Annotation of embedaddon/php/ext/pcre/pcrelib/HACKING, revision 1.1.1.1
1.1 misho 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:
8: Historical note 1
9: -----------------
10:
11: Many years ago I implemented some regular expression functions to an algorithm
12: suggested by Martin Richards. These were not Unix-like in form, and were quite
13: restricted in what they could do by comparison with Perl. The interesting part
14: about the algorithm was that the amount of space required to hold the compiled
15: form of an expression was known in advance. The code to apply an expression did
16: not operate by backtracking, as the original Henry Spencer code and current
17: Perl code does, but instead checked all possibilities simultaneously by keeping
18: a list of current states and checking all of them as it advanced through the
19: subject string. In the terminology of Jeffrey Friedl's book, it was a "DFA
20: algorithm", though it was not a traditional Finite State Machine (FSM). When
21: the pattern was all used up, all remaining states were possible matches, and
22: the one matching the longest subset of the subject string was chosen. This did
23: not necessarily maximize the individual wild portions of the pattern, as is
24: expected in Unix and Perl-style regular expressions.
25:
26:
27: Historical note 2
28: -----------------
29:
30: By contrast, the code originally written by Henry Spencer (which was
31: subsequently heavily modified for Perl) compiles the expression twice: once in
32: a dummy mode in order to find out how much store will be needed, and then for
33: real. (The Perl version probably doesn't do this any more; I'm talking about
34: the original library.) The execution function operates by backtracking and
35: maximizing (or, optionally, minimizing in Perl) the amount of the subject that
36: matches individual wild portions of the pattern. This is an "NFA algorithm" in
37: Friedl's terminology.
38:
39:
40: OK, here's the real stuff
41: -------------------------
42:
43: For the set of functions that form the "basic" PCRE library (which are
44: unrelated to those mentioned above), I tried at first to invent an algorithm
45: that used an amount of store bounded by a multiple of the number of characters
46: in the pattern, to save on compiling time. However, because of the greater
47: complexity in Perl regular expressions, I couldn't do this. In any case, a
48: first pass through the pattern is helpful for other reasons.
49:
50:
51: Computing the memory requirement: how it was
52: --------------------------------------------
53:
54: Up to and including release 6.7, PCRE worked by running a very degenerate first
55: pass to calculate a maximum store size, and then a second pass to do the real
56: compile - which might use a bit less than the predicted amount of memory. The
57: idea was that this would turn out faster than the Henry Spencer code because
58: the first pass is degenerate and the second pass can just store stuff straight
59: into the vector, which it knows is big enough.
60:
61:
62: Computing the memory requirement: how it is
63: -------------------------------------------
64:
65: By the time I was working on a potential 6.8 release, the degenerate first pass
66: had become very complicated and hard to maintain. Indeed one of the early
67: things I did for 6.8 was to fix Yet Another Bug in the memory computation. Then
68: I had a flash of inspiration as to how I could run the real compile function in
69: a "fake" mode that enables it to compute how much memory it would need, while
70: actually only ever using a few hundred bytes of working memory, and without too
71: many tests of the mode that might slow it down. So I re-factored the compiling
72: functions to work this way. This got rid of about 600 lines of source. It
73: should make future maintenance and development easier. As this was such a major
74: change, I never released 6.8, instead upping the number to 7.0 (other quite
75: major changes were also present in the 7.0 release).
76:
77: A side effect of this work was that the previous limit of 200 on the nesting
78: depth of parentheses was removed. However, there is a downside: pcre_compile()
79: runs more slowly than before (30% or more, depending on the pattern) because it
80: is doing a full analysis of the pattern. My hope was that this would not be a
81: big issue, and in the event, nobody has commented on it.
82:
83:
84: Traditional matching function
85: -----------------------------
86:
87: The "traditional", and original, matching function is called pcre_exec(), and
88: it implements an NFA algorithm, similar to the original Henry Spencer algorithm
89: and the way that Perl works. This is not surprising, since it is intended to be
90: as compatible with Perl as possible. This is the function most users of PCRE
91: will use most of the time.
92:
93:
94: Supplementary matching function
95: -------------------------------
96:
97: From PCRE 6.0, there is also a supplementary matching function called
98: pcre_dfa_exec(). This implements a DFA matching algorithm that searches
99: simultaneously for all possible matches that start at one point in the subject
100: string. (Going back to my roots: see Historical Note 1 above.) This function
101: intreprets the same compiled pattern data as pcre_exec(); however, not all the
102: facilities are available, and those that are do not always work in quite the
103: same way. See the user documentation for details.
104:
105: The algorithm that is used for pcre_dfa_exec() is not a traditional FSM,
106: because it may have a number of states active at one time. More work would be
107: needed at compile time to produce a traditional FSM where only one state is
108: ever active at once. I believe some other regex matchers work this way.
