1=head1 NAME 2 3perlreguts - Description of the Perl regular expression engine. 4 5=head1 DESCRIPTION 6 7This document is an attempt to shine some light on the guts of the regex 8engine and how it works. The regex engine represents a significant chunk 9of the perl codebase, but is relatively poorly understood. This document 10is a meagre attempt at addressing this situation. It is derived from the 11author's experience, comments in the source code, other papers on the 12regex engine, feedback on the perl5-porters mail list, and no doubt other 13places as well. 14 15B<NOTICE!> It should be clearly understood that the behavior and 16structures discussed in this represents the state of the engine as the 17author understood it at the time of writing. It is B<NOT> an API 18definition, it is purely an internals guide for those who want to hack 19the regex engine, or understand how the regex engine works. Readers of 20this document are expected to understand perl's regex syntax and its 21usage in detail. If you want to learn about the basics of Perl's 22regular expressions, see L<perlre>. And if you want to replace the 23regex engine with your own, see L<perlreapi>. 24 25=head1 OVERVIEW 26 27=head2 A quick note on terms 28 29There is some debate as to whether to say "regexp" or "regex". In this 30document we will use the term "regex" unless there is a special reason 31not to, in which case we will explain why. 32 33When speaking about regexes we need to distinguish between their source 34code 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 37correspond to the terms I<S-regex> and I<B-regex> that Mark Jason 38Dominus employs in his paper on "Rx" ([1] in L</REFERENCES>). 39 40=head2 What is a regular expression engine? 41 42A regular expression engine is a program that takes a set of constraints 43specified in a mini-language, and then applies those constraints to a 44target string, and determines whether or not the string satisfies the 45constraints. See L<perlre> for a full definition of the language. 46 47In less grandiose terms, the first part of the job is to turn a pattern into 48something the computer can efficiently use to find the matching point in 49the string, and the second part is performing the search itself. 50 51To do this we need to produce a program by parsing the text. We then 52need to execute the program to find the point in the string that 53matches. And we need to do the whole thing efficiently. 54 55=head2 Structure of a Regexp Program 56 57=head3 High Level 58 59Although it is a bit confusing and some people object to the terminology, it 60is worth taking a look at a comment that has 61been in F<regexp.h> for years: 62 63I<This is essentially a linear encoding of a nondeterministic 64finite-state machine (aka syntax charts or "railroad normal form" in 65parsing technology).> 66 67The term "railroad normal form" is a bit esoteric, with "syntax 68diagram/charts", or "railroad diagram/charts" being more common terms. 69Nevertheless it provides a useful mental image of a regex program: each 70node can be thought of as a unit of track, with a single entry and in 71most cases a single exit point (there are pieces of track that fork, but 72statistically not many), and the whole forms a layout with a 73single entry and single exit point. The matching process can be thought 74of as a car that moves along the track, with the particular route through 75the system being determined by the character read at each possible 76connector point. A car can fall off the track at any point but it may 77only proceed as long as it matches the track. 78 79Thus the pattern C</foo(?:\w+|\d+|\s+)bar/> can be thought of as the 80following chart: 81 82 [start] 83 | 84 <foo> 85 | 86 +-----+-----+ 87 | | | 88 <\w+> <\d+> <\s+> 89 | | | 90 +-----+-----+ 91 | 92 <bar> 93 | 94 [end] 95 96The truth of the matter is that perl's regular expressions these days are 97much more complex than this kind of structure, but visualising it this way 98can help when trying to get your bearings, and it matches the 99current implementation pretty closely. 100 101To be more precise, we will say that a regex program is an encoding 102of a graph. Each node in the graph corresponds to part of 103the original regex pattern, such as a literal string or a branch, 104and has a pointer to the nodes representing the next component 105to be matched. Since "node" and "opcode" already have other meanings in the 106perl source, we will call the nodes in a regex program "regops". 