1PERLREGUTS(1) Perl Programmers Reference Guide PERLREGUTS(1)
2
6 perlreguts - Description of the Perl regular expression engine.
7
9 This document is an attempt to shine some light on the guts of the
10 regex engine and how it works. The regex engine represents a
11 significant chunk of the perl codebase, but is relatively poorly
12 understood. This document is a meagre attempt at addressing this
13 situation. It is derived from the author's experience, comments in the
14 source code, other papers on the regex engine, feedback on the
15 perl5-porters mail list, and no doubt other places as well.
16 NOTICE! It should be clearly understood that the behavior and
18 structures discussed in this represents the state of the engine as the
19 author understood it at the time of writing. It is NOT an API
20 definition, it is purely an internals guide for those who want to hack
21 the regex engine, or understand how the regex engine works. Readers of
22 this document are expected to understand perl's regex syntax and its
23 usage in detail. If you want to learn about the basics of Perl's
24 regular expressions, see perlre. And if you want to replace the regex
25 engine with your own, see perlreapi.
26
28 A quick note on terms
29 There is some debate as to whether to say "regexp" or "regex". In this
30 document we will use the term "regex" unless there is a special reason
31 not to, in which case we will explain why.
32 When speaking about regexes we need to distinguish between their source
34 code form and their internal form. In this document we will use the
35 term "pattern" when we speak of their textual, source code form, and
36 the term "program" when we speak of their internal representation.
37 These correspond to the terms S-regex and B-regex that Mark Jason
38 Dominus employs in his paper on "Rx" ([1] in "REFERENCES").
39 What is a regular expression engine?
41 A regular expression engine is a program that takes a set of
42 constraints specified in a mini-language, and then applies those
43 constraints to a target string, and determines whether or not the
44 string satisfies the constraints. See perlre for a full definition of
45 the language.
46 In less grandiose terms, the first part of the job is to turn a pattern
48 into something the computer can efficiently use to find the matching
49 point in the string, and the second part is performing the search
50 itself.
51 To do this we need to produce a program by parsing the text. We then
53 need to execute the program to find the point in the string that
54 matches. And we need to do the whole thing efficiently.
55 Structure of a Regexp Program
57 High Level
58 Although it is a bit confusing and some people object to the
60 terminology, it is worth taking a look at a comment that has been in
61 regexp.h for years:
62 This is essentially a linear encoding of a nondeterministic finite-
64 state machine (aka syntax charts or "railroad normal form" in parsing
65 technology).
66 The term "railroad normal form" is a bit esoteric, with "syntax
68 diagram/charts", or "railroad diagram/charts" being more common terms.
69 Nevertheless it provides a useful mental image of a regex program: each
70 node can be thought of as a unit of track, with a single entry and in
71 most cases a single exit point (there are pieces of track that fork,
72 but statistically not many), and the whole forms a layout with a single
73 entry and single exit point. The matching process can be thought of as
74 a car that moves along the track, with the particular route through the
75 system being determined by the character read at each possible
76 connector point. A car can fall off the track at any point but it may
77 only proceed as long as it matches the track.
78 Thus the pattern "/foo(?:\w+|\d+|\s+)bar/" can be thought of as the
80 following chart:
81 [start]
83 |
84 <foo>
85 |
86 +-----+-----+
87 | | |
88 <\w+> <\d+> <\s+>
89 | | |
90 +-----+-----+
91 |
92 <bar>
93 |
94 [end]
95 The truth of the matter is that perl's regular expressions these days
97 are much more complex than this kind of structure, but visualising it
98 this way can help when trying to get your bearings, and it matches the
99 current implementation pretty closely.
100 To be more precise, we will say that a regex program is an encoding of
102 a graph. Each node in the graph corresponds to part of the original
103 regex pattern, such as a literal string or a branch, and has a pointer
104 to the nodes representing the next component to be matched. Since
105 "node" and "opcode" already have other meanings in the perl source, we
106 will call the nodes in a regex program "regops".
107 The program is represented by an array of "regnode" structures, one or
109 more of which represent a single regop of the program. Struct "regnode"
110 is the smallest struct needed, and has a field structure which is
111 shared with all the other larger structures.
