3 perlreguts - Description of the Perl regular expression engine.
7 This document is an attempt to shine some light on the guts of the regex
8 engine and how it works. The regex engine represents a significant chunk
9 of the perl codebase, but is relatively poorly understood. This document
10 is a meagre attempt at addressing this situation. It is derived from the
11 author's experience, comments in the source code, other papers on the
12 regex engine, feedback on the perl5-porters mail list, and no doubt other
15 B<NOTICE!> It should be clearly understood that the behavior and
16 structures discussed in this represents the state of the engine as the
17 author understood it at the time of writing. It is B<NOT> an API
18 definition, it is purely an internals guide for those who want to hack
19 the regex engine, or understand how the regex engine works. Readers of
20 this document are expected to understand perl's regex syntax and its
21 usage in detail. If you want to learn about the basics of Perl's
22 regular expressions, see L<perlre>. And if you want to replace the
23 regex engine with your own, see L<perlreapi>.
27 =head2 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.
33 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 term
35 "pattern" when we speak of their textual, source code form, and the term
36 "program" when we speak of their internal representation. These
37 correspond to the terms I<S-regex> and I<B-regex> that Mark Jason
38 Dominus employs in his paper on "Rx" ([1] in L</REFERENCES>).
40 =head2 What is a regular expression engine?
42 A regular expression engine is a program that takes a set of constraints
43 specified in a mini-language, and then applies those constraints to a
44 target string, and determines whether or not the string satisfies the
45 constraints. See L<perlre> for a full definition of the language.
47 In less grandiose terms, the first part of the job is to turn a pattern into
48 something the computer can efficiently use to find the matching point in
49 the string, and the second part is performing the search itself.
51 To do this we need to produce a program by parsing the text. We then
52 need to execute the program to find the point in the string that
53 matches. And we need to do the whole thing efficiently.
55 =head2 Structure of a Regexp Program
59 Although it is a bit confusing and some people object to the terminology, it
60 is worth taking a look at a comment that has
61 been in F<regexp.h> for years:
63 I<This is essentially a linear encoding of a nondeterministic
64 finite-state machine (aka syntax charts or "railroad normal form" in
67 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, but
72 statistically not many), and the whole forms a layout with a
73 single entry and single exit point. The matching process can be thought
74 of as a car that moves along the track, with the particular route through
75 the 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.
79 Thus the pattern C</foo(?:\w+|\d+|\s+)bar/> can be thought of as the
96 The truth of the matter is that perl's regular expressions these days are
97 much more complex than this kind of structure, but visualising it this way
98 can help when trying to get your bearings, and it matches the
99 current implementation pretty closely.
101 To be more precise, we will say that a regex program is an encoding
102 of a graph. Each node in the graph corresponds to part of
103 the original regex pattern, such as a literal string or a branch,
104 and has a pointer to the nodes representing the next component
105 to be matched. Since "node" and "opcode" already have other meanings in the
106 perl source, we will call the nodes in a regex program "regops".
108 The program is represented by an array of C<regnode> structures, one or
109 more of which represent a single regop of the program. Struct
110 C<regnode> is the smallest struct needed, and has a field structure which is
111 shared with all the other larger structures.
113 The "next" pointers of all regops except C<BRANCH> implement concatenation;
114 a "next" pointer with a C<BRANCH> on both ends of it is connecting two
115 alternatives. [Here we have one of the subtle syntax dependencies: an
116 individual C<BRANCH> (as opposed to a collection of them) is never
117 concatenated with anything because of operator precedence.]
119 The operand of some types of regop is a literal string; for others,
120 it is a regop leading into a sub-program. In particular, the operand
121 of a C<BRANCH> node is the first regop of the branch.
123 B<NOTE>: As the railroad metaphor suggests, this is B<not> a tree
124 structure: the tail of the branch connects to the thing following the
125 set of C<BRANCH>es. It is a like a single line of railway track that
126 splits as it goes into a station or railway yard and rejoins as it comes
131 The base structure of a regop is defined in F<regexp.h> as follows:
134 U8 flags; /* Various purposes, sometimes overridden */
135 U8 type; /* Opcode value as specified by regnodes.h */
136 U16 next_off; /* Offset in size regnode */
139 Other larger C<regnode>-like structures are defined in F<regcomp.h>. They
140 are almost like subclasses in that they have the same fields as
141 C<regnode>, with possibly additional fields following in
142 the structure, and in some cases the specific meaning (and name)
143 of some of base fields are overridden. The following is a more
144 complete description.