107 108The program is represented by an array of C<regnode> structures, one or 109more of which represent a single regop of the program. Struct 110C<regnode> is the smallest struct needed, and has a field structure which is 111shared with all the other larger structures. 112 113The "next" pointers of all regops except C<BRANCH> implement concatenation; 114a "next" pointer with a C<BRANCH> on both ends of it is connecting two 115alternatives. [Here we have one of the subtle syntax dependencies: an 116individual C<BRANCH> (as opposed to a collection of them) is never 117concatenated with anything because of operator precedence.] 118 119The operand of some types of regop is a literal string; for others, 120it is a regop leading into a sub-program. In particular, the operand 121of a C<BRANCH> node is the first regop of the branch. 122 123B<NOTE>: As the railroad metaphor suggests, this is B<not> a tree 124structure: the tail of the branch connects to the thing following the 125set of C<BRANCH>es. It is a like a single line of railway track that 126splits as it goes into a station or railway yard and rejoins as it comes 127out the other side. 128 129=head3 Regops 130 131The base structure of a regop is defined in F<regexp.h> as follows: 132 133 struct regnode { 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 */ 137 }; 138 139Other larger C<regnode>-like structures are defined in F<regcomp.h>. They 140are almost like subclasses in that they have the same fields as 141C<regnode>, with possibly additional fields following in 142the structure, and in some cases the specific meaning (and name) 143of some of base fields are overridden. The following is a more 144complete description. 145 146=over 4 147 148=item C<regnode_1> 149 150=item C<regnode_2> 151 152C<regnode_1> structures have the same header, followed by a single 153four-byte argument; C<regnode_2> structures contain two two-byte 154arguments instead: 155 156 regnode_1 U32 arg1; 157 regnode_2 U16 arg1; U16 arg2; 158 159=item C<regnode_string> 160 161C<regnode_string> structures, used for literal strings, follow the header 162with a one-byte length and then the string data. Strings are padded on 163the end with zero bytes so that the total length of the node is a 164multiple of four bytes: 165 166 regnode_string char string[1]; 167 U8 str_len; /* overrides flags */ 168 169=item C<regnode_charclass> 170 171Bracketed character classes are represented by C<regnode_charclass> 172structures, which have a four-byte argument and then a 32-byte (256-bit) 173bitmap indicating which characters in the Latin1 range are included in 174the class. 175 176 regnode_charclass U32 arg1; 177 char bitmap[ANYOF_BITMAP_SIZE]; 178 179Various flags whose names begin with C<ANYOF_> are used for special 180situations. Above Latin1 matches and things not known until run-time 181are stored in L</Perl's pprivate structure>. 182 183=item C<regnode_charclass_posixl> 184 185There is also a larger form of a char class structure used to represent 186POSIX char classes under C</l> matching, 187called C<regnode_charclass_posixl> which has an 188additional 32-bit bitmap indicating which POSIX char classes 189have been included. 190 191 regnode_charclass_posixl U32 arg1; 192 char bitmap[ANYOF_BITMAP_SIZE]; 193 U32 classflags; 194 195=back 196 197F<regnodes.h> defines an array called C<regarglen[]> which gives the size 198of each opcode in units of C<size regnode> (4-byte). A macro is used 199to calculate the size of an C<EXACT> node based on its C<str_len> field. 200 201The regops are defined in F<regnodes.h> which is generated from 202F<regcomp.sym> by F<regcomp.pl>. Currently the maximum possible number 203of distinct regops is restricted to 256, with about a quarter already 204used. 205 206A set of macros makes accessing the fields 207easier and more consistent. These include C<OP()>, which is used to determine 208the type of a C<regnode>-like structure; C<NEXT_OFF()>, which is the offset to 209the next node (more on this later); C<ARG()>, C<ARG1()>, C<ARG2()>, C<ARG_SET()>, 210and equivalents for reading and setting the arguments; and C<STR_LEN()>, 211C<STRING()> and C<OPERAND()> for manipulating strings and regop bearing 212types. 213 214=head3 What regop is next? 215 216There are three distinct concepts of "next" in the regex engine, and 217it is important to keep them clear. 