112 The "next" pointers of all regops except "BRANCH" implement
114 concatenation; a "next" pointer with a "BRANCH" on both ends of it is
115 connecting two alternatives. [Here we have one of the subtle syntax
116 dependencies: an individual "BRANCH" (as opposed to a collection of
117 them) is never concatenated with anything because of operator
118 precedence.]
119 The operand of some types of regop is a literal string; for others, it
121 is a regop leading into a sub-program. In particular, the operand of a
122 "BRANCH" node is the first regop of the branch.
123 NOTE: As the railroad metaphor suggests, this is not a tree structure:
125 the tail of the branch connects to the thing following the set of
126 "BRANCH"es. It is a like a single line of railway track that splits as
127 it goes into a station or railway yard and rejoins as it comes out the
128 other side.
129 Regops
131 The base structure of a regop is defined in regexp.h as follows:
133 struct regnode {
135 U8 flags; /* Various purposes, sometimes overridden */
136 U8 type; /* Opcode value as specified by regnodes.h */
137 U16 next_off; /* Offset in size regnode */
138 };
139 Other larger "regnode"-like structures are defined in regcomp.h. They
141 are almost like subclasses in that they have the same fields as
142 "regnode", with possibly additional fields following in the structure,
143 and in some cases the specific meaning (and name) of some of base
144 fields are overridden. The following is a more complete description.
145 "regnode_1"
147 "regnode_2"
148 "regnode_1" structures have the same header, followed by a single
149 four-byte argument; "regnode_2" structures contain two two-byte
150 arguments instead:
151 regnode_1 U32 arg1;
153 regnode_2 U16 arg1; U16 arg2;
154 "regnode_string"
156 "regnode_string" structures, used for literal strings, follow the
157 header with a one-byte length and then the string data. Strings are
158 padded on the end with zero bytes so that the total length of the
159 node is a multiple of four bytes:
160 regnode_string char string[1];
162 U8 str_len; /* overrides flags */
163 "regnode_charclass"
165 Character classes are represented by "regnode_charclass"
166 structures, which have a four-byte argument and then a 32-byte
167 (256-bit) bitmap indicating which characters are included in the
168 class.
169 regnode_charclass U32 arg1;
171 char bitmap[ANYOF_BITMAP_SIZE];
172 "regnode_charclass_class"
174 There is also a larger form of a char class structure used to
175 represent POSIX char classes called "regnode_charclass_class" which
176 has an additional 4-byte (32-bit) bitmap indicating which POSIX
177 char classes have been included.
178 regnode_charclass_class U32 arg1;
180 char bitmap[ANYOF_BITMAP_SIZE];
181 char classflags[ANYOF_CLASSBITMAP_SIZE];
182 regnodes.h defines an array called "regarglen[]" which gives the size
184 of each opcode in units of "size regnode" (4-byte). A macro is used to
185 calculate the size of an "EXACT" node based on its "str_len" field.
186 The regops are defined in regnodes.h which is generated from
188 regcomp.sym by regcomp.pl. Currently the maximum possible number of
189 distinct regops is restricted to 256, with about a quarter already
190 used.
191 A set of macros makes accessing the fields easier and more consistent.
193 These include "OP()", which is used to determine the type of a
194 "regnode"-like structure; "NEXT_OFF()", which is the offset to the next
195 node (more on this later); "ARG()", "ARG1()", "ARG2()", "ARG_SET()",
196 and equivalents for reading and setting the arguments; and "STR_LEN()",
197 "STRING()" and "OPERAND()" for manipulating strings and regop bearing
198 types.
199 What regop is next?
201 There are three distinct concepts of "next" in the regex engine, and it
203 is important to keep them clear.
204 · There is the "next regnode" from a given regnode, a value which is
206 rarely useful except that sometimes it matches up in terms of value
207 with one of the others, and that sometimes the code assumes this to
208 always be so.
209 · There is the "next regop" from a given regop/regnode. This is the
211 regop physically located after the current one, as determined by
212 the size of the current regop. This is often useful, such as when
213 dumping the structure we use this order to traverse. Sometimes the
214 code assumes that the "next regnode" is the same as the "next
215 regop", or in other words assumes that the sizeof a given regop
216 type is always going to be one regnode large.