109:
110:
111: Format of compiled patterns
112: ---------------------------
113:
114: The compiled form of a pattern is a vector of bytes, containing items of
115: variable length. The first byte in an item is an opcode, and the length of the
116: item is either implicit in the opcode or contained in the data bytes that
117: follow it.
118:
119: In many cases below LINK_SIZE data values are specified for offsets within the
120: compiled pattern. The default value for LINK_SIZE is 2, but PCRE can be
121: compiled to use 3-byte or 4-byte values for these offsets (impairing the
122: performance). This is necessary only when patterns whose compiled length is
123: greater than 64K are going to be processed. In this description, we assume the
124: "normal" compilation options. Data values that are counts (e.g. for
125: quantifiers) are always just two bytes long.
126:
127: A list of the opcodes follows:
128:
129: Opcodes with no following data
130: ------------------------------
131:
132: These items are all just one byte long
133:
134: OP_END end of pattern
135: OP_ANY match any one character other than newline
136: OP_ALLANY match any one character, including newline
137: OP_ANYBYTE match any single byte, even in UTF-8 mode
138: OP_SOD match start of data: \A
139: OP_SOM, start of match (subject + offset): \G
140: OP_SET_SOM, set start of match (\K)
141: OP_CIRC ^ (start of data, or after \n in multiline)
142: OP_NOT_WORD_BOUNDARY \W
143: OP_WORD_BOUNDARY \w
144: OP_NOT_DIGIT \D
145: OP_DIGIT \d
146: OP_NOT_HSPACE \H
147: OP_HSPACE \h
148: OP_NOT_WHITESPACE \S
149: OP_WHITESPACE \s
150: OP_NOT_VSPACE \V
151: OP_VSPACE \v
152: OP_NOT_WORDCHAR \W
153: OP_WORDCHAR \w
154: OP_EODN match end of data or \n at end: \Z
155: OP_EOD match end of data: \z
156: OP_DOLL $ (end of data, or before \n in multiline)
157: OP_EXTUNI match an extended Unicode character
158: OP_ANYNL match any Unicode newline sequence
159:
160: OP_ACCEPT ) These are Perl 5.10's "backtracking control
161: OP_COMMIT ) verbs". If OP_ACCEPT is inside capturing
162: OP_FAIL ) parentheses, it may be preceded by one or more
163: OP_PRUNE ) OP_CLOSE, followed by a 2-byte number,
164: OP_SKIP ) indicating which parentheses must be closed.
165:
166:
167: Backtracking control verbs with data
168: ------------------------------------
169:
170: OP_THEN is followed by a LINK_SIZE offset, which is the distance back to the
171: start of the current branch.
172:
173: OP_MARK is followed by the mark name, preceded by a one-byte length, and
174: followed by a binary zero. For (*PRUNE), (*SKIP), and (*THEN) with arguments,
175: the opcodes OP_PRUNE_ARG, OP_SKIP_ARG, and OP_THEN_ARG are used. For the first
176: two, the name follows immediately; for OP_THEN_ARG, it follows the LINK_SIZE
177: offset value.
178:
179:
180: Repeating single characters
181: ---------------------------
182:
183: The common repeats (*, +, ?) when applied to a single character use the
184: following opcodes:
185:
186: OP_STAR
187: OP_MINSTAR
188: OP_POSSTAR
189: OP_PLUS
190: OP_MINPLUS
191: OP_POSPLUS
192: OP_QUERY
193: OP_MINQUERY
194: OP_POSQUERY
195:
196: In ASCII mode, these are two-byte items; in UTF-8 mode, the length is variable.
197: Those with "MIN" in their name are the minimizing versions. Those with "POS" in
198: their names are possessive versions. Each is followed by the character that is
199: to be repeated. Other repeats make use of
200:
201: OP_UPTO
202: OP_MINUPTO
203: OP_POSUPTO
204: OP_EXACT
205:
206: which are followed by a two-byte count (most significant first) and the
207: repeated character. OP_UPTO matches from 0 to the given number. A repeat with a
208: non-zero minimum and a fixed maximum is coded as an OP_EXACT followed by an
209: OP_UPTO (or OP_MINUPTO or OPT_POSUPTO).
210:
211:
212: Repeating character types
213: -------------------------
214:
215: Repeats of things like \d are done exactly as for single characters, except
216: that instead of a character, the opcode for the type is stored in the data
217: byte. The opcodes are:
218:
219: OP_TYPESTAR
220: OP_TYPEMINSTAR
221: OP_TYPEPOSSTAR
222: OP_TYPEPLUS
223: OP_TYPEMINPLUS
224: OP_TYPEPOSPLUS
225: OP_TYPEQUERY
226: OP_TYPEMINQUERY
227: OP_TYPEPOSQUERY
228: OP_TYPEUPTO
229: OP_TYPEMINUPTO
230: OP_TYPEPOSUPTO
231: OP_TYPEEXACT
232:
233:
234: Match by Unicode property
235: -------------------------
236:
237: OP_PROP and OP_NOTPROP are used for positive and negative matches of a
238: character by testing its Unicode property (the \p and \P escape sequences).