218 219=over 4 220 221=item * 222 223There is the "next regnode" from a given regnode, a value which is 224rarely useful except that sometimes it matches up in terms of value 225with one of the others, and that sometimes the code assumes this to 226always be so. 227 228=item * 229 230There is the "next regop" from a given regop/regnode. This is the 231regop physically located after the current one, as determined by 232the size of the current regop. This is often useful, such as when 233dumping the structure we use this order to traverse. Sometimes the code 234assumes that the "next regnode" is the same as the "next regop", or in 235other words assumes that the sizeof a given regop type is always going 236to be one regnode large. 237 238=item * 239 240There is the "regnext" from a given regop. This is the regop which 241is reached by jumping forward by the value of C<NEXT_OFF()>, 242or in a few cases for longer jumps by the C<arg1> field of the C<regnode_1> 243structure. The subroutine C<regnext()> handles this transparently. 244This is the logical successor of the node, which in some cases, like 245that of the C<BRANCH> regop, has special meaning. 246 247=back 248 249=head1 Process Overview 250 251Broadly speaking, performing a match of a string against a pattern 252involves the following steps: 253 254=over 5 255 256=item A. Compilation 257 258=over 5 259 260=item 1. Parsing for size 261 262=item 2. Parsing for construction 263 264=item 3. Peep-hole optimisation and analysis 265 266=back 267 268=item B. Execution 269 270=over 5 271 272=item 4. Start position and no-match optimisations 273 274=item 5. Program execution 275 276=back 277 278=back 279 280 281Where these steps occur in the actual execution of a perl program is 282determined by whether the pattern involves interpolating any string 283variables. If interpolation occurs, then compilation happens at run time. If it 284does not, then compilation is performed at compile time. (The C</o> modifier changes this, 285as does C<qr//> to a certain extent.) The engine doesn't really care that 286much. 287 288=head2 Compilation 289 290This code resides primarily in F<regcomp.c>, along with the header files 291F<regcomp.h>, F<regexp.h> and F<regnodes.h>. 292 293Compilation starts with C<pregcomp()>, which is mostly an initialisation 294wrapper which farms work out to two other routines for the heavy lifting: the 295first is C<reg()>, which is the start point for parsing; the second, 296C<study_chunk()>, is responsible for optimisation. 297 298Initialisation in C<pregcomp()> mostly involves the creation and data-filling 299of a special structure, C<RExC_state_t> (defined in F<regcomp.c>). 300Almost all internally-used routines in F<regcomp.h> take a pointer to one 301of these structures as their first argument, with the name C<pRExC_state>. 302This structure is used to store the compilation state and contains many 303fields. Likewise there are many macros which operate on this 304variable: anything that looks like C<RExC_xxxx> is a macro that operates on 305this pointer/structure. 306 307=head3 Parsing for size 308 309In this pass the input pattern is parsed in order to calculate how much 310space is needed for each regop we would need to emit. The size is also 311used to determine whether long jumps will be required in the program. 312 313This stage is controlled by the macro C<SIZE_ONLY> being set. 314 315The parse proceeds pretty much exactly as it does during the 316construction phase, except that most routines are short-circuited to 317change the size field C<RExC_size> and not do anything else. 318 319=head3 Parsing for construction 320 321Once the size of the program has been determined, the pattern is parsed 322again, but this time for real. Now C<SIZE_ONLY> will be false, and the 323actual construction can occur. 324 325C<reg()> is the start of the parse process. It is responsible for 326parsing an arbitrary chunk of pattern up to either the end of the 327string, or the first closing parenthesis it encounters in the pattern. 328This means it can be used to parse the top-level regex, or any section 329inside of a grouping parenthesis. It also handles the "special parens" 330that perl's regexes have. For instance when parsing C</x(?:foo)y/> C<reg()> 331will at one point be called to parse from the "?" symbol up to and 332including the ")". 