152 C<regnode_1> structures have the same header, followed by a single
153 four-byte argument; C<regnode_2> structures contain two two-byte
157 regnode_2 U16 arg1; U16 arg2;
159 =item C<regnode_string>
161 C<regnode_string> structures, used for literal strings, follow the header
162 with a one-byte length and then the string data. Strings are padded on
163 the end with zero bytes so that the total length of the node is a
164 multiple of four bytes:
166 regnode_string char string[1];
167 U8 str_len; /* overrides flags */
169 =item C<regnode_charclass>
171 Bracketed character classes are represented by C<regnode_charclass>
172 structures, which have a four-byte argument and then a 32-byte (256-bit)
173 bitmap indicating which characters in the Latin1 range are included in
176 regnode_charclass U32 arg1;
177 char bitmap[ANYOF_BITMAP_SIZE];
179 Various flags whose names begin with C<ANYOF_> are used for special
180 situations. Above Latin1 matches and things not known until run-time
181 are stored in L</Perl's pprivate structure>.
183 =item C<regnode_charclass_posixl>
185 There is also a larger form of a char class structure used to represent
186 POSIX char classes under C</l> matching,
187 called C<regnode_charclass_posixl> which has an
188 additional 32-bit bitmap indicating which POSIX char classes
191 regnode_charclass_posixl U32 arg1;
192 char bitmap[ANYOF_BITMAP_SIZE];
197 F<regnodes.h> defines an array called C<regarglen[]> which gives the size
198 of each opcode in units of C<size regnode> (4-byte). A macro is used
199 to calculate the size of an C<EXACT> node based on its C<str_len> field.
201 The regops are defined in F<regnodes.h> which is generated from
202 F<regcomp.sym> by F<regcomp.pl>. Currently the maximum possible number
203 of distinct regops is restricted to 256, with about a quarter already
206 A set of macros makes accessing the fields
207 easier and more consistent. These include C<OP()>, which is used to determine
208 the type of a C<regnode>-like structure; C<NEXT_OFF()>, which is the offset to
209 the next node (more on this later); C<ARG()>, C<ARG1()>, C<ARG2()>, C<ARG_SET()>,
210 and equivalents for reading and setting the arguments; and C<STR_LEN()>,
211 C<STRING()> and C<OPERAND()> for manipulating strings and regop bearing
214 =head3 What regop is next?
216 There are three distinct concepts of "next" in the regex engine, and
217 it is important to keep them clear.
223 There is the "next regnode" from a given regnode, a value which is
224 rarely useful except that sometimes it matches up in terms of value
225 with one of the others, and that sometimes the code assumes this to
230 There is the "next regop" from a given regop/regnode. This is the
231 regop physically located after the current one, as determined by
232 the size of the current regop. This is often useful, such as when
233 dumping the structure we use this order to traverse. Sometimes the code
234 assumes that the "next regnode" is the same as the "next regop", or in
235 other words assumes that the sizeof a given regop type is always going
236 to be one regnode large.
240 There is the "regnext" from a given regop. This is the regop which
241 is reached by jumping forward by the value of C<NEXT_OFF()>,
242 or in a few cases for longer jumps by the C<arg1> field of the C<regnode_1>
243 structure. The subroutine C<regnext()> handles this transparently.
244 This is the logical successor of the node, which in some cases, like
245 that of the C<BRANCH> regop, has special meaning.
249 =head1 Process Overview
251 Broadly speaking, performing a match of a string against a pattern
252 involves the following steps:
260 =item 1. Parsing for size
262 =item 2. Parsing for construction
264 =item 3. Peep-hole optimisation and analysis
272 =item 4. Start position and no-match optimisations
274 =item 5. Program execution
281 Where these steps occur in the actual execution of a perl program is
282 determined by whether the pattern involves interpolating any string
283 variables. If interpolation occurs, then compilation happens at run time. If it
284 does not, then compilation is performed at compile time. (The C</o> modifier changes this,
285 as does C<qr//> to a certain extent.) The engine doesn't really care that
290 This code resides primarily in F<regcomp.c>, along with the header files
291 F<regcomp.h>, F<regexp.h> and F<regnodes.h>.
293 Compilation starts with C<pregcomp()>, which is mostly an initialisation
294 wrapper which farms work out to two other routines for the heavy lifting: the
295 first is C<reg()>, which is the start point for parsing; the second,
296 C<study_chunk()>, is responsible for optimisation.
298 Initialisation in C<pregcomp()> mostly involves the creation and data-filling
299 of a special structure, C<RExC_state_t> (defined in F<regcomp.c>).
