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1 nigel 41 Technical Notes about PCRE
2     --------------------------
4 nigel 91 These are very rough technical notes that record potentially useful information
5 ph10 612 about PCRE internals. For information about testing PCRE, see the pcretest
6     documentation and the comment at the head of the RunTest file.
7 nigel 91
8 ph10 550
9 nigel 75 Historical note 1
10     -----------------
12 nigel 41 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 nigel 63 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 nigel 75 subject string. In the terminology of Jeffrey Friedl's book, it was a "DFA
21 nigel 93 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 nigel 41
27 ph10 550
28 nigel 75 Historical note 2
29     -----------------
31 nigel 91 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 nigel 41
40 ph10 550
41 nigel 75 OK, here's the real stuff
42     -------------------------
44 nigel 77 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 nigel 93 first pass through the pattern is helpful for other reasons.
50 nigel 41
51 ph10 550
52 ph10 840 Support for 16-bit data strings
53     -------------------------------
55     From release 8.30, PCRE supports 16-bit as well as 8-bit data strings, by being
56     compilable in either 8-bit or 16-bit modes, or both. Thus, two different
57     libraries can be created. In the description that follows, the word "short" is
58     used for a 16-bit data quantity, and the word "unit" is used for a quantity
59     that is a byte in 8-bit mode and a short in 16-bit mode. However, so as not to
60     over-complicate the text, the names of PCRE functions are given in 8-bit form
61     only.
64 nigel 93 Computing the memory requirement: how it was
65     --------------------------------------------
67     Up to and including release 6.7, PCRE worked by running a very degenerate first
68     pass to calculate a maximum store size, and then a second pass to do the real
69     compile - which might use a bit less than the predicted amount of memory. The
70     idea was that this would turn out faster than the Henry Spencer code because
71     the first pass is degenerate and the second pass can just store stuff straight
72     into the vector, which it knows is big enough.
74 ph10 550
75 nigel 93 Computing the memory requirement: how it is
76     -------------------------------------------
78     By the time I was working on a potential 6.8 release, the degenerate first pass
79     had become very complicated and hard to maintain. Indeed one of the early
80     things I did for 6.8 was to fix Yet Another Bug in the memory computation. Then
81     I had a flash of inspiration as to how I could run the real compile function in
82     a "fake" mode that enables it to compute how much memory it would need, while
83     actually only ever using a few hundred bytes of working memory, and without too
84 ph10 602 many tests of the mode that might slow it down. So I refactored the compiling
85 nigel 93 functions to work this way. This got rid of about 600 lines of source. It
86     should make future maintenance and development easier. As this was such a major
87     change, I never released 6.8, instead upping the number to 7.0 (other quite
88 ph10 456 major changes were also present in the 7.0 release).
89 nigel 93
90 ph10 456 A side effect of this work was that the previous limit of 200 on the nesting
91 nigel 93 depth of parentheses was removed. However, there is a downside: pcre_compile()
92     runs more slowly than before (30% or more, depending on the pattern) because it
93 ph10 456 is doing a full analysis of the pattern. My hope was that this would not be a
94     big issue, and in the event, nobody has commented on it.
95 nigel 93
96 ph10 550
97 nigel 77 Traditional matching function
98     -----------------------------
100     The "traditional", and original, matching function is called pcre_exec(), and
101     it implements an NFA algorithm, similar to the original Henry Spencer algorithm
102 ph10 456 and the way that Perl works. This is not surprising, since it is intended to be
103     as compatible with Perl as possible. This is the function most users of PCRE
104 ph10 672 will use most of the time. From release 8.20, if PCRE is compiled with
105     just-in-time (JIT) support, and studying a compiled pattern with JIT is
106     successful, the JIT code is run instead of the normal pcre_exec() code, but the
107     result is the same.
108 nigel 77
109 ph10 550
110 nigel 77 Supplementary matching function
111     -------------------------------
113     From PCRE 6.0, there is also a supplementary matching function called
114     pcre_dfa_exec(). This implements a DFA matching algorithm that searches
115     simultaneously for all possible matches that start at one point in the subject
116     string. (Going back to my roots: see Historical Note 1 above.) This function
117     intreprets the same compiled pattern data as pcre_exec(); however, not all the
118 nigel 91 facilities are available, and those that are do not always work in quite the
119 nigel 77 same way. See the user documentation for details.
