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 Character classes are represented by C<regnode_charclass> structures,
172 which have a four-byte argument and then a 32-byte (256-bit) bitmap
173 indicating which characters are included in the class.
175 regnode_charclass U32 arg1;
176 char bitmap[ANYOF_BITMAP_SIZE];
178 =item C<regnode_charclass_class>
180 There is also a larger form of a char class structure used to represent
181 POSIX char classes called C<regnode_charclass_class> which has an
182 additional 4-byte (32-bit) bitmap indicating which POSIX char classes
185 regnode_charclass_class U32 arg1;
186 char bitmap[ANYOF_BITMAP_SIZE];
187 char classflags[ANYOF_CLASSBITMAP_SIZE];
191 F<regnodes.h> defines an array called C<regarglen[]> which gives the size
192 of each opcode in units of C<size regnode> (4-byte). A macro is used
193 to calculate the size of an C<EXACT> node based on its C<str_len> field.
195 The regops are defined in F<regnodes.h> which is generated from
196 F<regcomp.sym> by F<regcomp.pl>. Currently the maximum possible number
197 of distinct regops is restricted to 256, with about a quarter already
200 A set of macros makes accessing the fields
201 easier and more consistent. These include C<OP()>, which is used to determine
202 the type of a C<regnode>-like structure; C<NEXT_OFF()>, which is the offset to
203 the next node (more on this later); C<ARG()>, C<ARG1()>, C<ARG2()>, C<ARG_SET()>,
204 and equivalents for reading and setting the arguments; and C<STR_LEN()>,
205 C<STRING()> and C<OPERAND()> for manipulating strings and regop bearing
208 =head3 What regop is next?
210 There are three distinct concepts of "next" in the regex engine, and
211 it is important to keep them clear.
217 There is the "next regnode" from a given regnode, a value which is
218 rarely useful except that sometimes it matches up in terms of value
219 with one of the others, and that sometimes the code assumes this to
224 There is the "next regop" from a given regop/regnode. This is the
225 regop physically located after the current one, as determined by
226 the size of the current regop. This is often useful, such as when
227 dumping the structure we use this order to traverse. Sometimes the code
228 assumes that the "next regnode" is the same as the "next regop", or in
229 other words assumes that the sizeof a given regop type is always going
230 to be one regnode large.
234 There is the "regnext" from a given regop. This is the regop which
235 is reached by jumping forward by the value of C<NEXT_OFF()>,
236 or in a few cases for longer jumps by the C<arg1> field of the C<regnode_1>
237 structure. The subroutine C<regnext()> handles this transparently.
238 This is the logical successor of the node, which in some cases, like
239 that of the C<BRANCH> regop, has special meaning.
243 =head1 Process Overview
245 Broadly speaking, performing a match of a string against a pattern
246 involves the following steps:
254 =item 1. Parsing for size
256 =item 2. Parsing for construction
258 =item 3. Peep-hole optimisation and analysis
266 =item 4. Start position and no-match optimisations
268 =item 5. Program execution
275 Where these steps occur in the actual execution of a perl program is
276 determined by whether the pattern involves interpolating any string
277 variables. If interpolation occurs, then compilation happens at run time. If it
278 does not, then compilation is performed at compile time. (The C</o> modifier changes this,
279 as does C<qr//> to a certain extent.) The engine doesn't really care that
284 This code resides primarily in F<regcomp.c>, along with the header files
285 F<regcomp.h>, F<regexp.h> and F<regnodes.h>.
287 Compilation starts with C<pregcomp()>, which is mostly an initialisation
288 wrapper which farms work out to two other routines for the heavy lifting: the
289 first is C<reg()>, which is the start point for parsing; the second,
290 C<study_chunk()>, is responsible for optimisation.
292 Initialisation in C<pregcomp()> mostly involves the creation and data-filling
293 of a special structure, C<RExC_state_t> (defined in F<regcomp.c>).
