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1 nigel 41 Technical Notes about PCRE
2     --------------------------
3    
4     Many years ago I implemented some regular expression functions to an algorithm
5     suggested by Martin Richards. These were not Unix-like in form, and were quite
6     restricted in what they could do by comparison with Perl. The interesting part
7     about the algorithm was that the amount of space required to hold the compiled
8     form of an expression was known in advance. The code to apply an expression did
9 nigel 63 not operate by backtracking, as the original Henry Spencer code and current
10     Perl code does, but instead checked all possibilities simultaneously by keeping
11     a list of current states and checking all of them as it advanced through the
12     subject string. (In the terminology of Jeffrey Friedl's book, it was a "DFA
13     algorithm".) When the pattern was all used up, all remaining states were
14     possible matches, and the one matching the longest subset of the subject string
15     was chosen. This did not necessarily maximize the individual wild portions of
16     the pattern, as is expected in Unix and Perl-style regular expressions.
17 nigel 41
18     By contrast, the code originally written by Henry Spencer and subsequently
19     heavily modified for Perl actually compiles the expression twice: once in a
20     dummy mode in order to find out how much store will be needed, and then for
21     real. The execution function operates by backtracking and maximizing (or,
22     optionally, minimizing in Perl) the amount of the subject that matches
23     individual wild portions of the pattern. This is an "NFA algorithm" in Friedl's
24     terminology.
25    
26 nigel 43 For the set of functions that forms PCRE (which are unrelated to those
27     mentioned above), I tried at first to invent an algorithm that used an amount
28     of store bounded by a multiple of the number of characters in the pattern, to
29     save on compiling time. However, because of the greater complexity in Perl
30     regular expressions, I couldn't do this. In any case, a first pass through the
31 nigel 63 pattern is needed, for a number of reasons. PCRE works by running a very
32     degenerate first pass to calculate a maximum store size, and then a second pass
33     to do the real compile - which may use a bit less than the predicted amount of
34     store. The idea is that this is going to turn out faster because the first pass
35     is degenerate and the second pass can just store stuff straight into the
36     vector. It does make the compiling functions bigger, of course, but they have
37     got quite big anyway to handle all the Perl stuff.
38 nigel 41
39     The compiled form of a pattern is a vector of bytes, containing items of
40     variable length. The first byte in an item is an opcode, and the length of the
41     item is either implicit in the opcode or contained in the data bytes which
42     follow it. A list of all the opcodes follows:
43    
44     Opcodes with no following data
45     ------------------------------
46    
47     These items are all just one byte long
48    
49     OP_END end of pattern
50     OP_ANY match any character
51 nigel 71 OP_ANYBYTE match any single byte, even in UTF-8 mode
52 nigel 41 OP_SOD match start of data: \A
53 nigel 71 OP_SOM, start of match (subject + offset): \G
54 nigel 41 OP_CIRC ^ (start of data, or after \n in multiline)
55     OP_NOT_WORD_BOUNDARY \W
56     OP_WORD_BOUNDARY \w
57     OP_NOT_DIGIT \D
58     OP_DIGIT \d
59     OP_NOT_WHITESPACE \S
60     OP_WHITESPACE \s
61     OP_NOT_WORDCHAR \W
62     OP_WORDCHAR \w
63     OP_EODN match end of data or \n at end: \Z
64     OP_EOD match end of data: \z
65     OP_DOLL $ (end of data, or before \n in multiline)
66    
67    
68     Repeating single characters
69     ---------------------------
70    
71     The common repeats (*, +, ?) when applied to a single character appear as
72     two-byte items using the following opcodes:
73    
74     OP_STAR
75     OP_MINSTAR
76     OP_PLUS
77     OP_MINPLUS
78     OP_QUERY
79     OP_MINQUERY
80    
81     Those with "MIN" in their name are the minimizing versions. Each is followed by
82     the character that is to be repeated. Other repeats make use of
83    
84     OP_UPTO
85     OP_MINUPTO
86     OP_EXACT
87    
88     which are followed by a two-byte count (most significant first) and the
89     repeated character. OP_UPTO matches from 0 to the given number. A repeat with a
90     non-zero minimum and a fixed maximum is coded as an OP_EXACT followed by an