217 · There is the "regnext" from a given regop. This is the regop which
219 is reached by jumping forward by the value of "NEXT_OFF()", or in a
220 few cases for longer jumps by the "arg1" field of the "regnode_1"
221 structure. The subroutine "regnext()" handles this transparently.
222 This is the logical successor of the node, which in some cases,
223 like that of the "BRANCH" regop, has special meaning.
224
226 Broadly speaking, performing a match of a string against a pattern
227 involves the following steps:
228 A. Compilation
230 1. Parsing for size
231 2. Parsing for construction
232 3. Peep-hole optimisation and analysis
233 B. Execution
234 4. Start position and no-match optimisations
235 5. Program execution
236 Where these steps occur in the actual execution of a perl program is
238 determined by whether the pattern involves interpolating any string
239 variables. If interpolation occurs, then compilation happens at run
240 time. If it does not, then compilation is performed at compile time.
241 (The "/o" modifier changes this, as does "qr//" to a certain extent.)
242 The engine doesn't really care that much.
243 Compilation
245 This code resides primarily in regcomp.c, along with the header files
246 regcomp.h, regexp.h and regnodes.h.
247 Compilation starts with "pregcomp()", which is mostly an initialisation
249 wrapper which farms work out to two other routines for the heavy
250 lifting: the first is "reg()", which is the start point for parsing;
251 the second, "study_chunk()", is responsible for optimisation.
252 Initialisation in "pregcomp()" mostly involves the creation and data-
254 filling of a special structure, "RExC_state_t" (defined in regcomp.c).
255 Almost all internally-used routines in regcomp.h take a pointer to one
256 of these structures as their first argument, with the name
257 "pRExC_state". This structure is used to store the compilation state
258 and contains many fields. Likewise there are many macros which operate
259 on this variable: anything that looks like "RExC_xxxx" is a macro that
260 operates on this pointer/structure.
261 Parsing for size
263 In this pass the input pattern is parsed in order to calculate how much
265 space is needed for each regop we would need to emit. The size is also
266 used to determine whether long jumps will be required in the program.
267 This stage is controlled by the macro "SIZE_ONLY" being set.
269 The parse proceeds pretty much exactly as it does during the
271 construction phase, except that most routines are short-circuited to
272 change the size field "RExC_size" and not do anything else.
273 Parsing for construction
275 Once the size of the program has been determined, the pattern is parsed
277 again, but this time for real. Now "SIZE_ONLY" will be false, and the
278 actual construction can occur.
279 "reg()" is the start of the parse process. It is responsible for
281 parsing an arbitrary chunk of pattern up to either the end of the
282 string, or the first closing parenthesis it encounters in the pattern.
283 This means it can be used to parse the top-level regex, or any section
284 inside of a grouping parenthesis. It also handles the "special parens"
285 that perl's regexes have. For instance when parsing "/x(?:foo)y/"
286 "reg()" will at one point be called to parse from the "?" symbol up to
287 and including the ")".
288 Additionally, "reg()" is responsible for parsing the one or more
290 branches from the pattern, and for "finishing them off" by correctly
291 setting their next pointers. In order to do the parsing, it repeatedly
292 calls out to "regbranch()", which is responsible for handling up to the
293 first "|" symbol it sees.
294 "regbranch()" in turn calls "regpiece()" which handles "things"
296 followed by a quantifier. In order to parse the "things", "regatom()"
297 is called. This is the lowest level routine, which parses out constant
298 strings, character classes, and the various special symbols like "$".
299 If "regatom()" encounters a "(" character it in turn calls "reg()".
300 The routine "regtail()" is called by both "reg()" and "regbranch()" in
302 order to "set the tail pointer" correctly. When executing and we get to
303 the end of a branch, we need to go to the node following the grouping
304 parens. When parsing, however, we don't know where the end will be
305 until we get there, so when we do we must go back and update the
306 offsets as appropriate. "regtail" is used to make this easier.
