes are handled specially (see above). This applies to
214 OP_CLASS and OP_REF. In both cases, the repeat information follows the base
215 item. The matching code looks at the following opcode to see if it is one of
216
217 OP_CRSTAR
218 OP_CRMINSTAR
219 OP_CRPLUS
220 OP_CRMINPLUS
221 OP_CRQUERY
222 OP_CRMINQUERY
223 OP_CRRANGE
224 OP_CRMINRANGE
225
226 All but the last two are just single-byte items. The others are followed by
227 four bytes of data, comprising the minimum and maximum repeat counts.
228
229
230 Brackets and alternation
231 ------------------------
232
233 A pair of non-capturing (round) brackets is wrapped round each expression at
234 compile time, so alternation always happens in the context of brackets.
235
236 Non-capturing brackets use the opcode OP_BRA, while capturing brackets use
237 OP_BRA+1, OP_BRA+2, etc. [Note for North Americans: "bracket" to some English
238 speakers, including myself, can be round, square, curly, or pointy. Hence this
239 usage.]
240
241 Originally PCRE was limited to 99 capturing brackets (so as not to use up all
242 the opcodes). From release 3.5, there is no limit. What happens is that the
243 first ones, up to EXTRACT_BASIC_MAX are handled with separate opcodes, as
244 above. If there are more, the opcode is set to EXTRACT_BASIC_MAX+1, and the
245 first operation in the bracket is OP_BRANUMBER, followed by a 2-byte bracket
246 number. This opcode is ignored while matching, but is fished out when handling
247 the bracket itself. (They could have all been done like this, but I was making
248 minimal changes.)
249
250 A bracket opcode is followed by LINK_SIZE bytes which give the offset to the
251 next alternative OP_ALT or, if there aren't any branches, to the matching
252 OP_KET opcode. Each OP_ALT is followed by LINK_SIZE bytes giving the offset to
253 the next one, or to the OP_KET opcode.
254
255 OP_KET is used for subpatterns that do not repeat indefinitely, while
256 OP_KETRMIN and OP_KETRMAX are used for indefinite repetitions, minimally or
257 maximally respectively. All three are followed by LINK_SIZE bytes giving (as a
258 positive number) the offset back to the matching OP_BRA opcode.
259
260 If a subpattern is quantified such that it is permitted to match zero times, it
261 is preceded by one of OP_BRAZERO or OP_BRAMINZERO. These are single-byte
262 opcodes which tell the matcher that skipping this subpattern entirely is a
263 valid branch.
264
265 A subpattern with an indefinite maximum repetition is replicated in the
266 compiled data its minimum number of times (or once with OP_BRAZERO if the
267 minimum is zero), with the final copy terminating with OP_KETRMIN or OP_KETRMAX
268 as appropriate.
269
270 A subpattern with a bounded maximum repetition is replicated in a nested
271 fashion up to the maximum number of times, with OP_BRAZERO or OP_BRAMINZERO
272 before each replication after the minimum, so that, for example, (abc){2,5} is
273 compiled as (abc)(abc)((abc)((abc)(abc)?)?)?.
274
275
276 Assertions
277 ----------
278
279 Forward assertions are just like other subpatterns, but starting with one of
280 the opcodes OP_ASSERT or OP_ASSERT_NOT. Backward assertions use the opcodes
281 OP_ASSERTBACK and OP_ASSERTBACK_NOT, and the first opcode inside the assertion
282 is OP_REVERSE, followed by a two byte count of the number of characters to move
283 back the pointer in the subject string. When operating in UTF-8 mode, the count
284 is a character count rather than a byte count. A separate count is present in
285 each alternative of a lookbehind assertion, allowing them to have different
286 fixed lengths.
287
288
289 Once-only subpatterns
290 ---------------------
291
292 These are also just like other subpatterns, but they start with the opcode
293 OP_ONCE.
294
295
296 Conditional subpatterns
297 -----------------------
298
299 These are like other subpatterns, but they start with the opcode OP_COND. If
300 the condition is a back reference, this is stored at the start of the
301 subpattern using the opcode OP_CREF followed by two bytes containing the
302 reference number. If the condition is "in recursion" (coded as "(?(R)"), the
303 same scheme is used, with a "reference number" of 0xffff. Otherwise, a
304 conditional subpattern always starts with one of the assertions.
305
306
307 Recursion
308 ---------
309
310 Recursion either matches the current regex, or some subexpression. The opcode
311 OP_RECURSE is followed by an value which is the offset to the starting bracket
312 from the start of the whole pattern.
313
314
315 Callout
316 -------
317
318 OP_CALLOUT is followed by one byte of data that holds a callout number in the
319 range 0 to 254 for manual callouts, or 255 for an automatic callout. In both
320 cases there follows a two-byte value giving the offset in the pattern to the
321 start of the following item, and another two-byte item giving the length of the
322 next item.
323
324
325 Changing options
326 ----------------
327
328 If any of the /i, /m, or /s options are changed within a pattern, an OP_OPT
329 opcode is compiled, followed by one byte containing the new settings of these
330 flags. If there are several alternatives, there is an occurrence of OP_OPT at
331 the start of all those following the first options change, to set appropriate
332 options for the start of the alternative. Immediately after the end of the
333 group there is another such item to reset the flags to their previous values. A
334 change of flag right at the very start of the pattern can be handled entirely
335 at compile time, and so does not cause anything to be put into the compiled
336 data.
337
338 Philip Hazel
339 March 2005

webmaster@exim.org
ViewVC Help
Powered by ViewVC 1.1.12