121 nigel 93 The algorithm that is used for pcre_dfa_exec() is not a traditional FSM,
122     because it may have a number of states active at one time. More work would be
123     needed at compile time to produce a traditional FSM where only one state is
124     ever active at once. I believe some other regex matchers work this way.
127 ph10 602 Changeable options
128     ------------------
130     The /i, /m, or /s options (PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL) may
131     change in the middle of patterns. From PCRE 8.13, their processing is handled
132     entirely at compile time by generating different opcodes for the different
133     settings. The runtime functions do not need to keep track of an options state
134     any more.
137 nigel 77 Format of compiled patterns
138     ---------------------------
140 ph10 840 The compiled form of a pattern is a vector of units (bytes in 8-bit mode, or
141     shorts in 16-bit mode), containing items of variable length. The first unit in
142     an item contains an opcode, and the length of the item is either implicit in
143     the opcode or contained in the data that follows it.
144 nigel 41
145 ph10 840 In many cases listed below, LINK_SIZE data values are specified for offsets
146     within the compiled pattern. LINK_SIZE always specifies a number of bytes. The
147     default value for LINK_SIZE is 2, but PCRE can be compiled to use 3-byte or
148     4-byte values for these offsets, although this impairs the performance. (3-byte
149     LINK_SIZE values are available only in 8-bit mode.) Specifing a LINK_SIZE
150     larger than 2 is necessary only when patterns whose compiled length is greater
151     than 64K are going to be processed. In this description, we assume the "normal"
152     compilation options. Data values that are counts (e.g. for quantifiers) are
153     always just two bytes long (one short in 16-bit mode).
154 nigel 75
155 nigel 41 Opcodes with no following data
156     ------------------------------
158 ph10 840 These items are all just one unit long
159 nigel 41
160     OP_END end of pattern
161 ph10 342 OP_ANY match any one character other than newline
162     OP_ALLANY match any one character, including newline
163 nigel 75 OP_ANYBYTE match any single byte, even in UTF-8 mode
164 nigel 41 OP_SOD match start of data: \A
165 nigel 71 OP_SOM, start of match (subject + offset): \G
166 ph10 181 OP_SET_SOM, set start of match (\K)
167 ph10 602 OP_CIRC ^ (start of data)
168     OP_CIRCM ^ multiline mode (start of data or after newline)
169 nigel 41 OP_NOT_WORD_BOUNDARY \W
171     OP_NOT_DIGIT \D
172     OP_DIGIT \d
173 ph10 181 OP_NOT_HSPACE \H
174     OP_HSPACE \h
175 nigel 41 OP_NOT_WHITESPACE \S
176     OP_WHITESPACE \s
177 ph10 181 OP_NOT_VSPACE \V
178     OP_VSPACE \v
179 nigel 41 OP_NOT_WORDCHAR \W
180     OP_WORDCHAR \w
181     OP_EODN match end of data or \n at end: \Z
182     OP_EOD match end of data: \z
183 ph10 602 OP_DOLL $ (end of data, or before final newline)
184     OP_DOLLM $ multiline mode (end of data or before newline)
185 nigel 75 OP_EXTUNI match an extended Unicode character
186 nigel 93 OP_ANYNL match any Unicode newline sequence
187 nigel 75
188 ph10 550 OP_ACCEPT ) These are Perl 5.10's "backtracking control
189     OP_COMMIT ) verbs". If OP_ACCEPT is inside capturing
190     OP_FAIL ) parentheses, it may be preceded by one or more
191     OP_PRUNE ) OP_CLOSE, followed by a 2-byte number,
192     OP_SKIP ) indicating which parentheses must be closed.
193 ph10 212
194 nigel 41
195 ph10 716 Backtracking control verbs with (optional) data
196     -----------------------------------------------
197 ph10 550
198 ph10 716 (*THEN) without an argument generates the opcode OP_THEN and no following data.