294 Almost all internally-used routines in F<regcomp.h> take a pointer to one
295 of these structures as their first argument, with the name C<pRExC_state>.
296 This structure is used to store the compilation state and contains many
297 fields. Likewise there are many macros which operate on this
298 variable: anything that looks like C<RExC_xxxx> is a macro that operates on
299 this pointer/structure.
301 =head3 Parsing for size
303 In this pass the input pattern is parsed in order to calculate how much
304 space is needed for each regop we would need to emit. The size is also
305 used to determine whether long jumps will be required in the program.
307 This stage is controlled by the macro C<SIZE_ONLY> being set.
309 The parse proceeds pretty much exactly as it does during the
310 construction phase, except that most routines are short-circuited to
311 change the size field C<RExC_size> and not do anything else.
313 =head3 Parsing for construction
315 Once the size of the program has been determined, the pattern is parsed
316 again, but this time for real. Now C<SIZE_ONLY> will be false, and the
317 actual construction can occur.
319 C<reg()> is the start of the parse process. It is responsible for
320 parsing an arbitrary chunk of pattern up to either the end of the
321 string, or the first closing parenthesis it encounters in the pattern.
322 This means it can be used to parse the top-level regex, or any section
323 inside of a grouping parenthesis. It also handles the "special parens"
324 that perl's regexes have. For instance when parsing C</x(?:foo)y/> C<reg()>
325 will at one point be called to parse from the "?" symbol up to and
328 Additionally, C<reg()> is responsible for parsing the one or more
329 branches from the pattern, and for "finishing them off" by correctly
330 setting their next pointers. In order to do the parsing, it repeatedly
331 calls out to C<regbranch()>, which is responsible for handling up to the
332 first C<|> symbol it sees.
334 C<regbranch()> in turn calls C<regpiece()> which
335 handles "things" followed by a quantifier. In order to parse the
336 "things", C<regatom()> is called. This is the lowest level routine, which
337 parses out constant strings, character classes, and the
338 various special symbols like C<$>. If C<regatom()> encounters a "("
339 character it in turn calls C<reg()>.
341 The routine C<regtail()> is called by both C<reg()> and C<regbranch()>
342 in order to "set the tail pointer" correctly. When executing and
343 we get to the end of a branch, we need to go to the node following the
344 grouping parens. When parsing, however, we don't know where the end will
345 be until we get there, so when we do we must go back and update the
346 offsets as appropriate. C<regtail> is used to make this easier.
348 A subtlety of the parsing process means that a regex like C</foo/> is
349 originally parsed into an alternation with a single branch. It is only
350 afterwards that the optimiser converts single branch alternations into the
353 =head3 Parse Call Graph and a Grammar
355 The call graph looks like this:
357 reg() # parse a top level regex, or inside of
359 regbranch() # parse a single branch of an alternation
360 regpiece() # parse a pattern followed by a quantifier
361 regatom() # parse a simple pattern
362 regclass() # used to handle a class
363 reg() # used to handle a parenthesised
367 regtail() # finish off the branch
369 regtail() # finish off the branch sequence. Tie each
370 # branch's tail to the tail of the
372 # (NEW) In Debug mode this is
375 A grammar form might be something like this:
377 atom : constant | class
378 quant : '*' | '+' | '?' | '{min,max}'
384 group : '(' branch ')'
391 In the 5.9.x development version of perl you can C<< use re Debug => 'PARSE' >>
392 to see some trace information about the parse process. We will start with some
393 simple patterns and build up to more complex patterns.
395 So when we parse C</foo/> we see something like the following table. The
396 left shows what is being parsed, and the number indicates where the next regop
397 would go. The stuff on the right is the trace output of the graph. The
398 names are chosen to be short to make it less dense on the screen. 'tsdy'
399 is a special form of C<regtail()> which does some extra analysis.