307 A subtlety of the parsing process means that a regex like "/foo/" is
309 originally parsed into an alternation with a single branch. It is only
310 afterwards that the optimiser converts single branch alternations into
311 the simpler form.
312 Parse Call Graph and a Grammar
314 The call graph looks like this:
316 reg() # parse a top level regex, or inside of parens
318 regbranch() # parse a single branch of an alternation
319 regpiece() # parse a pattern followed by a quantifier
320 regatom() # parse a simple pattern
321 regclass() # used to handle a class
322 reg() # used to handle a parenthesised subpattern
323 ....
324 ...
325 regtail() # finish off the branch
326 ...
327 regtail() # finish off the branch sequence. Tie each
328 # branch's tail to the tail of the sequence
329 # (NEW) In Debug mode this is
330 # regtail_study().
331 A grammar form might be something like this:
333 atom : constant | class
335 quant : '*' | '+' | '?' | '{min,max}'
336 _branch: piece
337 | piece _branch
338 | nothing
339 branch: _branch
340 | _branch '|' branch
341 group : '(' branch ')'
342 _piece: atom | group
343 piece : _piece
344 | _piece quant
345 Debug Output
347 In the 5.9.x development version of perl you can "use re Debug =>
349 'PARSE'" to see some trace information about the parse process. We will
350 start with some simple patterns and build up to more complex patterns.
351 So when we parse "/foo/" we see something like the following table. The
353 left shows what is being parsed, and the number indicates where the
354 next regop would go. The stuff on the right is the trace output of the
355 graph. The names are chosen to be short to make it less dense on the
356 screen. 'tsdy' is a special form of "regtail()" which does some extra
357 analysis.
358 >foo< 1 reg
360 brnc
361 piec
362 atom
363 >< 4 tsdy~ EXACT <foo> (EXACT) (1)
364 ~ attach to END (3) offset to 2
365 The resulting program then looks like:
367 1: EXACT <foo>(3)
369 3: END(0)
370 As you can see, even though we parsed out a branch and a piece, it was
372 ultimately only an atom. The final program shows us how things work. We
373 have an "EXACT" regop, followed by an "END" regop. The number in parens
374 indicates where the "regnext" of the node goes. The "regnext" of an
375 "END" regop is unused, as "END" regops mean we have successfully
376 matched. The number on the left indicates the position of the regop in
377 the regnode array.
378 Now let's try a harder pattern. We will add a quantifier, so now we
380 have the pattern "/foo+/". We will see that "regbranch()" calls
381 "regpiece()" twice.
382 >foo+< 1 reg
384 brnc
385 piec
386 atom
387 >o+< 3 piec
388 atom
389 >< 6 tail~ EXACT <fo> (1)
390 7 tsdy~ EXACT <fo> (EXACT) (1)
391 ~ PLUS (END) (3)
392 ~ attach to END (6) offset to 3
393 And we end up with the program:
395 1: EXACT <fo>(3)
397 3: PLUS(6)
398 4: EXACT <o>(0)
399 6: END(0)
400 Now we have a special case. The "EXACT" regop has a "regnext" of 0.
402 This is because if it matches it should try to match itself again. The
403 "PLUS" regop handles the actual failure of the "EXACT" regop and acts
404 appropriately (going to regnode 6 if the "EXACT" matched at least once,
405 or failing if it didn't).