199 ph10 840 OP_MARK is followed by the mark name, preceded by a one-unit length, and
200 ph10 716 followed by a binary zero. For (*PRUNE), (*SKIP), and (*THEN) with arguments,
201     the opcodes OP_PRUNE_ARG, OP_SKIP_ARG, and OP_THEN_ARG are used, with the name
202     following in the same format.
203 ph10 550
205 ph10 602 Matching literal characters
206     ---------------------------
208     The OP_CHAR opcode is followed by a single character that is to be matched
209 ph10 840 casefully. For caseless matching, OP_CHARI is used. In UTF-8 or UTF-16 modes,
210     the character may be more than one unit long.
211 ph10 602
213 nigel 41 Repeating single characters
214     ---------------------------
216 ph10 840 The common repeats (*, +, ?), when applied to a single character, use the
217 ph10 602 following opcodes, which come in caseful and caseless versions:
218 nigel 41
219 ph10 602 Caseful Caseless
229 nigel 41
230 ph10 840 Each opcode is followed by the character that is to be repeated. In ASCII mode,
231     these are two-unit items; in UTF-8 or UTF-16 modes, the length is variable.
232     Those with "MIN" in their names are the minimizing versions. Those with "POS"
233     in their names are possessive versions. Other repeats make use of these
234     opcodes:
235 nigel 41
236 ph10 602 Caseful Caseless
241 nigel 41
242 ph10 840 Each of these is followed by a two-byte (one short) count (most significant
243     byte first in 8-bit mode) and then the repeated character. OP_UPTO matches from
244     0 to the given number. A repeat with a non-zero minimum and a fixed maximum is
245     coded as an OP_EXACT followed by an OP_UPTO (or OP_MINUPTO or OPT_POSUPTO).
246 nigel 41
248     Repeating character types
249     -------------------------
251     Repeats of things like \d are done exactly as for single characters, except
252     that instead of a character, the opcode for the type is stored in the data
253 ph10 840 unit. The opcodes are:
254 nigel 41
257 nigel 93 OP_TYPEPOSSTAR
258 nigel 41 OP_TYPEPLUS
260 nigel 93 OP_TYPEPOSPLUS
261 nigel 41 OP_TYPEQUERY
263 nigel 93 OP_TYPEPOSQUERY
264 nigel 41 OP_TYPEUPTO
266 nigel 93 OP_TYPEPOSUPTO
267 nigel 41 OP_TYPEEXACT
270 nigel 75 Match by Unicode property
271     -------------------------
273     OP_PROP and OP_NOTPROP are used for positive and negative matches of a
274     character by testing its Unicode property (the \p and \P escape sequences).
275 ph10 840 Each is followed by two units that encode the desired property as a type and a
276 nigel 91 value.
277 nigel 75
278 ph10 840 Repeats of these items use the OP_TYPESTAR etc. set of opcodes, followed by
279     three units: OP_PROP or OP_NOTPROP, and then the desired property type and
280 nigel 91 value.
281 nigel 75
283 nigel 41 Character classes
284     -----------------
286 ph10 840 If there is only one character in the class, OP_CHAR or OP_CHARI is used for a
287     positive class, and OP_NOT or OP_NOTI for a negative one (that is, for
288     something like [^a]). However, OP_NOT[I] can be used only with single-unit
289     characters, so in UTF-8 (UTF-16) mode, the use of OP_NOT[I] applies only to
290     characters whose code points are no greater than 127 (0xffff).
291 nigel 41
292 ph10 840 Another set of 13 repeating opcodes (called OP_NOTSTAR etc.) are used for
293     repeated, negated, single-character classes. The normal single-character
294     opcodes (OP_STAR, etc.) are used for repeated positive single-character
295     classes.
296 nigel 63
297 ph10 602 When there is more than one character in a class and all the characters are
298     less than 256, OP_CLASS is used for a positive class, and OP_NCLASS for a
299 ph10 840 negative one. In either case, the opcode is followed by a 32-byte (16-short)
300     bit map containing a 1 bit for every character that is acceptable. The bits are
301     counted from the least significant end of each unit. In caseless mode, bits for
302     both cases are set.
303 nigel 41
304 ph10 840 The reason for having both OP_CLASS and OP_NCLASS is so that, in UTF-8/16 mode,
305     subject characters with values greater than 255 can be handled correctly. For
306 ph10 602 OP_CLASS they do not match, whereas for OP_NCLASS they do.