91     OP_UPTO (or OP_MINUPTO).
92    
93    
94     Repeating character types
95     -------------------------
96    
97     Repeats of things like \d are done exactly as for single characters, except
98     that instead of a character, the opcode for the type is stored in the data
99     byte. The opcodes are:
100    
101     OP_TYPESTAR
102     OP_TYPEMINSTAR
103     OP_TYPEPLUS
104     OP_TYPEMINPLUS
105     OP_TYPEQUERY
106     OP_TYPEMINQUERY
107     OP_TYPEUPTO
108     OP_TYPEMINUPTO
109     OP_TYPEEXACT
110    
111    
112     Matching a character string
113     ---------------------------
114    
115     The OP_CHARS opcode is followed by a one-byte count and then that number of
116     characters. If there are more than 255 characters in sequence, successive
117     instances of OP_CHARS are used.
118    
119    
120     Character classes
121     -----------------
122    
123 nigel 71 If there is only one character, OP_CHARS is used for a positive class,
124 nigel 63 and OP_NOT for a negative one (that is, for something like [^a]). However, in
125     UTF-8 mode, this applies only to characters with values < 128, because OP_NOT
126     is confined to single bytes.
127 nigel 41
128 nigel 63 Another set of repeating opcodes (OP_NOTSTAR etc.) are used for a repeated,
129     negated, single-character class. The normal ones (OP_STAR etc.) are used for a
130     repeated positive single-character class.
131    
132 nigel 71 When there's more than one character in a class and all the characters are less
133     than 256, OP_CLASS is used for a positive class, and OP_NCLASS for a negative
134     one. In either case, the opcode is followed by a 32-byte bit map containing a 1
135     bit for every character that is acceptable. The bits are counted from the least
136     significant end of each byte.
137 nigel 41
138 nigel 71 The reason for having both OP_CLASS and OP_NCLASS is so that, in UTF-8 mode,
139     subject characters with values greater than 256 can be handled correctly. For
140     OP_CLASS they don't match, whereas for OP_NCLASS they do.
141    
142 nigel 63 For classes containing characters with values > 255, OP_XCLASS is used. It
143     optionally uses a bit map (if any characters lie within it), followed by a list
144     of pairs and single characters. There is a flag character than indicates
145     whether it's a positive or a negative class.
146 nigel 41
147 nigel 63
148 nigel 41 Back references
149     ---------------
150    
151 nigel 53 OP_REF is followed by two bytes containing the reference number.
152 nigel 41
153    
154     Repeating character classes and back references
155     -----------------------------------------------
156    
157     Single-character classes are handled specially (see above). This applies to
158     OP_CLASS and OP_REF. In both cases, the repeat information follows the base
159     item. The matching code looks at the following opcode to see if it is one of
160    
161     OP_CRSTAR
162     OP_CRMINSTAR
163     OP_CRPLUS
164     OP_CRMINPLUS
165     OP_CRQUERY
166     OP_CRMINQUERY
167     OP_CRRANGE
168     OP_CRMINRANGE
169    
170     All but the last two are just single-byte items. The others are followed by
171     four bytes of data, comprising the minimum and maximum repeat counts.
172    
173    
174     Brackets and alternation
175     ------------------------
176    
177 nigel 43 A pair of non-capturing (round) brackets is wrapped round each expression at
178 nigel 41 compile time, so alternation always happens in the context of brackets.
179 nigel 53
180 nigel 43 Non-capturing brackets use the opcode OP_BRA, while capturing brackets use
181 nigel 41 OP_BRA+1, OP_BRA+2, etc. [Note for North Americans: "bracket" to some English
182 nigel 43 speakers, including myself, can be round, square, curly, or pointy. Hence this
183     usage.]
184 nigel 41
185 nigel 53 Originally PCRE was limited to 99 capturing brackets (so as not to use up all
186     the opcodes). From release 3.5, there is no limit. What happens is that the
187     first ones, up to EXTRACT_BASIC_MAX are handled with separate opcodes, as
188     above. If there are more, the opcode is set to EXTRACT_BASIC_MAX+1, and the
189     first operation in the bracket is OP_BRANUMBER, followed by a 2-byte bracket
190     number. This opcode is ignored while matching, but is fished out when handling
191     the bracket itself. (They could have all been done like this, but I was making
192     minimal changes.)