239: Each is followed by two bytes that encode the desired property as a type and a
240: value.
241:
242: Repeats of these items use the OP_TYPESTAR etc. set of opcodes, followed by
243: three bytes: OP_PROP or OP_NOTPROP and then the desired property type and
244: value.
245:
246:
247: Matching literal characters
248: ---------------------------
249:
250: The OP_CHAR opcode is followed by a single character that is to be matched
251: casefully. For caseless matching, OP_CHARNC is used. In UTF-8 mode, the
252: character may be more than one byte long. (Earlier versions of PCRE used
253: multi-character strings, but this was changed to allow some new features to be
254: added.)
255:
256:
257: Character classes
258: -----------------
259:
260: If there is only one character, OP_CHAR or OP_CHARNC is used for a positive
261: class, and OP_NOT for a negative one (that is, for something like [^a]).
262: However, in UTF-8 mode, the use of OP_NOT applies only to characters with
263: values < 128, because OP_NOT is confined to single bytes.
264:
265: Another set of repeating opcodes (OP_NOTSTAR etc.) are used for a repeated,
266: negated, single-character class. The normal ones (OP_STAR etc.) are used for a
267: repeated positive single-character class.
268:
269: When there's more than one character in a class and all the characters are less
270: than 256, OP_CLASS is used for a positive class, and OP_NCLASS for a negative
271: one. In either case, the opcode is followed by a 32-byte bit map containing a 1
272: bit for every character that is acceptable. The bits are counted from the least
273: significant end of each byte.
274:
275: The reason for having both OP_CLASS and OP_NCLASS is so that, in UTF-8 mode,
276: subject characters with values greater than 256 can be handled correctly. For
277: OP_CLASS they don't match, whereas for OP_NCLASS they do.
278:
279: For classes containing characters with values > 255, OP_XCLASS is used. It
280: optionally uses a bit map (if any characters lie within it), followed by a list
281: of pairs and single characters. There is a flag character than indicates
282: whether it's a positive or a negative class.
283:
284:
285: Back references
286: ---------------
287:
288: OP_REF is followed by two bytes containing the reference number.
289:
290:
291: Repeating character classes and back references
292: -----------------------------------------------
293:
294: Single-character classes are handled specially (see above). This section
295: applies to OP_CLASS and OP_REF. In both cases, the repeat information follows
296: the base item. The matching code looks at the following opcode to see if it is
297: one of
298:
299: OP_CRSTAR
300: OP_CRMINSTAR
301: OP_CRPLUS
302: OP_CRMINPLUS
303: OP_CRQUERY
304: OP_CRMINQUERY
305: OP_CRRANGE
306: OP_CRMINRANGE
307:
308: All but the last two are just single-byte items. The others are followed by
309: four bytes of data, comprising the minimum and maximum repeat counts. There are
310: no special possessive opcodes for these repeats; a possessive repeat is
311: compiled into an atomic group.
312:
313:
314: Brackets and alternation
315: ------------------------
316:
317: A pair of non-capturing (round) brackets is wrapped round each expression at
318: compile time, so alternation always happens in the context of brackets.
319:
320: [Note for North Americans: "bracket" to some English speakers, including
321: myself, can be round, square, curly, or pointy. Hence this usage.]
322:
323: Non-capturing brackets use the opcode OP_BRA. Originally PCRE was limited to 99
324: capturing brackets and it used a different opcode for each one. From release
325: 3.5, the limit was removed by putting the bracket number into the data for
326: higher-numbered brackets. From release 7.0 all capturing brackets are handled
327: this way, using the single opcode OP_CBRA.
328:
329: A bracket opcode is followed by LINK_SIZE bytes which give the offset to the
330: next alternative OP_ALT or, if there aren't any branches, to the matching
331: OP_KET opcode. Each OP_ALT is followed by LINK_SIZE bytes giving the offset to
332: the next one, or to the OP_KET opcode. For capturing brackets, the bracket
333: number immediately follows the offset, always as a 2-byte item.
334:
335: OP_KET is used for subpatterns that do not repeat indefinitely, while
336: OP_KETRMIN and OP_KETRMAX are used for indefinite repetitions, minimally or
337: maximally respectively. All three are followed by LINK_SIZE bytes giving (as a
338: positive number) the offset back to the matching bracket opcode.