307 nigel 71
308 ph10 840 For classes containing characters with values greater than 255, OP_XCLASS is
309     used. It optionally uses a bit map (if any characters lie within it), followed
310     by a list of pairs (for a range) and single characters. In caseless mode, both
311     cases are explicitly listed. There is a flag character than indicates whether
312     it is a positive or a negative class.
313 nigel 41
314 nigel 63
315 nigel 41 Back references
316     ---------------
318 ph10 840 OP_REF (caseful) or OP_REFI (caseless) is followed by two bytes (one short)
319     containing the reference number.
320 nigel 41
322     Repeating character classes and back references
323     -----------------------------------------------
325 nigel 93 Single-character classes are handled specially (see above). This section
326 ph10 602 applies to OP_CLASS and OP_REF[I]. In both cases, the repeat information
327     follows the base item. The matching code looks at the following opcode to see
328     if it is one of
329 nigel 41
330     OP_CRSTAR
332     OP_CRPLUS
334     OP_CRQUERY
336     OP_CRRANGE
339 ph10 840 All but the last two are just single-unit items. The others are followed by
340     four bytes (two shorts) of data, comprising the minimum and maximum repeat
341     counts. There are no special possessive opcodes for these repeats; a possessive
342     repeat is compiled into an atomic group.
343 nigel 41
345     Brackets and alternation
346     ------------------------
348 nigel 43 A pair of non-capturing (round) brackets is wrapped round each expression at
349 nigel 41 compile time, so alternation always happens in the context of brackets.
350 nigel 53
351 nigel 93 [Note for North Americans: "bracket" to some English speakers, including
352 ph10 840 myself, can be round, square, curly, or pointy. Hence this usage rather than
353     "parentheses".]
354 nigel 41
355 nigel 93 Non-capturing brackets use the opcode OP_BRA. Originally PCRE was limited to 99
356     capturing brackets and it used a different opcode for each one. From release
357     3.5, the limit was removed by putting the bracket number into the data for
358     higher-numbered brackets. From release 7.0 all capturing brackets are handled
359     this way, using the single opcode OP_CBRA.
360 nigel 53
361 nigel 77 A bracket opcode is followed by LINK_SIZE bytes which give the offset to the
362     next alternative OP_ALT or, if there aren't any branches, to the matching
363     OP_KET opcode. Each OP_ALT is followed by LINK_SIZE bytes giving the offset to
364 nigel 93 the next one, or to the OP_KET opcode. For capturing brackets, the bracket
365 ph10 840 number immediately follows the offset, always as a 2-byte (one short) item.
366 nigel 41
367 ph10 840 OP_KET is used for subpatterns that do not repeat indefinitely, and
368 nigel 41 OP_KETRMIN and OP_KETRMAX are used for indefinite repetitions, minimally or
369 ph10 604 maximally respectively (see below for possessive repetitions). All three are
370     followed by LINK_SIZE bytes giving (as a positive number) the offset back to
371     the matching bracket opcode.
372 nigel 41
373     If a subpattern is quantified such that it is permitted to match zero times, it
374 ph10 335 is preceded by one of OP_BRAZERO, OP_BRAMINZERO, or OP_SKIPZERO. These are
375 ph10 840 single-unit opcodes that tell the matcher that skipping the following
376 ph10 335 subpattern entirely is a valid branch. In the case of the first two, not
377     skipping the pattern is also valid (greedy and non-greedy). The third is used
378     when a pattern has the quantifier {0,0}. It cannot be entirely discarded,
379     because it may be called as a subroutine from elsewhere in the regex.
380 nigel 41
381     A subpattern with an indefinite maximum repetition is replicated in the
382 nigel 75 compiled data its minimum number of times (or once with OP_BRAZERO if the
383     minimum is zero), with the final copy terminating with OP_KETRMIN or OP_KETRMAX
384     as appropriate.
385 nigel 41
386     A subpattern with a bounded maximum repetition is replicated in a nested
387 nigel 75 fashion up to the maximum number of times, with OP_BRAZERO or OP_BRAMINZERO
388     before each replication after the minimum, so that, for example, (abc){2,5} is
389 nigel 93 compiled as (abc)(abc)((abc)((abc)(abc)?)?)?, except that each bracketed group
390     has the same number.