406 Now for something much more complex: "/x(?:foo*|b[a][rR])(foo|bar)$/"
408 >x(?:foo*|b... 1 reg
410 brnc
411 piec
412 atom
413 >(?:foo*|b[... 3 piec
414 atom
415 >?:foo*|b[a... reg
416 >foo*|b[a][... brnc
417 piec
418 atom
419 >o*|b[a][rR... 5 piec
420 atom
421 >|b[a][rR])... 8 tail~ EXACT <fo> (3)
422 >b[a][rR])(... 9 brnc
423 10 piec
424 atom
425 >[a][rR])(f... 12 piec
426 atom
427 >a][rR])(fo... clas
428 >[rR])(foo|... 14 tail~ EXACT <b> (10)
429 piec
430 atom
431 >rR])(foo|b... clas
432 >)(foo|bar)... 25 tail~ EXACT <a> (12)
433 tail~ BRANCH (3)
434 26 tsdy~ BRANCH (END) (9)
435 ~ attach to TAIL (25) offset to 16
436 tsdy~ EXACT <fo> (EXACT) (4)
437 ~ STAR (END) (6)
438 ~ attach to TAIL (25) offset to 19
439 tsdy~ EXACT <b> (EXACT) (10)
440 ~ EXACT <a> (EXACT) (12)
441 ~ ANYOF[Rr] (END) (14)
442 ~ attach to TAIL (25) offset to 11
443 >(foo|bar)$< tail~ EXACT <x> (1)
444 piec
445 atom
446 >foo|bar)$< reg
447 28 brnc
448 piec
449 atom
450 >|bar)$< 31 tail~ OPEN1 (26)
451 >bar)$< brnc
452 32 piec
453 atom
454 >)$< 34 tail~ BRANCH (28)
455 36 tsdy~ BRANCH (END) (31)
456 ~ attach to CLOSE1 (34) offset to 3
457 tsdy~ EXACT <foo> (EXACT) (29)
458 ~ attach to CLOSE1 (34) offset to 5
459 tsdy~ EXACT <bar> (EXACT) (32)
460 ~ attach to CLOSE1 (34) offset to 2
461 >$< tail~ BRANCH (3)
462 ~ BRANCH (9)
463 ~ TAIL (25)
464 piec
465 atom
466 >< 37 tail~ OPEN1 (26)
467 ~ BRANCH (28)
468 ~ BRANCH (31)
469 ~ CLOSE1 (34)
470 38 tsdy~ EXACT <x> (EXACT) (1)
471 ~ BRANCH (END) (3)
472 ~ BRANCH (END) (9)
473 ~ TAIL (END) (25)
474 ~ OPEN1 (END) (26)
475 ~ BRANCH (END) (28)
476 ~ BRANCH (END) (31)
477 ~ CLOSE1 (END) (34)
478 ~ EOL (END) (36)
479 ~ attach to END (37) offset to 1
480 Resulting in the program
482 1: EXACT <x>(3)
484 3: BRANCH(9)
485 4: EXACT <fo>(6)
486 6: STAR(26)
487 7: EXACT <o>(0)
488 9: BRANCH(25)
489 10: EXACT <ba>(14)
490 12: OPTIMIZED (2 nodes)
491 14: ANYOF[Rr](26)
492 25: TAIL(26)
493 26: OPEN1(28)
494 28: TRIE-EXACT(34)
495 [StS:1 Wds:2 Cs:6 Uq:5 #Sts:7 Mn:3 Mx:3 Stcls:bf]
496 <foo>
497 <bar>
498 30: OPTIMIZED (4 nodes)
499 34: CLOSE1(36)
500 36: EOL(37)
501 37: END(0)
502 Here we can see a much more complex program, with various optimisations
504 in play. At regnode 10 we see an example where a character class with
505 only one character in it was turned into an "EXACT" node. We can also
506 see where an entire alternation was turned into a "TRIE-EXACT" node. As
507 a consequence, some of the regnodes have been marked as optimised away.
508 We can see that the "$" symbol has been converted into an "EOL" regop,
509 a special piece of code that looks for "\n" or the end of the string.
510 The next pointer for "BRANCH"es is interesting in that it points at
512 where execution should go if the branch fails. When executing, if the
513 engine tries to traverse from a branch to a "regnext" that isn't a
514 branch then the engine will know that the entire set of branches has
515 failed.
516 Peep-hole Optimisation and Analysis
518 The regular expression engine can be a weighty tool to wield. On long
520 strings and complex patterns it can end up having to do a lot of work
521 to find a match, and even more to decide that no match is possible.
522 Consider a situation like the following pattern.
523 'ababababababababababab' =~ /(a|b)*z/
525 The "(a|b)*" part can match at every char in the string, and then fail
527 every time because there is no "z" in the string. So obviously we can
528 avoid using the regex engine unless there is a "z" in the string.