333 334Additionally, C<reg()> is responsible for parsing the one or more 335branches from the pattern, and for "finishing them off" by correctly 336setting their next pointers. In order to do the parsing, it repeatedly 337calls out to C<regbranch()>, which is responsible for handling up to the 338first C<|> symbol it sees. 339 340C<regbranch()> in turn calls C<regpiece()> which 341handles "things" followed by a quantifier. In order to parse the 342"things", C<regatom()> is called. This is the lowest level routine, which 343parses out constant strings, character classes, and the 344various special symbols like C<$>. If C<regatom()> encounters a "(" 345character it in turn calls C<reg()>. 346 347The routine C<regtail()> is called by both C<reg()> and C<regbranch()> 348in order to "set the tail pointer" correctly. When executing and 349we get to the end of a branch, we need to go to the node following the 350grouping parens. When parsing, however, we don't know where the end will 351be until we get there, so when we do we must go back and update the 352offsets as appropriate. C<regtail> is used to make this easier. 353 354A subtlety of the parsing process means that a regex like C</foo/> is 355originally parsed into an alternation with a single branch. It is only 356afterwards that the optimiser converts single branch alternations into the 357simpler form. 358 359=head3 Parse Call Graph and a Grammar 360 361The call graph looks like this: 362 363 reg() # parse a top level regex, or inside of 364 # parens 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 370 # subpattern 371 .... 372 ... 373 regtail() # finish off the branch 374 ... 375 regtail() # finish off the branch sequence. Tie each 376 # branch's tail to the tail of the 377 # sequence 378 # (NEW) In Debug mode this is 379 # regtail_study(). 380 381A grammar form might be something like this: 382 383 atom : constant | class 384 quant : '*' | '+' | '?' | '{min,max}' 385 _branch: piece 386 | piece _branch 387 | nothing 388 branch: _branch 389 | _branch '|' branch 390 group : '(' branch ')' 391 _piece: atom | group 392 piece : _piece 393 | _piece quant 394 395=head3 Parsing complications 396 397The implication of the above description is that a pattern containing nested 398parentheses will result in a call graph which cycles through C<reg()>, 399C<regbranch()>, C<regpiece()>, C<regatom()>, C<reg()>, C<regbranch()> I<etc> 400multiple times, until the deepest level of nesting is reached. All the above 401routines return a pointer to a C<regnode>, which is usually the last regnode 402added to the program. However, one complication is that reg() returns NULL 403for parsing C<(?:)> syntax for embedded modifiers, setting the flag 404C<TRYAGAIN>. The C<TRYAGAIN> propagates upwards until it is captured, in 405some cases by C<regatom()>, but otherwise unconditionally by 406C<regbranch()>. Hence it will never be returned by C<regbranch()> to 407C<reg()>. This flag permits patterns such as C<(?i)+> to be detected as 408errors (I<Quantifier follows nothing in regex; marked by <-- HERE in m/(?i)+ 409<-- HERE />). 410 411Another complication is that the representation used for the program differs 412if it needs to store Unicode, but it's not always possible to know for sure 413whether it does until midway through parsing. The Unicode representation for 414the program is larger, and cannot be matched as efficiently. (See L</Unicode 415and Localisation Support> below for more details as to why.) If the pattern 416contains literal Unicode, it's obvious that the program needs to store 417Unicode. Otherwise, the parser optimistically assumes that the more 418efficient representation can be used, and starts sizing on this basis. 419However, if it then encounters something in the pattern which must be stored 420as Unicode, such as an C<\x{...}> escape sequence representing a character 421literal, then this means that all previously calculated sizes need to be 422redone, using values appropriate for the Unicode representation. Currently, 423all regular expression constructions which can trigger this are parsed by code 424in C<regatom()>. 425 426To 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 429request 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 431Unicode, and restarts the sizing pass. It is also possible for constructions 432within run-time code blocks to turn out to need Unicode representation., 433which is signalled by C<S_compile_runtime_code()> returning false to 434C<Perl_re_op_compile()>. 