391 nigel 41
392 nigel 93 When a repeated subpattern has an unbounded upper limit, it is checked to see
393     whether it could match an empty string. If this is the case, the opcode in the
394     final replication is changed to OP_SBRA or OP_SCBRA. This tells the matcher
395     that it needs to check for matching an empty string when it hits OP_KETRMIN or
396     OP_KETRMAX, and if so, to break the loop.
397 nigel 41
398 ph10 604 Possessive brackets
399     -------------------
400 nigel 93
401 ph10 604 When a repeated group (capturing or non-capturing) is marked as possessive by
402     the "+" notation, e.g. (abc)++, different opcodes are used. Their names all
403     have POS on the end, e.g. OP_BRAPOS instead of OP_BRA and OP_SCPBRPOS instead
404     of OP_SCBRA. The end of such a group is marked by OP_KETRPOS. If the minimum
405     repetition is zero, the group is preceded by OP_BRAPOSZERO.
408 nigel 41 Assertions
409     ----------
411     Forward assertions are just like other subpatterns, but starting with one of
412     the opcodes OP_ASSERT or OP_ASSERT_NOT. Backward assertions use the opcodes
413     OP_ASSERTBACK and OP_ASSERTBACK_NOT, and the first opcode inside the assertion
414 ph10 840 is OP_REVERSE, followed by a two byte (one short) count of the number of
415     characters to move back the pointer in the subject string. In ASCII mode, the
416     count is a number of units, but in UTF-8/16 mode each character may occupy more
417     than one unit. A separate count is present in each alternative of a lookbehind
418     assertion, allowing them to have different fixed lengths.
419 nigel 41
421 nigel 93 Once-only (atomic) subpatterns
422     ------------------------------
423 nigel 41
424     These are also just like other subpatterns, but they start with the opcode
425 nigel 93 OP_ONCE. The check for matching an empty string in an unbounded repeat is
426     handled entirely at runtime, so there is just this one opcode.
427 nigel 41
429     Conditional subpatterns
430     -----------------------
432 nigel 93 These are like other subpatterns, but they start with the opcode OP_COND, or
433     OP_SCOND for one that might match an empty string in an unbounded repeat. If
434 nigel 41 the condition is a back reference, this is stored at the start of the
435 ph10 840 subpattern using the opcode OP_CREF followed by two bytes (one short)
436     containing the reference number. OP_NCREF is used instead if the reference was
437     generated by name (so that the runtime code knows to check for duplicate
438     names).
439 nigel 41
440 ph10 460 If the condition is "in recursion" (coded as "(?(R)"), or "in recursion of
441     group x" (coded as "(?(Rx)"), the group number is stored at the start of the
442     subpattern using the opcode OP_RREF or OP_NRREF (cf OP_NCREF), and a value of
443 ph10 840 zero for "the whole pattern". For a DEFINE condition, just the single unit
444 ph10 460 OP_DEF is used (it has no associated data). Otherwise, a conditional subpattern
445     always starts with one of the assertions.
446 nigel 41
447 ph10 460
448 nigel 71 Recursion
449     ---------
451     Recursion either matches the current regex, or some subexpression. The opcode
452     OP_RECURSE is followed by an value which is the offset to the starting bracket
453 nigel 87 from the start of the whole pattern. From release 6.5, OP_RECURSE is
454     automatically wrapped inside OP_ONCE brackets (because otherwise some patterns
455     broke it). OP_RECURSE is also used for "subroutine" calls, even though they
456     are not strictly a recursion.
457 nigel 71
459     Callout
460     -------
462 ph10 840 OP_CALLOUT is followed by one unit of data that holds a callout number in the
463 nigel 75 range 0 to 254 for manual callouts, or 255 for an automatic callout. In both
464 ph10 840 cases there follows a two-byte (one short) value giving the offset in the
465     pattern to the start of the following item, and another two-byte (one short)
466     item giving the length of the next item.
467 nigel 71
469 nigel 41 Philip Hazel
470 ph10 840 December 2011

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