529 Likewise in a pattern like:
530 /foo(\w+)bar/
532 In this case we know that the string must contain a "foo" which must be
534 followed by "bar". We can use Fast Boyer-Moore matching as implemented
535 in "fbm_instr()" to find the location of these strings. If they don't
536 exist then we don't need to resort to the much more expensive regex
537 engine. Even better, if they do exist then we can use their positions
538 to reduce the search space that the regex engine needs to cover to
539 determine if the entire pattern matches.
540 There are various aspects of the pattern that can be used to facilitate
542 optimisations along these lines:
543 · anchored fixed strings
545 · floating fixed strings
547 · minimum and maximum length requirements
549 · start class
551 · Beginning/End of line positions
553 Another form of optimisation that can occur is the post-parse "peep-
555 hole" optimisation, where inefficient constructs are replaced by more
556 efficient constructs. The "TAIL" regops which are used during parsing
557 to mark the end of branches and the end of groups are examples of this.
558 These regops are used as place-holders during construction and "always
559 match" so they can be "optimised away" by making the things that point
560 to the "TAIL" point to the thing that "TAIL" points to, thus "skipping"
561 the node.
562 Another optimisation that can occur is that of ""EXACT" merging" which
564 is where two consecutive "EXACT" nodes are merged into a single regop.
565 An even more aggressive form of this is that a branch sequence of the
566 form "EXACT BRANCH ... EXACT" can be converted into a "TRIE-EXACT"
567 regop.
568 All of this occurs in the routine "study_chunk()" which uses a special
570 structure "scan_data_t" to store the analysis that it has performed,
571 and does the "peep-hole" optimisations as it goes.
572 The code involved in "study_chunk()" is extremely cryptic. Be careful.
574 :-)
575 Execution
577 Execution of a regex generally involves two phases, the first being
578 finding the start point in the string where we should match from, and
579 the second being running the regop interpreter.
580 If we can tell that there is no valid start point then we don't bother
582 running interpreter at all. Likewise, if we know from the analysis
583 phase that we cannot detect a short-cut to the start position, we go
584 straight to the interpreter.
585 The two entry points are "re_intuit_start()" and "pregexec()". These
587 routines have a somewhat incestuous relationship with overlap between
588 their functions, and "pregexec()" may even call "re_intuit_start()" on
589 its own. Nevertheless other parts of the perl source code may call into
590 either, or both.
591 Execution of the interpreter itself used to be recursive, but thanks to
593 the efforts of Dave Mitchell in the 5.9.x development track, that has
594 changed: now an internal stack is maintained on the heap and the
595 routine is fully iterative. This can make it tricky as the code is
596 quite conservative about what state it stores, with the result that two
597 consecutive lines in the code can actually be running in totally
598 different contexts due to the simulated recursion.
599 Start position and no-match optimisations
601 "re_intuit_start()" is responsible for handling start points and no-
603 match optimisations as determined by the results of the analysis done
604 by "study_chunk()" (and described in "Peep-hole Optimisation and
605 Analysis").
606 The basic structure of this routine is to try to find the start- and/or
608 end-points of where the pattern could match, and to ensure that the
609 string is long enough to match the pattern. It tries to use more
610 efficient methods over less efficient methods and may involve
611 considerable cross-checking of constraints to find the place in the
612 string that matches. For instance it may try to determine that a given
613 fixed string must be not only present but a certain number of chars
614 before the end of the string, or whatever.
615 It calls several other routines, such as "fbm_instr()" which does Fast
617 Boyer Moore matching and "find_byclass()" which is responsible for
618 finding the start using the first mandatory regop in the program.
619 When the optimisation criteria have been satisfied, "reg_try()" is
621 called to perform the match.
622 Program execution
624 "pregexec()" is the main entry point for running a regex. It contains
626 support for initialising the regex interpreter's state, running
627 "re_intuit_start()" if needed, and running the interpreter on the
628 string from various start positions as needed. When it is necessary to
629 use the regex interpreter "pregexec()" calls "regtry()".
630 "regtry()" is the entry point into the regex interpreter. It expects as
632 arguments a pointer to a "regmatch_info" structure and a pointer to a
633 string. It returns an integer 1 for success and a 0 for failure. It
634 is basically a set-up wrapper around "regmatch()".
