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 171Character classes are represented by C<regnode_charclass> structures, 172which have a four-byte argument and then a 32-byte (256-bit) bitmap 173indicating which characters are included in the class. 174 175 regnode_charclass U32 arg1; 176 char bitmap[ANYOF_BITMAP_SIZE]; 177 178=item C<regnode_charclass_class> 179 180There is also a larger form of a char class structure used to represent 181POSIX char classes called C<regnode_charclass_class> which has an 182additional 4-byte (32-bit) bitmap indicating which POSIX char classes 183have been included. 184 185 regnode_charclass_class U32 arg1; 186 char bitmap[ANYOF_BITMAP_SIZE]; 187 char classflags[ANYOF_CLASSBITMAP_SIZE]; 188 189=back 190 191F<regnodes.h> defines an array called C<regarglen[]> which gives the size 192of each opcode in units of C<size regnode> (4-byte). A macro is used 193to calculate the size of an C<EXACT> node based on its C<str_len> field. 194 195The regops are defined in F<regnodes.h> which is generated from 196F<regcomp.sym> by F<regcomp.pl>. Currently the maximum possible number 197of distinct regops is restricted to 256, with about a quarter already 198used. 199 200A set of macros makes accessing the fields 201easier and more consistent. These include C<OP()>, which is used to determine 202the type of a C<regnode>-like structure; C<NEXT_OFF()>, which is the offset to 203the next node (more on this later); C<ARG()>, C<ARG1()>, C<ARG2()>, C<ARG_SET()>, 204and equivalents for reading and setting the arguments; and C<STR_LEN()>, 205C<STRING()> and C<OPERAND()> for manipulating strings and regop bearing 206types. 207 208=head3 What regop is next? 209 210There are three distinct concepts of "next" in the regex engine, and 211it is important to keep them clear. 212 213=over 4 214 215=item * 216 217There is the "next regnode" from a given regnode, a value which is 218rarely useful except that sometimes it matches up in terms of value 219with one of the others, and that sometimes the code assumes this to 220always be so. 221 222=item * 223 224There is the "next regop" from a given regop/regnode. This is the 225regop physically located after the current one, as determined by 226the size of the current regop. This is often useful, such as when 227dumping the structure we use this order to traverse. Sometimes the code 228assumes that the "next regnode" is the same as the "next regop", or in 229other words assumes that the sizeof a given regop type is always going 230to be one regnode large. 231 232=item * 233 234There is the "regnext" from a given regop. This is the regop which 235is reached by jumping forward by the value of C<NEXT_OFF()>, 236or in a few cases for longer jumps by the C<arg1> field of the C<regnode_1> 237structure. The subroutine C<regnext()> handles this transparently. 238This is the logical successor of the node, which in some cases, like 239that of the C<BRANCH> regop, has special meaning. 240 241=back 242 243=head1 Process Overview 244 245Broadly speaking, performing a match of a string against a pattern 246involves the following steps: 247 248=over 5 249 250=item A. Compilation 251 252=over 5 253 254=item 1. Parsing for size 255 256=item 2. Parsing for construction 257 258=item 3. Peep-hole optimisation and analysis 259 260=back 261 262=item B. Execution 263 264=over 5 265 266=item 4. Start position and no-match optimisations 267 268=item 5. Program execution 269 270=back 271 272=back 273 274 275Where these steps occur in the actual execution of a perl program is 276determined by whether the pattern involves interpolating any string 277variables. If interpolation occurs, then compilation happens at run time. If it 278does not, then compilation is performed at compile time. (The C</o> modifier changes this, 279as does C<qr//> to a certain extent.) The engine doesn't really care that 280much. 281 282=head2 Compilation 283 284This code resides primarily in F<regcomp.c>, along with the header files 285F<regcomp.h>, F<regexp.h> and F<regnodes.h>. 286 287Compilation starts with C<pregcomp()>, which is mostly an initialisation 288wrapper which farms work out to two other routines for the heavy lifting: the 289first is C<reg()>, which is the start point for parsing; the second, 290C<study_chunk()>, is responsible for optimisation. 291 292Initialisation in C<pregcomp()> mostly involves the creation and data-filling 293of a special structure, C<RExC_state_t> (defined in F<regcomp.c>). 294Almost all internally-used routines in F<regcomp.h> take a pointer to one 295of these structures as their first argument, with the name C<pRExC_state>. 296This structure is used to store the compilation state and contains many 297fields. Likewise there are many macros which operate on this 298variable: anything that looks like C<RExC_xxxx> is a macro that operates on 299this pointer/structure. 300 301=head3 Parsing for size 302 303In this pass the input pattern is parsed in order to calculate how much 304space is needed for each regop we would need to emit. The size is also 305used to determine whether long jumps will be required in the program. 306 307This stage is controlled by the macro C<SIZE_ONLY> being set. 308 309The parse proceeds pretty much exactly as it does during the 310construction phase, except that most routines are short-circuited to 311change the size field C<RExC_size> and not do anything else. 312 313=head3 Parsing for construction 314 315Once the size of the program has been determined, the pattern is parsed 316again, but this time for real. Now C<SIZE_ONLY> will be false, and the 317actual construction can occur. 318 319C<reg()> is the start of the parse process. It is responsible for 320parsing an arbitrary chunk of pattern up to either the end of the 321string, or the first closing parenthesis it encounters in the pattern. 322This means it can be used to parse the top-level regex, or any section 323inside of a grouping parenthesis. It also handles the "special parens" 324that perl's regexes have. For instance when parsing C</x(?:foo)y/> C<reg()> 325will at one point be called to parse from the "?" symbol up to and 326including the ")". 