339:
340: If a subpattern is quantified such that it is permitted to match zero times, it
341: is preceded by one of OP_BRAZERO, OP_BRAMINZERO, or OP_SKIPZERO. These are
342: single-byte opcodes that tell the matcher that skipping the following
343: subpattern entirely is a valid branch. In the case of the first two, not
344: skipping the pattern is also valid (greedy and non-greedy). The third is used
345: when a pattern has the quantifier {0,0}. It cannot be entirely discarded,
346: because it may be called as a subroutine from elsewhere in the regex.
347:
348: A subpattern with an indefinite maximum repetition is replicated in the
349: compiled data its minimum number of times (or once with OP_BRAZERO if the
350: minimum is zero), with the final copy terminating with OP_KETRMIN or OP_KETRMAX
351: as appropriate.
352:
353: A subpattern with a bounded maximum repetition is replicated in a nested
354: fashion up to the maximum number of times, with OP_BRAZERO or OP_BRAMINZERO
355: before each replication after the minimum, so that, for example, (abc){2,5} is
356: compiled as (abc)(abc)((abc)((abc)(abc)?)?)?, except that each bracketed group
357: has the same number.
358:
359: When a repeated subpattern has an unbounded upper limit, it is checked to see
360: whether it could match an empty string. If this is the case, the opcode in the
361: final replication is changed to OP_SBRA or OP_SCBRA. This tells the matcher
362: that it needs to check for matching an empty string when it hits OP_KETRMIN or
363: OP_KETRMAX, and if so, to break the loop.
364:
365:
366: Assertions
367: ----------
368:
369: Forward assertions are just like other subpatterns, but starting with one of
370: the opcodes OP_ASSERT or OP_ASSERT_NOT. Backward assertions use the opcodes
371: OP_ASSERTBACK and OP_ASSERTBACK_NOT, and the first opcode inside the assertion
372: is OP_REVERSE, followed by a two byte count of the number of characters to move
373: back the pointer in the subject string. When operating in UTF-8 mode, the count
374: is a character count rather than a byte count. A separate count is present in
375: each alternative of a lookbehind assertion, allowing them to have different
376: fixed lengths.
377:
378:
379: Once-only (atomic) subpatterns
380: ------------------------------
381:
382: These are also just like other subpatterns, but they start with the opcode
383: OP_ONCE. The check for matching an empty string in an unbounded repeat is
384: handled entirely at runtime, so there is just this one opcode.
385:
386:
387: Conditional subpatterns
388: -----------------------
389:
390: These are like other subpatterns, but they start with the opcode OP_COND, or
391: OP_SCOND for one that might match an empty string in an unbounded repeat. If
392: the condition is a back reference, this is stored at the start of the
393: subpattern using the opcode OP_CREF followed by two bytes containing the
394: reference number. OP_NCREF is used instead if the reference was generated by
395: name (so that the runtime code knows to check for duplicate names).
396:
397: If the condition is "in recursion" (coded as "(?(R)"), or "in recursion of
398: group x" (coded as "(?(Rx)"), the group number is stored at the start of the
399: subpattern using the opcode OP_RREF or OP_NRREF (cf OP_NCREF), and a value of
400: zero for "the whole pattern". For a DEFINE condition, just the single byte
401: OP_DEF is used (it has no associated data). Otherwise, a conditional subpattern
402: always starts with one of the assertions.
403:
404:
405: Recursion
406: ---------
407:
408: Recursion either matches the current regex, or some subexpression. The opcode
409: OP_RECURSE is followed by an value which is the offset to the starting bracket
410: from the start of the whole pattern. From release 6.5, OP_RECURSE is
411: automatically wrapped inside OP_ONCE brackets (because otherwise some patterns
412: broke it). OP_RECURSE is also used for "subroutine" calls, even though they
413: are not strictly a recursion.
414:
415:
416: Callout
417: -------
418:
419: OP_CALLOUT is followed by one byte of data that holds a callout number in the
420: range 0 to 254 for manual callouts, or 255 for an automatic callout. In both
421: cases there follows a two-byte value giving the offset in the pattern to the
422: start of the following item, and another two-byte item giving the length of the
423: next item.
424:
425:
426: Changing options
427: ----------------
428:
429: If any of the /i, /m, or /s options are changed within a pattern, an OP_OPT
430: opcode is compiled, followed by one byte containing the new settings of these
431: flags. If there are several alternatives, there is an occurrence of OP_OPT at
432: the start of all those following the first options change, to set appropriate
433: options for the start of the alternative. Immediately after the end of the
434: group there is another such item to reset the flags to their previous values. A
435: change of flag right at the very start of the pattern can be handled entirely
436: at compile time, and so does not cause anything to be put into the compiled
437: data.
438:
439: Philip Hazel
440: October 2010
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