635 "regmatch" is the main "recursive loop" of the interpreter. It is
637 basically a giant switch statement that implements a state machine,
638 where the possible states are the regops themselves, plus a number of
639 additional intermediate and failure states. A few of the states are
640 implemented as subroutines but the bulk are inline code.
641
643 Unicode and Localisation Support
644 When dealing with strings containing characters that cannot be
645 represented using an eight-bit character set, perl uses an internal
646 representation that is a permissive version of Unicode's UTF-8
647 encoding[2]. This uses single bytes to represent characters from the
648 ASCII character set, and sequences of two or more bytes for all other
649 characters. (See perlunitut for more information about the relationship
650 between UTF-8 and perl's encoding, utf8. The difference isn't important
651 for this discussion.)
652 No matter how you look at it, Unicode support is going to be a pain in
654 a regex engine. Tricks that might be fine when you have 256 possible
655 characters often won't scale to handle the size of the UTF-8 character
656 set. Things you can take for granted with ASCII may not be true with
657 Unicode. For instance, in ASCII, it is safe to assume that
658 "sizeof(char1) == sizeof(char2)", but in UTF-8 it isn't. Unicode case
659 folding is vastly more complex than the simple rules of ASCII, and even
660 when not using Unicode but only localised single byte encodings, things
661 can get tricky (for example, LATIN SMALL LETTER SHARP S (U+00DF, ss)
662 should match 'SS' in localised case-insensitive matching).
663 Making things worse is that UTF-8 support was a later addition to the
665 regex engine (as it was to perl) and this necessarily made things a
666 lot more complicated. Obviously it is easier to design a regex engine
667 with Unicode support in mind from the beginning than it is to retrofit
668 it to one that wasn't.
669 Nearly all regops that involve looking at the input string have two
671 cases, one for UTF-8, and one not. In fact, it's often more complex
672 than that, as the pattern may be UTF-8 as well.
673 Care must be taken when making changes to make sure that you handle
675 UTF-8 properly, both at compile time and at execution time, including
676 when the string and pattern are mismatched.
677 The following comment in regcomp.h gives an example of exactly how
679 tricky this can be:
680 Two problematic code points in Unicode casefolding of EXACT nodes:
682 U+0390 - GREEK SMALL LETTER IOTA WITH DIALYTIKA AND TONOS
684 U+03B0 - GREEK SMALL LETTER UPSILON WITH DIALYTIKA AND TONOS
685 which casefold to
687 Unicode UTF-8
689 U+03B9 U+0308 U+0301 0xCE 0xB9 0xCC 0x88 0xCC 0x81
691 U+03C5 U+0308 U+0301 0xCF 0x85 0xCC 0x88 0xCC 0x81
692 This means that in case-insensitive matching (or "loose matching",
694 as Unicode calls it), an EXACTF of length six (the UTF-8 encoded
695 byte length of the above casefolded versions) can match a target
696 string of length two (the byte length of UTF-8 encoded U+0390 or
697 U+03B0). This would rather mess up the minimum length computation.
698 What we'll do is to look for the tail four bytes, and then peek
700 at the preceding two bytes to see whether we need to decrease
701 the minimum length by four (six minus two).
702 Thanks to the design of UTF-8, there cannot be false matches:
704 A sequence of valid UTF-8 bytes cannot be a subsequence of
705 another valid sequence of UTF-8 bytes.
706 Base Structures
708 The "regexp" structure described in perlreapi is common to all regex
709 engines. Two of its fields that are intended for the private use of the
710 regex engine that compiled the pattern. These are the "intflags" and
711 pprivate members. The "pprivate" is a void pointer to an arbitrary
712 structure whose use and management is the responsibility of the
713 compiling engine. perl will never modify either of these values. In the
714 case of the stock engine the structure pointed to by "pprivate" is
715 called "regexp_internal".
716 Its "pprivate" and "intflags" fields contain data specific to each
718 engine.
719 There are two structures used to store a compiled regular expression.
721 One, the "regexp" structure described in perlreapi is populated by the
722 engine currently being. used and some of its fields read by perl to
723 implement things such as the stringification of "qr//".