435 436The restart was previously implemented using a C<longjmp> in C<regatom()> 437back to a C<setjmp> in C<Perl_re_op_compile()>, but this proved to be 438problematic as the latter is a large function containing many automatic 439variables, which interact badly with the emergent control flow of C<setjmp>. 440 441=head3 Debug Output 442 443In the 5.9.x development version of perl you can C<< use re Debug => 'PARSE' >> 444to see some trace information about the parse process. We will start with some 445simple patterns and build up to more complex patterns. 446 447So when we parse C</foo/> we see something like the following table. The 448left shows what is being parsed, and the number indicates where the next regop 449would go. The stuff on the right is the trace output of the graph. The 450names are chosen to be short to make it less dense on the screen. 'tsdy' 451is a special form of C<regtail()> which does some extra analysis. 452 453 >foo< 1 reg 454 brnc 455 piec 456 atom 457 >< 4 tsdy~ EXACT <foo> (EXACT) (1) 458 ~ attach to END (3) offset to 2 459 460The resulting program then looks like: 461 462 1: EXACT <foo>(3) 463 3: END(0) 464 465As you can see, even though we parsed out a branch and a piece, it was ultimately 466only an atom. The final program shows us how things work. We have an C<EXACT> regop, 467followed by an C<END> regop. The number in parens indicates where the C<regnext> of 468the node goes. The C<regnext> of an C<END> regop is unused, as C<END> regops mean 469we have successfully matched. The number on the left indicates the position of 470the regop in the regnode array. 471 472Now let's try a harder pattern. We will add a quantifier, so now we have the pattern 473C</foo+/>. We will see that C<regbranch()> calls C<regpiece()> twice. 474 475 >foo+< 1 reg 476 brnc 477 piec 478 atom 479 >o+< 3 piec 480 atom 481 >< 6 tail~ EXACT <fo> (1) 482 7 tsdy~ EXACT <fo> (EXACT) (1) 483 ~ PLUS (END) (3) 484 ~ attach to END (6) offset to 3 485 486And we end up with the program: 487 488 1: EXACT <fo>(3) 489 3: PLUS(6) 490 4: EXACT <o>(0) 491 6: END(0) 492 493Now we have a special case. The C<EXACT> regop has a C<regnext> of 0. This is 494because if it matches it should try to match itself again. The C<PLUS> regop 495handles the actual failure of the C<EXACT> regop and acts appropriately (going 496to regnode 6 if the C<EXACT> matched at least once, or failing if it didn't). 497 498Now for something much more complex: C</x(?:foo*|b[a][rR])(foo|bar)$/> 499 500 >x(?:foo*|b... 1 reg 501 brnc 502 piec 503 atom 504 >(?:foo*|b[... 3 piec 505 atom 506 >?:foo*|b[a... reg 507 >foo*|b[a][... brnc 508 piec 509 atom 510 >o*|b[a][rR... 5 piec 511 atom 512 >|b[a][rR])... 8 tail~ EXACT <fo> (3) 513 >b[a][rR])(... 9 brnc 514 10 piec 515 atom 516 >[a][rR])(f... 12 piec 517 atom 518 >a][rR])(fo... clas 519 >[rR])(foo|... 14 tail~ EXACT <b> (10) 520 piec 521 atom 522 >rR])(foo|b... clas 523 >)(foo|bar)... 25 tail~ EXACT <a> (12) 524 tail~ BRANCH (3) 525 26 tsdy~ BRANCH (END) (9) 526 ~ attach to TAIL (25) offset to 16 527 tsdy~ EXACT <fo> (EXACT) (4) 528 ~ STAR (END) (6) 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)$< tail~ EXACT <x> (1) 535 piec 536 atom 537 >foo|bar)$< reg 538 28 brnc 539 piec 540 atom 541 >|bar)$< 31 tail~ OPEN1 (26) 542 >bar)$< brnc 543 32 piec 544 atom 545 >)$< 34 tail~ BRANCH (28) 546 36 tsdy~ BRANCH (END) (31) 547 ~ attach to CLOSE1 (34) offset to 3 548 tsdy~ EXACT <foo> (EXACT) (29) 549 ~ attach to CLOSE1 (34) offset to 5 550 tsdy~ EXACT <bar> (EXACT) (32) 551 ~ attach to CLOSE1 (34) offset to 2 552 >$< tail~ BRANCH (3) 553 ~ BRANCH (9) 554 ~ TAIL (25) 555 piec 556 atom 557 >< 37 tail~ OPEN1 (26) 558 ~ BRANCH (28) 559 ~ BRANCH (31) 560 ~ CLOSE1 (34) 561 38 tsdy~ EXACT <x> (EXACT) (1) 562 ~ BRANCH (END) (3) 563 ~ BRANCH (END) (9) 564 ~ TAIL (END) (25) 565 ~ OPEN1 (END) (26) 566 ~ BRANCH (END) (28) 567 ~ BRANCH (END) (31) 568 ~ CLOSE1 (END) (34) 569 ~ EOL (END) (36) 570 ~ attach to END (37) offset to 1 571 572Resulting in the program 573 574 1: EXACT <x>(3) 575 3: BRANCH(9) 576 4: EXACT <fo>(6) 577 6: STAR(26) 578 7: EXACT <o>(0) 579 9: BRANCH(25) 580 10: EXACT <ba>(14) 581 12: OPTIMIZED (2 nodes) 582 14: ANYOF[Rr](26) 583 25: TAIL(26) 584 26: OPEN1(28) 585 28: TRIE-EXACT(34) 586 [StS:1 Wds:2 Cs:6 Uq:5 #Sts:7 Mn:3 Mx:3 Stcls:bf] 587 <foo> 588 <bar> 589 30: OPTIMIZED (4 nodes) 590 34: CLOSE1(36) 591 36: EOL(37) 592 37: END(0) 593 594Here we can see a much more complex program, with various optimisations in 595play. At regnode 10 we see an example where a character class with only 596one character in it was turned into an C<EXACT> node. We can also see where 597an entire alternation was turned into a C<TRIE-EXACT> node. As a consequence, 598some of the regnodes have been marked as optimised away. We can see that 599the C<$> symbol has been converted into an C<EOL> regop, a special piece of 600code that looks for C<\n> or the end of the string. 