405 >< 4 tsdy~ EXACT <foo> (EXACT) (1)
406 ~ attach to END (3) offset to 2
408 The resulting program then looks like:
413 As you can see, even though we parsed out a branch and a piece, it was ultimately
414 only an atom. The final program shows us how things work. We have an C<EXACT> regop,
415 followed by an C<END> regop. The number in parens indicates where the C<regnext> of
416 the node goes. The C<regnext> of an C<END> regop is unused, as C<END> regops mean
417 we have successfully matched. The number on the left indicates the position of
418 the regop in the regnode array.
420 Now let's try a harder pattern. We will add a quantifier, so now we have the pattern
421 C</foo+/>. We will see that C<regbranch()> calls C<regpiece()> twice.
429 >< 6 tail~ EXACT <fo> (1)
430 7 tsdy~ EXACT <fo> (EXACT) (1)
432 ~ attach to END (6) offset to 3
434 And we end up with the program:
441 Now we have a special case. The C<EXACT> regop has a C<regnext> of 0. This is
442 because if it matches it should try to match itself again. The C<PLUS> regop
443 handles the actual failure of the C<EXACT> regop and acts appropriately (going
444 to regnode 6 if the C<EXACT> matched at least once, or failing if it didn't).
446 Now for something much more complex: C</x(?:foo*|b[a][rR])(foo|bar)$/>
452 >(?:foo*|b[... 3 piec
458 >o*|b[a][rR... 5 piec
460 >|b[a][rR])... 8 tail~ EXACT <fo> (3)
461 >b[a][rR])(... 9 brnc
464 >[a][rR])(f... 12 piec
467 >[rR])(foo|... 14 tail~ EXACT <b> (10)
471 >)(foo|bar)... 25 tail~ EXACT <a> (12)
473 26 tsdy~ BRANCH (END) (9)
474 ~ attach to TAIL (25) offset to 16
475 tsdy~ EXACT <fo> (EXACT) (4)
477 ~ attach to TAIL (25) offset to 19
478 tsdy~ EXACT <b> (EXACT) (10)
479 ~ EXACT <a> (EXACT) (12)
480 ~ ANYOF[Rr] (END) (14)
481 ~ attach to TAIL (25) offset to 11
482 >(foo|bar)$< tail~ EXACT <x> (1)
489 >|bar)$< 31 tail~ OPEN1 (26)
493 >)$< 34 tail~ BRANCH (28)
494 36 tsdy~ BRANCH (END) (31)
495 ~ attach to CLOSE1 (34) offset to 3
496 tsdy~ EXACT <foo> (EXACT) (29)
497 ~ attach to CLOSE1 (34) offset to 5
498 tsdy~ EXACT <bar> (EXACT) (32)
499 ~ attach to CLOSE1 (34) offset to 2
505 >< 37 tail~ OPEN1 (26)
509 38 tsdy~ EXACT <x> (EXACT) (1)
518 ~ attach to END (37) offset to 1
520 Resulting in the program
529 12: OPTIMIZED (2 nodes)
534 [StS:1 Wds:2 Cs:6 Uq:5 #Sts:7 Mn:3 Mx:3 Stcls:bf]
537 30: OPTIMIZED (4 nodes)
542 Here we can see a much more complex program, with various optimisations in
543 play. At regnode 10 we see an example where a character class with only
544 one character in it was turned into an C<EXACT> node. We can also see where
545 an entire alternation was turned into a C<TRIE-EXACT> node. As a consequence,
546 some of the regnodes have been marked as optimised away. We can see that
547 the C<$> symbol has been converted into an C<EOL> regop, a special piece of
548 code that looks for C<\n> or the end of the string.
550 The next pointer for C<BRANCH>es is interesting in that it points at where
551 execution should go if the branch fails. When executing, if the engine
552 tries to traverse from a branch to a C<regnext> that isn't a branch then
553 the engine will know that the entire set of branches has failed.
555 =head3 Peep-hole Optimisation and Analysis
557 The regular expression engine can be a weighty tool to wield. On long
558 strings and complex patterns it can end up having to do a lot of work
559 to find a match, and even more to decide that no match is possible.