724 The other structure is pointed to be the "regexp" struct's "pprivate"
726 and is in addition to "intflags" in the same struct considered to be
727 the property of the regex engine which compiled the regular expression;
728 The regexp structure contains all the data that perl needs to be aware
730 of to properly work with the regular expression. It includes data about
731 optimisations that perl can use to determine if the regex engine should
732 really be used, and various other control info that is needed to
733 properly execute patterns in various contexts such as is the pattern
734 anchored in some way, or what flags were used during the compile, or
735 whether the program contains special constructs that perl needs to be
736 aware of.
737 In addition it contains two fields that are intended for the private
739 use of the regex engine that compiled the pattern. These are the
740 "intflags" and pprivate members. The "pprivate" is a void pointer to an
741 arbitrary structure whose use and management is the responsibility of
742 the compiling engine. perl will never modify either of these values.
743 As mentioned earlier, in the case of the default engines, the
745 "pprivate" will be a pointer to a regexp_internal structure which holds
746 the compiled program and any additional data that is private to the
747 regex engine implementation.
748 Perl's "pprivate" structure
750 The following structure is used as the "pprivate" struct by perl's
752 regex engine. Since it is specific to perl it is only of curiosity
753 value to other engine implementations.
754 typedef struct regexp_internal {
756 regexp_paren_ofs *swap; /* Swap copy of *startp / *endp */
757 U32 *offsets; /* offset annotations 20001228 MJD
758 data about mapping the program to the
759 string*/
760 regnode *regstclass; /* Optional startclass as identified or constructed
761 by the optimiser */
762 struct reg_data *data; /* Additional miscellaneous data used by the program.
763 Used to make it easier to clone and free arbitrary
764 data that the regops need. Often the ARG field of
765 a regop is an index into this structure */
766 regnode program[1]; /* Unwarranted chumminess with compiler. */
767 } regexp_internal;
768 "swap"
770 "swap" formerly was an extra set of startp/endp stored in a
771 "regexp_paren_ofs" struct. This was used when the last successful
772 match was from the same pattern as the current pattern, so that a
773 partial match didn't overwrite the previous match's results, but
774 it caused a problem with re-entrant code such as trying to build
775 the UTF-8 swashes. Currently unused and left for backward
776 compatibility with 5.10.0.
777 "offsets"
779 Offsets holds a mapping of offset in the "program" to offset in
780 the "precomp" string. This is only used by ActiveState's visual
781 regex debugger.
782 "regstclass"
784 Special regop that is used by "re_intuit_start()" to check if a
785 pattern can match at a certain position. For instance if the regex
786 engine knows that the pattern must start with a 'Z' then it can
787 scan the string until it finds one and then launch the regex
788 engine from there. The routine that handles this is called
789 "find_by_class()". Sometimes this field points at a regop embedded
790 in the program, and sometimes it points at an independent
791 synthetic regop that has been constructed by the optimiser.
792 "data"
794 This field points at a reg_data structure, which is defined as
795 follows
796 struct reg_data {
798 U32 count;
799 U8 *what;
800 void* data[1];
801 };
802 This structure is used for handling data structures that the regex
804 engine needs to handle specially during a clone or free operation
805 on the compiled product. Each element in the data array has a
806 corresponding element in the what array. During compilation regops
807 that need special structures stored will add an element to each
808 array using the add_data() routine and then store the index in the
809 regop.
810 "program"
812 Compiled program. Inlined into the structure so the entire struct
813 can be treated as a single blob.
814
816 perlreapi
817 perlre
819 perlunitut
821
823 by Yves Orton, 2006.
824 With excerpts from Perl, and contributions and suggestions from Ronald
826 J. Kimball, Dave Mitchell, Dominic Dunlop, Mark Jason Dominus, Stephen
827 McCamant, and David Landgren.
828
830 Same terms as Perl.
831
833 [1] <http://perl.plover.com/Rx/paper/>
834 [2] <http://www.unicode.org>
836perl v5.16.3 2013-03-04 PERLREGUTS(1)