300 Almost all internally-used routines in F<regcomp.h> take a pointer to one
301 of these structures as their first argument, with the name C<pRExC_state>.
302 This structure is used to store the compilation state and contains many
303 fields. Likewise there are many macros which operate on this
304 variable: anything that looks like C<RExC_xxxx> is a macro that operates on
305 this pointer/structure.
307 =head3 Parsing for size
309 In this pass the input pattern is parsed in order to calculate how much
310 space is needed for each regop we would need to emit. The size is also
311 used to determine whether long jumps will be required in the program.
313 This stage is controlled by the macro C<SIZE_ONLY> being set.
315 The parse proceeds pretty much exactly as it does during the
316 construction phase, except that most routines are short-circuited to
317 change the size field C<RExC_size> and not do anything else.
319 =head3 Parsing for construction
321 Once the size of the program has been determined, the pattern is parsed
322 again, but this time for real. Now C<SIZE_ONLY> will be false, and the
323 actual construction can occur.
325 C<reg()> is the start of the parse process. It is responsible for
326 parsing an arbitrary chunk of pattern up to either the end of the
327 string, or the first closing parenthesis it encounters in the pattern.
328 This means it can be used to parse the top-level regex, or any section
329 inside of a grouping parenthesis. It also handles the "special parens"
330 that perl's regexes have. For instance when parsing C</x(?:foo)y/> C<reg()>
331 will at one point be called to parse from the "?" symbol up to and
334 Additionally, C<reg()> is responsible for parsing the one or more
335 branches from the pattern, and for "finishing them off" by correctly
336 setting their next pointers. In order to do the parsing, it repeatedly
337 calls out to C<regbranch()>, which is responsible for handling up to the
338 first C<|> symbol it sees.
340 C<regbranch()> in turn calls C<regpiece()> which
341 handles "things" followed by a quantifier. In order to parse the
342 "things", C<regatom()> is called. This is the lowest level routine, which
343 parses out constant strings, character classes, and the
344 various special symbols like C<$>. If C<regatom()> encounters a "("
345 character it in turn calls C<reg()>.
347 The routine C<regtail()> is called by both C<reg()> and C<regbranch()>
348 in order to "set the tail pointer" correctly. When executing and
349 we get to the end of a branch, we need to go to the node following the
350 grouping parens. When parsing, however, we don't know where the end will
351 be until we get there, so when we do we must go back and update the
352 offsets as appropriate. C<regtail> is used to make this easier.
354 A subtlety of the parsing process means that a regex like C</foo/> is
355 originally parsed into an alternation with a single branch. It is only
356 afterwards that the optimiser converts single branch alternations into the
359 =head3 Parse Call Graph and a Grammar
361 The call graph looks like this:
363 reg() # parse a top level regex, or inside of
365 regbranch() # parse a single branch of an alternation
366 regpiece() # parse a pattern followed by a quantifier
367 regatom() # parse a simple pattern
368 regclass() # used to handle a class
369 reg() # used to handle a parenthesised
373 regtail() # finish off the branch
375 regtail() # finish off the branch sequence. Tie each
376 # branch's tail to the tail of the
378 # (NEW) In Debug mode this is
381 A grammar form might be something like this:
383 atom : constant | class
384 quant : '*' | '+' | '?' | '{min,max}'
390 group : '(' branch ')'
395 =head3 Parsing complications
397 The implication of the above description is that a pattern containing nested
398 parentheses will result in a call graph which cycles through C<reg()>,
399 C<regbranch()>, C<regpiece()>, C<regatom()>, C<reg()>, C<regbranch()> I<etc>
400 multiple times, until the deepest level of nesting is reached. All the above
401 routines return a pointer to a C<regnode>, which is usually the last regnode
402 added to the program. However, one complication is that reg() returns NULL
403 for parsing C<(?:)> syntax for embedded modifiers, setting the flag
404 C<TRYAGAIN>. The C<TRYAGAIN> propagates upwards until it is captured, in
405 some cases by C<regatom()>, but otherwise unconditionally by
406 C<regbranch()>. Hence it will never be returned by C<regbranch()> to
407 C<reg()>. This flag permits patterns such as C<(?i)+> to be detected as
408 errors (I<Quantifier follows nothing in regex; marked by <-- HERE in m/(?i)+
411 Another complication is that the representation used for the program differs
412 if it needs to store Unicode, but it's not always possible to know for sure
413 whether it does until midway through parsing. The Unicode representation for
414 the program is larger, and cannot be matched as efficiently. (See L</Unicode
415 and Localisation Support> below for more details as to why.) If the pattern
416 contains literal Unicode, it's obvious that the program needs to store
417 Unicode. Otherwise, the parser optimistically assumes that the more
418 efficient representation can be used, and starts sizing on this basis.