560 Consider a situation like the following pattern.
562 'ababababababababababab' =~ /(a|b)*z/
564 The C<(a|b)*> part can match at every char in the string, and then fail
565 every time because there is no C<z> in the string. So obviously we can
566 avoid using the regex engine unless there is a C<z> in the string.
567 Likewise in a pattern like:
571 In this case we know that the string must contain a C<foo> which must be
572 followed by C<bar>. We can use Fast Boyer-Moore matching as implemented
573 in C<fbm_instr()> to find the location of these strings. If they don't exist
574 then we don't need to resort to the much more expensive regex engine.
575 Even better, if they do exist then we can use their positions to
576 reduce the search space that the regex engine needs to cover to determine
577 if the entire pattern matches.
579 There are various aspects of the pattern that can be used to facilitate
580 optimisations along these lines:
584 =item * anchored fixed strings
586 =item * floating fixed strings
588 =item * minimum and maximum length requirements
592 =item * Beginning/End of line positions
596 Another form of optimisation that can occur is the post-parse "peep-hole"
597 optimisation, where inefficient constructs are replaced by more efficient
598 constructs. The C<TAIL> regops which are used during parsing to mark the end
599 of branches and the end of groups are examples of this. These regops are used
600 as place-holders during construction and "always match" so they can be
601 "optimised away" by making the things that point to the C<TAIL> point to the
602 thing that C<TAIL> points to, thus "skipping" the node.
604 Another optimisation that can occur is that of "C<EXACT> merging" which is
605 where two consecutive C<EXACT> nodes are merged into a single
606 regop. An even more aggressive form of this is that a branch
607 sequence of the form C<EXACT BRANCH ... EXACT> can be converted into a
610 All of this occurs in the routine C<study_chunk()> which uses a special
611 structure C<scan_data_t> to store the analysis that it has performed, and
612 does the "peep-hole" optimisations as it goes.
614 The code involved in C<study_chunk()> is extremely cryptic. Be careful. :-)
618 Execution of a regex generally involves two phases, the first being
619 finding the start point in the string where we should match from,
620 and the second being running the regop interpreter.
622 If we can tell that there is no valid start point then we don't bother running
623 interpreter at all. Likewise, if we know from the analysis phase that we
624 cannot detect a short-cut to the start position, we go straight to the
627 The two entry points are C<re_intuit_start()> and C<pregexec()>. These routines
628 have a somewhat incestuous relationship with overlap between their functions,
629 and C<pregexec()> may even call C<re_intuit_start()> on its own. Nevertheless
630 other parts of the perl source code may call into either, or both.
632 Execution of the interpreter itself used to be recursive, but thanks to the
633 efforts of Dave Mitchell in the 5.9.x development track, that has changed: now an
634 internal stack is maintained on the heap and the routine is fully
635 iterative. This can make it tricky as the code is quite conservative
636 about what state it stores, with the result that two consecutive lines in the
637 code can actually be running in totally different contexts due to the
640 =head3 Start position and no-match optimisations
642 C<re_intuit_start()> is responsible for handling start points and no-match
643 optimisations as determined by the results of the analysis done by
644 C<study_chunk()> (and described in L<Peep-hole Optimisation and Analysis>).
646 The basic structure of this routine is to try to find the start- and/or
647 end-points of where the pattern could match, and to ensure that the string
648 is long enough to match the pattern. It tries to use more efficient
649 methods over less efficient methods and may involve considerable
650 cross-checking of constraints to find the place in the string that matches.
651 For instance it may try to determine that a given fixed string must be
652 not only present but a certain number of chars before the end of the
655 It calls several other routines, such as C<fbm_instr()> which does
656 Fast Boyer Moore matching and C<find_byclass()> which is responsible for
657 finding the start using the first mandatory regop in the program.
659 When the optimisation criteria have been satisfied, C<reg_try()> is called
660 to perform the match.