601 602The next pointer for C<BRANCH>es is interesting in that it points at where 603execution should go if the branch fails. When executing, if the engine 604tries to traverse from a branch to a C<regnext> that isn't a branch then 605the engine will know that the entire set of branches has failed. 606 607=head3 Peep-hole Optimisation and Analysis 608 609The regular expression engine can be a weighty tool to wield. On long 610strings and complex patterns it can end up having to do a lot of work 611to find a match, and even more to decide that no match is possible. 612Consider a situation like the following pattern. 613 614 'ababababababababababab' =~ /(a|b)*z/ 615 616The C<(a|b)*> part can match at every char in the string, and then fail 617every time because there is no C<z> in the string. So obviously we can 618avoid using the regex engine unless there is a C<z> in the string. 619Likewise in a pattern like: 620 621 /foo(\w+)bar/ 622 623In this case we know that the string must contain a C<foo> which must be 624followed by C<bar>. We can use Fast Boyer-Moore matching as implemented 625in C<fbm_instr()> to find the location of these strings. If they don't exist 626then we don't need to resort to the much more expensive regex engine. 627Even better, if they do exist then we can use their positions to 628reduce the search space that the regex engine needs to cover to determine 629if the entire pattern matches. 630 631There are various aspects of the pattern that can be used to facilitate 632optimisations along these lines: 633 634=over 5 635 636=item * anchored fixed strings 637 638=item * floating fixed strings 639 640=item * minimum and maximum length requirements 641 642=item * start class 643 644=item * Beginning/End of line positions 645 646=back 647 648Another form of optimisation that can occur is the post-parse "peep-hole" 649optimisation, where inefficient constructs are replaced by more efficient 650constructs. The C<TAIL> regops which are used during parsing to mark the end 651of branches and the end of groups are examples of this. These regops are used 652as 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 654thing that C<TAIL> points to, thus "skipping" the node. 655 656Another optimisation that can occur is that of "C<EXACT> merging" which is 657where two consecutive C<EXACT> nodes are merged into a single 658regop. An even more aggressive form of this is that a branch 659sequence of the form C<EXACT BRANCH ... EXACT> can be converted into a 660C<TRIE-EXACT> regop. 661 662All of this occurs in the routine C<study_chunk()> which uses a special 663structure C<scan_data_t> to store the analysis that it has performed, and 664does the "peep-hole" optimisations as it goes. 665 666The code involved in C<study_chunk()> is extremely cryptic. Be careful. :-) 667 668=head2 Execution 669 670Execution of a regex generally involves two phases, the first being 671finding the start point in the string where we should match from, 672and the second being running the regop interpreter. 673 674If we can tell that there is no valid start point then we don't bother running 675the interpreter at all. Likewise, if we know from the analysis phase that we 676cannot detect a short-cut to the start position, we go straight to the 677interpreter. 678 679The two entry points are C<re_intuit_start()> and C<pregexec()>. These routines 680have a somewhat incestuous relationship with overlap between their functions, 681and C<pregexec()> may even call C<re_intuit_start()> on its own. Nevertheless 682other parts of the perl source code may call into either, or both. 683 684Execution of the interpreter itself used to be recursive, but thanks to the 685efforts of Dave Mitchell in the 5.9.x development track, that has changed: now an 686internal stack is maintained on the heap and the routine is fully 687iterative. This can make it tricky as the code is quite conservative 688about what state it stores, with the result that two consecutive lines in the 689code can actually be running in totally different contexts due to the 690simulated recursion. 