419 However, if it then encounters something in the pattern which must be stored
420 as Unicode, such as an C<\x{...}> escape sequence representing a character
421 literal, then this means that all previously calculated sizes need to be
422 redone, using values appropriate for the Unicode representation. Currently,
423 all regular expression constructions which can trigger this are parsed by code
426 To avoid wasted work when a restart is needed, the sizing pass is abandoned
427 - C<regatom()> immediately returns NULL, setting the flag C<RESTART_UTF8>.
428 (This action is encapsulated using the macro C<REQUIRE_UTF8>.) This restart
429 request is propagated up the call chain in a similar fashion, until it is
430 "caught" in C<Perl_re_op_compile()>, which marks the pattern as containing
431 Unicode, and restarts the sizing pass. It is also possible for constructions
432 within run-time code blocks to turn out to need Unicode representation.,
433 which is signalled by C<S_compile_runtime_code()> returning false to
434 C<Perl_re_op_compile()>.
436 The restart was previously implemented using a C<longjmp> in C<regatom()>
437 back to a C<setjmp> in C<Perl_re_op_compile()>, but this proved to be
438 problematic as the latter is a large function containing many automatic
439 variables, which interact badly with the emergent control flow of C<setjmp>.
443 In the 5.9.x development version of perl you can C<< use re Debug => 'PARSE' >>
444 to see some trace information about the parse process. We will start with some
445 simple patterns and build up to more complex patterns.
447 So when we parse C</foo/> we see something like the following table. The
448 left shows what is being parsed, and the number indicates where the next regop
449 would go. The stuff on the right is the trace output of the graph. The
450 names are chosen to be short to make it less dense on the screen. 'tsdy'
451 is a special form of C<regtail()> which does some extra analysis.
457 >< 4 tsdy~ EXACT <foo> (EXACT) (1)
458 ~ attach to END (3) offset to 2
460 The resulting program then looks like:
465 As you can see, even though we parsed out a branch and a piece, it was ultimately
466 only an atom. The final program shows us how things work. We have an C<EXACT> regop,
467 followed by an C<END> regop. The number in parens indicates where the C<regnext> of
468 the node goes. The C<regnext> of an C<END> regop is unused, as C<END> regops mean
469 we have successfully matched. The number on the left indicates the position of
470 the regop in the regnode array.
472 Now let's try a harder pattern. We will add a quantifier, so now we have the pattern
473 C</foo+/>. We will see that C<regbranch()> calls C<regpiece()> twice.
481 >< 6 tail~ EXACT <fo> (1)
482 7 tsdy~ EXACT <fo> (EXACT) (1)
484 ~ attach to END (6) offset to 3
486 And we end up with the program:
493 Now we have a special case. The C<EXACT> regop has a C<regnext> of 0. This is
494 because if it matches it should try to match itself again. The C<PLUS> regop
495 handles the actual failure of the C<EXACT> regop and acts appropriately (going
496 to regnode 6 if the C<EXACT> matched at least once, or failing if it didn't).