662 =head3 Program execution
664 C<pregexec()> is the main entry point for running a regex. It contains
665 support for initialising the regex interpreter's state, running
666 C<re_intuit_start()> if needed, and running the interpreter on the string
667 from various start positions as needed. When it is necessary to use
668 the regex interpreter C<pregexec()> calls C<regtry()>.
670 C<regtry()> is the entry point into the regex interpreter. It expects
671 as arguments a pointer to a C<regmatch_info> structure and a pointer to
672 a string. It returns an integer 1 for success and a 0 for failure.
673 It is basically a set-up wrapper around C<regmatch()>.
675 C<regmatch> is the main "recursive loop" of the interpreter. It is
676 basically a giant switch statement that implements a state machine, where
677 the possible states are the regops themselves, plus a number of additional
678 intermediate and failure states. A few of the states are implemented as
679 subroutines but the bulk are inline code.
683 =head2 Unicode and Localisation Support
685 When dealing with strings containing characters that cannot be represented
686 using an eight-bit character set, perl uses an internal representation
687 that is a permissive version of Unicode's UTF-8 encoding[2]. This uses single
688 bytes to represent characters from the ASCII character set, and sequences
689 of two or more bytes for all other characters. (See L<perlunitut>
690 for more information about the relationship between UTF-8 and perl's
691 encoding, utf8. The difference isn't important for this discussion.)
693 No matter how you look at it, Unicode support is going to be a pain in a
694 regex engine. Tricks that might be fine when you have 256 possible
695 characters often won't scale to handle the size of the UTF-8 character
696 set. Things you can take for granted with ASCII may not be true with
697 Unicode. For instance, in ASCII, it is safe to assume that
698 C<sizeof(char1) == sizeof(char2)>, but in UTF-8 it isn't. Unicode case folding is
699 vastly more complex than the simple rules of ASCII, and even when not
700 using Unicode but only localised single byte encodings, things can get
701 tricky (for example, B<LATIN SMALL LETTER SHARP S> (U+00DF, E<szlig>)
702 should match 'SS' in localised case-insensitive matching).
704 Making things worse is that UTF-8 support was a later addition to the
705 regex engine (as it was to perl) and this necessarily made things a lot
706 more complicated. Obviously it is easier to design a regex engine with
707 Unicode support in mind from the beginning than it is to retrofit it to
710 Nearly all regops that involve looking at the input string have
711 two cases, one for UTF-8, and one not. In fact, it's often more complex
712 than that, as the pattern may be UTF-8 as well.
714 Care must be taken when making changes to make sure that you handle
715 UTF-8 properly, both at compile time and at execution time, including
716 when the string and pattern are mismatched.
718 The following comment in F<regcomp.h> gives an example of exactly how
721 Two problematic code points in Unicode casefolding of EXACT nodes:
723 U+0390 - GREEK SMALL LETTER IOTA WITH DIALYTIKA AND TONOS
724 U+03B0 - GREEK SMALL LETTER UPSILON WITH DIALYTIKA AND TONOS
730 U+03B9 U+0308 U+0301 0xCE 0xB9 0xCC 0x88 0xCC 0x81
731 U+03C5 U+0308 U+0301 0xCF 0x85 0xCC 0x88 0xCC 0x81
733 This means that in case-insensitive matching (or "loose matching",
734 as Unicode calls it), an EXACTF of length six (the UTF-8 encoded
735 byte length of the above casefolded versions) can match a target
736 string of length two (the byte length of UTF-8 encoded U+0390 or
737 U+03B0). This would rather mess up the minimum length computation.
739 What we'll do is to look for the tail four bytes, and then peek
740 at the preceding two bytes to see whether we need to decrease
741 the minimum length by four (six minus two).
743 Thanks to the design of UTF-8, there cannot be false matches:
744 A sequence of valid UTF-8 bytes cannot be a subsequence of
745 another valid sequence of UTF-8 bytes.