691 692=head3 Start position and no-match optimisations 693 694C<re_intuit_start()> is responsible for handling start points and no-match 695optimisations as determined by the results of the analysis done by 696C<study_chunk()> (and described in L<Peep-hole Optimisation and Analysis>). 697 698The basic structure of this routine is to try to find the start- and/or 699end-points of where the pattern could match, and to ensure that the string 700is long enough to match the pattern. It tries to use more efficient 701methods over less efficient methods and may involve considerable 702cross-checking of constraints to find the place in the string that matches. 703For instance it may try to determine that a given fixed string must be 704not only present but a certain number of chars before the end of the 705string, or whatever. 706 707It calls several other routines, such as C<fbm_instr()> which does 708Fast Boyer Moore matching and C<find_byclass()> which is responsible for 709finding the start using the first mandatory regop in the program. 710 711When the optimisation criteria have been satisfied, C<reg_try()> is called 712to perform the match. 713 714=head3 Program execution 715 716C<pregexec()> is the main entry point for running a regex. It contains 717support for initialising the regex interpreter's state, running 718C<re_intuit_start()> if needed, and running the interpreter on the string 719from various start positions as needed. When it is necessary to use 720the regex interpreter C<pregexec()> calls C<regtry()>. 721 722C<regtry()> is the entry point into the regex interpreter. It expects 723as arguments a pointer to a C<regmatch_info> structure and a pointer to 724a string. It returns an integer 1 for success and a 0 for failure. 725It is basically a set-up wrapper around C<regmatch()>. 726 727C<regmatch> is the main "recursive loop" of the interpreter. It is 728basically a giant switch statement that implements a state machine, where 729the possible states are the regops themselves, plus a number of additional 730intermediate and failure states. A few of the states are implemented as 731subroutines but the bulk are inline code. 732 733=head1 MISCELLANEOUS 734 735=head2 Unicode and Localisation Support 736 737When dealing with strings containing characters that cannot be represented 738using an eight-bit character set, perl uses an internal representation 739that is a permissive version of Unicode's UTF-8 encoding[2]. This uses single 740bytes to represent characters from the ASCII character set, and sequences 741of two or more bytes for all other characters. (See L<perlunitut> 742for more information about the relationship between UTF-8 and perl's 743encoding, utf8. The difference isn't important for this discussion.) 744 745No matter how you look at it, Unicode support is going to be a pain in a 746regex engine. Tricks that might be fine when you have 256 possible 747characters often won't scale to handle the size of the UTF-8 character 748set. Things you can take for granted with ASCII may not be true with 749Unicode. For instance, in ASCII, it is safe to assume that 750C<sizeof(char1) == sizeof(char2)>, but in UTF-8 it isn't. Unicode case folding is 751vastly more complex than the simple rules of ASCII, and even when not 752using Unicode but only localised single byte encodings, things can get 753tricky (for example, B<LATIN SMALL LETTER SHARP S> (U+00DF, E<szlig>) 754should match 'SS' in localised case-insensitive matching). 755 756Making things worse is that UTF-8 support was a later addition to the 757regex engine (as it was to perl) and this necessarily made things a lot 758more complicated. Obviously it is easier to design a regex engine with 759Unicode support in mind from the beginning than it is to retrofit it to 760one that wasn't. 761 762Nearly all regops that involve looking at the input string have 763two cases, one for UTF-8, and one not. In fact, it's often more complex 764than that, as the pattern may be UTF-8 as well. 765 766Care must be taken when making changes to make sure that you handle 767UTF-8 properly, both at compile time and at execution time, including 768when the string and pattern are mismatched. 769 770=head2 Base Structures 771 772The C<regexp> structure described in L<perlreapi> is common to all 773regex engines. Two of its fields are intended for the private use 774of the regex engine that compiled the pattern. These are the 775C<intflags> and pprivate members. The C<pprivate> is a void pointer to 776an arbitrary structure whose use and management is the responsibility 777of the compiling engine. perl will never modify either of these 778values. In the case of the stock engine the structure pointed to by 779C<pprivate> is called C<regexp_internal>. 