193    
194 nigel 41 A bracket opcode is followed by two bytes which give the offset to the next
195     alternative OP_ALT or, if there aren't any branches, to the matching KET
196     opcode. Each OP_ALT is followed by two bytes giving the offset to the next one,
197     or to the KET opcode.
198    
199     OP_KET is used for subpatterns that do not repeat indefinitely, while
200     OP_KETRMIN and OP_KETRMAX are used for indefinite repetitions, minimally or
201     maximally respectively. All three are followed by two bytes giving (as a
202     positive number) the offset back to the matching BRA opcode.
203    
204     If a subpattern is quantified such that it is permitted to match zero times, it
205     is preceded by one of OP_BRAZERO or OP_BRAMINZERO. These are single-byte
206     opcodes which tell the matcher that skipping this subpattern entirely is a
207     valid branch.
208    
209     A subpattern with an indefinite maximum repetition is replicated in the
210     compiled data its minimum number of times (or once with a BRAZERO if the
211     minimum is zero), with the final copy terminating with a KETRMIN or KETRMAX as
212     appropriate.
213    
214     A subpattern with a bounded maximum repetition is replicated in a nested
215     fashion up to the maximum number of times, with BRAZERO or BRAMINZERO before
216     each replication after the minimum, so that, for example, (abc){2,5} is
217 nigel 53 compiled as (abc)(abc)((abc)((abc)(abc)?)?)?. The 99 and 200 bracket limits do
218     not apply to these internally generated brackets.
219 nigel 41
220    
221     Assertions
222     ----------
223    
224     Forward assertions are just like other subpatterns, but starting with one of
225     the opcodes OP_ASSERT or OP_ASSERT_NOT. Backward assertions use the opcodes
226     OP_ASSERTBACK and OP_ASSERTBACK_NOT, and the first opcode inside the assertion
227     is OP_REVERSE, followed by a two byte count of the number of characters to move
228 nigel 49 back the pointer in the subject string. When operating in UTF-8 mode, the count
229     is a character count rather than a byte count. A separate count is present in
230     each alternative of a lookbehind assertion, allowing them to have different
231     fixed lengths.
232 nigel 41
233    
234     Once-only subpatterns
235     ---------------------
236    
237     These are also just like other subpatterns, but they start with the opcode
238     OP_ONCE.
239    
240    
241     Conditional subpatterns
242     -----------------------
243    
244     These are like other subpatterns, but they start with the opcode OP_COND. If
245     the condition is a back reference, this is stored at the start of the
246 nigel 53 subpattern using the opcode OP_CREF followed by two bytes containing the
247 nigel 63 reference number. If the condition is "in recursion" (coded as "(?(R)"), the
248     same scheme is used, with a "reference number" of 0xffff. Otherwise, a
249     conditional subpattern always starts with one of the assertions.
250 nigel 41
251    
252 nigel 71 Recursion
253     ---------
254    
255     Recursion either matches the current regex, or some subexpression. The opcode
256     OP_RECURSE is followed by an value which is the offset to the starting bracket
257     from the start of the whole pattern.
258    
259    
260     Callout
261     -------
262    
263     OP_CALLOUT is followed by one byte of data that holds a callout number in the
264     range 0 to 255.
265    
266    
267 nigel 41 Changing options
268     ----------------
269    
270 nigel 63 If any of the /i, /m, or /s options are changed within a pattern, an OP_OPT
271     opcode is compiled, followed by one byte containing the new settings of these
272     flags. If there are several alternatives, there is an occurrence of OP_OPT at
273     the start of all those following the first options change, to set appropriate
274     options for the start of the alternative. Immediately after the end of the
275     group there is another such item to reset the flags to their previous values. A
276     change of flag right at the very start of the pattern can be handled entirely
277     at compile time, and so does not cause anything to be put into the compiled
278     data.
279 nigel 41
280     Philip Hazel
281 nigel 71 August 2003

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