748 =head2 Base Structures
750 The C<regexp> structure described in L<perlreapi> is common to all
751 regex engines. Two of its fields that are intended for the private use
752 of the regex engine that compiled the pattern. These are the
753 C<intflags> and pprivate members. The C<pprivate> is a void pointer to
754 an arbitrary structure whose use and management is the responsibility
755 of the compiling engine. perl will never modify either of these
756 values. In the case of the stock engine the structure pointed to by
757 C<pprivate> is called C<regexp_internal>.
759 Its C<pprivate> and C<intflags> fields contain data
760 specific to each engine.
762 There are two structures used to store a compiled regular expression.
763 One, the C<regexp> structure described in L<perlreapi> is populated by
764 the engine currently being. used and some of its fields read by perl to
765 implement things such as the stringification of C<qr//>.
768 The other structure is pointed to be the C<regexp> struct's
769 C<pprivate> and is in addition to C<intflags> in the same struct
770 considered to be the property of the regex engine which compiled the
773 The regexp structure contains all the data that perl needs to be aware of
774 to properly work with the regular expression. It includes data about
775 optimisations that perl can use to determine if the regex engine should
776 really be used, and various other control info that is needed to properly
777 execute patterns in various contexts such as is the pattern anchored in
778 some way, or what flags were used during the compile, or whether the
779 program contains special constructs that perl needs to be aware of.
781 In addition it contains two fields that are intended for the private use
782 of the regex engine that compiled the pattern. These are the C<intflags>
783 and pprivate members. The C<pprivate> is a void pointer to an arbitrary
784 structure whose use and management is the responsibility of the compiling
785 engine. perl will never modify either of these values.
787 As mentioned earlier, in the case of the default engines, the C<pprivate>
788 will be a pointer to a regexp_internal structure which holds the compiled
789 program and any additional data that is private to the regex engine
792 =head3 Perl's C<pprivate> structure
794 The following structure is used as the C<pprivate> struct by perl's
795 regex engine. Since it is specific to perl it is only of curiosity
796 value to other engine implementations.
798 typedef struct regexp_internal {
799 U32 *offsets; /* offset annotations 20001228 MJD
800 * data about mapping the program to
802 regnode *regstclass; /* Optional startclass as identified or
803 * constructed by the optimiser */
804 struct reg_data *data; /* Additional miscellaneous data used
805 * by the program. Used to make it
806 * easier to clone and free arbitrary
807 * data that the regops need. Often the
808 * ARG field of a regop is an index
809 * into this structure */
810 regnode program[1]; /* Unwarranted chumminess with
818 Offsets holds a mapping of offset in the C<program>
819 to offset in the C<precomp> string. This is only used by ActiveState's
820 visual regex debugger.
824 Special regop that is used by C<re_intuit_start()> to check if a pattern
825 can match at a certain position. For instance if the regex engine knows
826 that the pattern must start with a 'Z' then it can scan the string until
827 it finds one and then launch the regex engine from there. The routine
828 that handles this is called C<find_by_class()>. Sometimes this field
829 points at a regop embedded in the program, and sometimes it points at
830 an independent synthetic regop that has been constructed by the optimiser.
834 This field points at a reg_data structure, which is defined as follows
842 This structure is used for handling data structures that the regex engine
843 needs to handle specially during a clone or free operation on the compiled
844 product. Each element in the data array has a corresponding element in the
845 what array. During compilation regops that need special structures stored
846 will add an element to each array using the add_data() routine and then store
847 the index in the regop.
851 Compiled program. Inlined into the structure so the entire struct can be
852 treated as a single blob.
868 With excerpts from Perl, and contributions and suggestions from
869 Ronald J. Kimball, Dave Mitchell, Dominic Dunlop, Mark Jason Dominus,
870 Stephen McCamant, and David Landgren.
878 [1] L<http://perl.plover.com/Rx/paper/>
880 [2] L<http://www.unicode.org>