780 781Its C<pprivate> and C<intflags> fields contain data 782specific to each engine. 783 784There are two structures used to store a compiled regular expression. 785One, the C<regexp> structure described in L<perlreapi> is populated by 786the engine currently being. used and some of its fields read by perl to 787implement things such as the stringification of C<qr//>. 788 789 790The other structure is pointed to by the C<regexp> struct's 791C<pprivate> and is in addition to C<intflags> in the same struct 792considered to be the property of the regex engine which compiled the 793regular expression; 794 795The regexp structure contains all the data that perl needs to be aware of 796to properly work with the regular expression. It includes data about 797optimisations that perl can use to determine if the regex engine should 798really be used, and various other control info that is needed to properly 799execute patterns in various contexts such as is the pattern anchored in 800some way, or what flags were used during the compile, or whether the 801program contains special constructs that perl needs to be aware of. 802 803In addition it contains two fields that are intended for the private use 804of the regex engine that compiled the pattern. These are the C<intflags> 805and pprivate members. The C<pprivate> is a void pointer to an arbitrary 806structure whose use and management is the responsibility of the compiling 807engine. perl will never modify either of these values. 808 809As mentioned earlier, in the case of the default engines, the C<pprivate> 810will be a pointer to a regexp_internal structure which holds the compiled 811program and any additional data that is private to the regex engine 812implementation. 813 814=head3 Perl's C<pprivate> structure 815 816The following structure is used as the C<pprivate> struct by perl's 817regex engine. Since it is specific to perl it is only of curiosity 818value to other engine implementations. 819 820 typedef struct regexp_internal { 821 U32 *offsets; /* offset annotations 20001228 MJD 822 * data about mapping the program to 823 * the string*/ 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 833 * compiler. */ 834 } regexp_internal; 835 836=over 5 837 838=item C<offsets> 839 840Offsets holds a mapping of offset in the C<program> 841to offset in the C<precomp> string. This is only used by ActiveState's 842visual regex debugger. 843 844=item C<regstclass> 845 846Special regop that is used by C<re_intuit_start()> to check if a pattern 847can match at a certain position. For instance if the regex engine knows 848that the pattern must start with a 'Z' then it can scan the string until 849it finds one and then launch the regex engine from there. The routine 850that handles this is called C<find_by_class()>. Sometimes this field 851points at a regop embedded in the program, and sometimes it points at 852an independent synthetic regop that has been constructed by the optimiser. 853 854=item C<data> 855 856This field points at a C<reg_data> structure, which is defined as follows 857 858 struct reg_data { 859 U32 count; 860 U8 *what; 861 void* data[1]; 862 }; 863 864This structure is used for handling data structures that the regex engine 865needs to handle specially during a clone or free operation on the compiled 866product. Each element in the data array has a corresponding element in the 867what array. During compilation regops that need special structures stored 868will add an element to each array using the add_data() routine and then store 869the index in the regop. 870 871=item C<program> 872 873Compiled program. Inlined into the structure so the entire struct can be 874treated as a single blob. 875 876=back 877 878=head1 SEE ALSO 879 880L<perlreapi> 881 882L<perlre> 883 884L<perlunitut> 885 886=head1 AUTHOR 887 888by Yves Orton, 2006. 889 890With excerpts from Perl, and contributions and suggestions from 891Ronald J. Kimball, Dave Mitchell, Dominic Dunlop, Mark Jason Dominus, 892Stephen McCamant, and David Landgren. 893 894=head1 LICENCE 895 896Same terms as Perl. 897 898=head1 REFERENCES 899 900[1] L<http://perl.plover.com/Rx/paper/> 901 902[2] L<http://www.unicode.org> 903 904=cut 905