498 Now for something much more complex: C</x(?:foo*|b[a][rR])(foo|bar)$/>
504 >(?:foo*|b[... 3 piec
510 >o*|b[a][rR... 5 piec
512 >|b[a][rR])... 8 tail~ EXACT <fo> (3)
513 >b[a][rR])(... 9 brnc
516 >[a][rR])(f... 12 piec
519 >[rR])(foo|... 14 tail~ EXACT <b> (10)
523 >)(foo|bar)... 25 tail~ EXACT <a> (12)
525 26 tsdy~ BRANCH (END) (9)
526 ~ attach to TAIL (25) offset to 16
527 tsdy~ EXACT <fo> (EXACT) (4)
529 ~ attach to TAIL (25) offset to 19
530 tsdy~ EXACT <b> (EXACT) (10)
531 ~ EXACT <a> (EXACT) (12)
532 ~ ANYOF[Rr] (END) (14)
533 ~ attach to TAIL (25) offset to 11
534 >(foo|bar)$< tail~ EXACT <x> (1)
541 >|bar)$< 31 tail~ OPEN1 (26)
545 >)$< 34 tail~ BRANCH (28)
546 36 tsdy~ BRANCH (END) (31)
547 ~ attach to CLOSE1 (34) offset to 3
548 tsdy~ EXACT <foo> (EXACT) (29)
549 ~ attach to CLOSE1 (34) offset to 5
550 tsdy~ EXACT <bar> (EXACT) (32)
551 ~ attach to CLOSE1 (34) offset to 2
557 >< 37 tail~ OPEN1 (26)
561 38 tsdy~ EXACT <x> (EXACT) (1)
570 ~ attach to END (37) offset to 1
572 Resulting in the program
581 12: OPTIMIZED (2 nodes)
586 [StS:1 Wds:2 Cs:6 Uq:5 #Sts:7 Mn:3 Mx:3 Stcls:bf]
589 30: OPTIMIZED (4 nodes)
594 Here we can see a much more complex program, with various optimisations in
595 play. At regnode 10 we see an example where a character class with only
596 one character in it was turned into an C<EXACT> node. We can also see where
597 an entire alternation was turned into a C<TRIE-EXACT> node. As a consequence,
598 some of the regnodes have been marked as optimised away. We can see that
599 the C<$> symbol has been converted into an C<EOL> regop, a special piece of
600 code that looks for C<\n> or the end of the string.
602 The next pointer for C<BRANCH>es is interesting in that it points at where
603 execution should go if the branch fails. When executing, if the engine
604 tries to traverse from a branch to a C<regnext> that isn't a branch then
605 the engine will know that the entire set of branches has failed.
607 =head3 Peep-hole Optimisation and Analysis
609 The regular expression engine can be a weighty tool to wield. On long
610 strings and complex patterns it can end up having to do a lot of work
611 to find a match, and even more to decide that no match is possible.
612 Consider a situation like the following pattern.
614 'ababababababababababab' =~ /(a|b)*z/
616 The C<(a|b)*> part can match at every char in the string, and then fail
617 every time because there is no C<z> in the string. So obviously we can
618 avoid using the regex engine unless there is a C<z> in the string.
619 Likewise in a pattern like:
623 In this case we know that the string must contain a C<foo> which must be
624 followed by C<bar>. We can use Fast Boyer-Moore matching as implemented
625 in C<fbm_instr()> to find the location of these strings. If they don't exist
626 then we don't need to resort to the much more expensive regex engine.
627 Even better, if they do exist then we can use their positions to
628 reduce the search space that the regex engine needs to cover to determine
629 if the entire pattern matches.
631 There are various aspects of the pattern that can be used to facilitate
632 optimisations along these lines:
636 =item * anchored fixed strings
638 =item * floating fixed strings
640 =item * minimum and maximum length requirements
644 =item * Beginning/End of line positions
648 Another form of optimisation that can occur is the post-parse "peep-hole"
649 optimisation, where inefficient constructs are replaced by more efficient
650 constructs. The C<TAIL> regops which are used during parsing to mark the end
651 of branches and the end of groups are examples of this. These regops are used
652 as place-holders during construction and "always match" so they can be
653 "optimised away" by making the things that point to the C<TAIL> point to the
654 thing that C<TAIL> points to, thus "skipping" the node.
656 Another optimisation that can occur is that of "C<EXACT> merging" which is
657 where two consecutive C<EXACT> nodes are merged into a single
658 regop. An even more aggressive form of this is that a branch
659 sequence of the form C<EXACT BRANCH ... EXACT> can be converted into a
662 All of this occurs in the routine C<study_chunk()> which uses a special
663 structure C<scan_data_t> to store the analysis that it has performed, and
664 does the "peep-hole" optimisations as it goes.
666 The code involved in C<study_chunk()> is extremely cryptic. Be careful. :-)
670 Execution of a regex generally involves two phases, the first being
671 finding the start point in the string where we should match from,
672 and the second being running the regop interpreter.
674 If we can tell that there is no valid start point then we don't bother running
675 the interpreter at all. Likewise, if we know from the analysis phase that we
676 cannot detect a short-cut to the start position, we go straight to the
679 The two entry points are C<re_intuit_start()> and C<pregexec()>. These routines
680 have a somewhat incestuous relationship with overlap between their functions,
681 and C<pregexec()> may even call C<re_intuit_start()> on its own. Nevertheless
682 other parts of the perl source code may call into either, or both.
684 Execution of the interpreter itself used to be recursive, but thanks to the
685 efforts of Dave Mitchell in the 5.9.x development track, that has changed: now an
686 internal stack is maintained on the heap and the routine is fully
687 iterative. This can make it tricky as the code is quite conservative
688 about what state it stores, with the result that two consecutive lines in the
689 code can actually be running in totally different contexts due to the
692 =head3 Start position and no-match optimisations
694 C<re_intuit_start()> is responsible for handling start points and no-match
695 optimisations as determined by the results of the analysis done by
696 C<study_chunk()> (and described in L</Peep-hole Optimisation and Analysis>).
698 The basic structure of this routine is to try to find the start- and/or
699 end-points of where the pattern could match, and to ensure that the string
700 is long enough to match the pattern. It tries to use more efficient
701 methods over less efficient methods and may involve considerable
702 cross-checking of constraints to find the place in the string that matches.
703 For instance it may try to determine that a given fixed string must be
704 not only present but a certain number of chars before the end of the
707 It calls several other routines, such as C<fbm_instr()> which does
708 Fast Boyer Moore matching and C<find_byclass()> which is responsible for
709 finding the start using the first mandatory regop in the program.
711 When the optimisation criteria have been satisfied, C<reg_try()> is called
712 to perform the match.
714 =head3 Program execution
716 C<pregexec()> is the main entry point for running a regex. It contains
717 support for initialising the regex interpreter's state, running
718 C<re_intuit_start()> if needed, and running the interpreter on the string
719 from various start positions as needed. When it is necessary to use
720 the regex interpreter C<pregexec()> calls C<regtry()>.
722 C<regtry()> is the entry point into the regex interpreter. It expects
723 as arguments a pointer to a C<regmatch_info> structure and a pointer to
724 a string. It returns an integer 1 for success and a 0 for failure.
725 It is basically a set-up wrapper around C<regmatch()>.
727 C<regmatch> is the main "recursive loop" of the interpreter. It is
728 basically a giant switch statement that implements a state machine, where
729 the possible states are the regops themselves, plus a number of additional
730 intermediate and failure states. A few of the states are implemented as
731 subroutines but the bulk are inline code.
735 =head2 Unicode and Localisation Support
737 When dealing with strings containing characters that cannot be represented
738 using an eight-bit character set, perl uses an internal representation
739 that is a permissive version of Unicode's UTF-8 encoding[2]. This uses single
740 bytes to represent characters from the ASCII character set, and sequences
741 of two or more bytes for all other characters. (See L<perlunitut>
742 for more information about the relationship between UTF-8 and perl's
743 encoding, utf8. The difference isn't important for this discussion.)
745 No matter how you look at it, Unicode support is going to be a pain in a
746 regex engine. Tricks that might be fine when you have 256 possible
747 characters often won't scale to handle the size of the UTF-8 character
748 set. Things you can take for granted with ASCII may not be true with
749 Unicode. For instance, in ASCII, it is safe to assume that
750 C<sizeof(char1) == sizeof(char2)>, but in UTF-8 it isn't. Unicode case folding is
751 vastly more complex than the simple rules of ASCII, and even when not
752 using Unicode but only localised single byte encodings, things can get
753 tricky (for example, B<LATIN SMALL LETTER SHARP S> (U+00DF, E<szlig>)
754 should match 'SS' in localised case-insensitive matching).
756 Making things worse is that UTF-8 support was a later addition to the
757 regex engine (as it was to perl) and this necessarily made things a lot
758 more complicated. Obviously it is easier to design a regex engine with
759 Unicode support in mind from the beginning than it is to retrofit it to
762 Nearly all regops that involve looking at the input string have
763 two cases, one for UTF-8, and one not. In fact, it's often more complex
764 than that, as the pattern may be UTF-8 as well.
766 Care must be taken when making changes to make sure that you handle
767 UTF-8 properly, both at compile time and at execution time, including
768 when the string and pattern are mismatched.
770 =head2 Base Structures
772 The C<regexp> structure described in L<perlreapi> is common to all
773 regex engines. Two of its fields are intended for the private use
774 of the regex engine that compiled the pattern. These are the
775 C<intflags> and pprivate members. The C<pprivate> is a void pointer to
776 an arbitrary structure whose use and management is the responsibility
777 of the compiling engine. perl will never modify either of these
778 values. In the case of the stock engine the structure pointed to by
779 C<pprivate> is called C<regexp_internal>.
781 Its C<pprivate> and C<intflags> fields contain data
782 specific to each engine.
784 There are two structures used to store a compiled regular expression.
785 One, the C<regexp> structure described in L<perlreapi> is populated by
786 the engine currently being. used and some of its fields read by perl to
787 implement things such as the stringification of C<qr//>.
790 The other structure is pointed to by the C<regexp> struct's
791 C<pprivate> and is in addition to C<intflags> in the same struct
792 considered to be the property of the regex engine which compiled the
795 The regexp structure contains all the data that perl needs to be aware of
796 to properly work with the regular expression. It includes data about
797 optimisations that perl can use to determine if the regex engine should
798 really be used, and various other control info that is needed to properly
799 execute patterns in various contexts such as is the pattern anchored in
800 some way, or what flags were used during the compile, or whether the
801 program contains special constructs that perl needs to be aware of.
803 In addition it contains two fields that are intended for the private use
804 of the regex engine that compiled the pattern. These are the C<intflags>
805 and pprivate members. The C<pprivate> is a void pointer to an arbitrary
806 structure whose use and management is the responsibility of the compiling
807 engine. perl will never modify either of these values.
809 As mentioned earlier, in the case of the default engines, the C<pprivate>
810 will be a pointer to a regexp_internal structure which holds the compiled
811 program and any additional data that is private to the regex engine
814 =head3 Perl's C<pprivate> structure
816 The following structure is used as the C<pprivate> struct by perl's
817 regex engine. Since it is specific to perl it is only of curiosity
818 value to other engine implementations.
820 typedef struct regexp_internal {
821 U32 *offsets; /* offset annotations 20001228 MJD
822 * data about mapping the program to
824 regnode *regstclass; /* Optional startclass as identified or
825 * constructed by the optimiser */
826 struct reg_data *data; /* Additional miscellaneous data used
827 * by the program. Used to make it
828 * easier to clone and free arbitrary
829 * data that the regops need. Often the
830 * ARG field of a regop is an index
831 * into this structure */
832 regnode program[1]; /* Unwarranted chumminess with
840 Offsets holds a mapping of offset in the C<program>
841 to offset in the C<precomp> string. This is only used by ActiveState's
842 visual regex debugger.
846 Special regop that is used by C<re_intuit_start()> to check if a pattern
847 can match at a certain position. For instance if the regex engine knows
848 that the pattern must start with a 'Z' then it can scan the string until
849 it finds one and then launch the regex engine from there. The routine
850 that handles this is called C<find_by_class()>. Sometimes this field
851 points at a regop embedded in the program, and sometimes it points at
852 an independent synthetic regop that has been constructed by the optimiser.
856 This field points at a C<reg_data> structure, which is defined as follows
864 This structure is used for handling data structures that the regex engine
865 needs to handle specially during a clone or free operation on the compiled
866 product. Each element in the data array has a corresponding element in the
867 what array. During compilation regops that need special structures stored
868 will add an element to each array using the add_data() routine and then store
869 the index in the regop.
873 Compiled program. Inlined into the structure so the entire struct can be
874 treated as a single blob.
890 With excerpts from Perl, and contributions and suggestions from
891 Ronald J. Kimball, Dave Mitchell, Dominic Dunlop, Mark Jason Dominus,
892 Stephen McCamant, and David Landgren.
900 [1] L<http://perl.plover.com/Rx/paper/>
902 [2] L<http://www.unicode.org>