327 328Additionally, C<reg()> is responsible for parsing the one or more 329branches from the pattern, and for "finishing them off" by correctly 330setting their next pointers. In order to do the parsing, it repeatedly 331calls out to C<regbranch()>, which is responsible for handling up to the 332first C<|> symbol it sees. 333 334C<regbranch()> in turn calls C<regpiece()> which 335handles "things" followed by a quantifier. In order to parse the 336"things", C<regatom()> is called. This is the lowest level routine, which 337parses out constant strings, character classes, and the 338various special symbols like C<$>. If C<regatom()> encounters a "(" 339character it in turn calls C<reg()>. 340 341The routine C<regtail()> is called by both C<reg()> and C<regbranch()> 342in order to "set the tail pointer" correctly. When executing and 343we get to the end of a branch, we need to go to the node following the 344grouping parens. When parsing, however, we don't know where the end will 345be until we get there, so when we do we must go back and update the 346offsets as appropriate. C<regtail> is used to make this easier. 347 348A subtlety of the parsing process means that a regex like C</foo/> is 349originally parsed into an alternation with a single branch. It is only 350afterwards that the optimiser converts single branch alternations into the 351simpler form. 352 353=head3 Parse Call Graph and a Grammar 354 355The call graph looks like this: 356 357 reg() # parse a top level regex, or inside of 358 # parens 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 364 # subpattern 365 .... 366 ... 367 regtail() # finish off the branch 368 ... 369 regtail() # finish off the branch sequence. Tie each 370 # branch's tail to the tail of the 371 # sequence 372 # (NEW) In Debug mode this is 373 # regtail_study(). 374 375A grammar form might be something like this: 376 377 atom : constant | class 378 quant : '*' | '+' | '?' | '{min,max}' 379 _branch: piece 380 | piece _branch 381 | nothing 382 branch: _branch 383 | _branch '|' branch 384 group : '(' branch ')' 385 _piece: atom | group 386 piece : _piece 387 | _piece quant 388 389=head3 Parsing complications 390 391The implication of the above description is that a pattern containing nested 392parentheses will result in a call graph which cycles through C<reg()>, 393C<regbranch()>, C<regpiece()>, C<regatom()>, C<reg()>, C<regbranch()> I<etc> 394multiple times, until the deepest level of nesting is reached. All the above 395routines return a pointer to a C<regnode>, which is usually the last regnode 396added to the program. However, one complication is that reg() returns NULL 397for parsing C<(?:)> syntax for embedded modifiers, setting the flag 398C<TRYAGAIN>. The C<TRYAGAIN> propagates upwards until it is captured, in 399some cases by by C<regatom()>, but otherwise unconditionally by 400C<regbranch()>. Hence it will never be returned by C<regbranch()> to 401C<reg()>. This flag permits patterns such as C<(?i)+> to be detected as 402errors (I<Quantifier follows nothing in regex; marked by <-- HERE in m/(?i)+ 403<-- HERE />). 404 405Another complication is that the representation used for the program differs 406if it needs to store Unicode, but it's not always possible to know for sure 407whether it does until midway through parsing. The Unicode representation for 408the program is larger, and cannot be matched as efficiently. (See L</Unicode 409and Localisation Support> below for more details as to why.) If the pattern 410contains literal Unicode, it's obvious that the program needs to store 411Unicode. Otherwise, the parser optimistically assumes that the more 412efficient representation can be used, and starts sizing on this basis. 413However, if it then encounters something in the pattern which must be stored 414as Unicode, such as an C<\x{...}> escape sequence representing a character 415literal, then this means that all previously calculated sizes need to be 416redone, using values appropriate for the Unicode representation. Currently, 417all regular expression constructions which can trigger this are parsed by code 418in C<regatom()>. 419 420To avoid wasted work when a restart is needed, the sizing pass is abandoned 421- C<regatom()> immediately returns NULL, setting the flag C<RESTART_UTF8>. 422(This action is encapsulated using the macro C<REQUIRE_UTF8>.) This restart 423request is propagated up the call chain in a similar fashion, until it is 424"caught" in C<Perl_re_op_compile()>, which marks the pattern as containing 425Unicode, and restarts the sizing pass. It is also possible for constructions 426within run-time code blocks to turn out to need Unicode representation., 427which is signalled by C<S_compile_runtime_code()> returning false to 428C<Perl_re_op_compile()>. 429 430The restart was previously implemented using a C<longjmp> in C<regatom()> 431back to a C<setjmp> in C<Perl_re_op_compile()>, but this proved to be 432problematic as the latter is a large function containing many automatic 433variables, which interact badly with the emergent control flow of C<setjmp>. 434 435=head3 Debug Output 436 437In the 5.9.x development version of perl you can C<< use re Debug => 'PARSE' >> 438to see some trace information about the parse process. We will start with some 439simple patterns and build up to more complex patterns. 440 441So when we parse C</foo/> we see something like the following table. The 442left shows what is being parsed, and the number indicates where the next regop 443would go. The stuff on the right is the trace output of the graph. The 444names are chosen to be short to make it less dense on the screen. 'tsdy' 445is a special form of C<regtail()> which does some extra analysis. 446 447 >foo< 1 reg 448 brnc 449 piec 450 atom 451 >< 4 tsdy~ EXACT <foo> (EXACT) (1) 452 ~ attach to END (3) offset to 2 453 454The resulting program then looks like: 455 456 1: EXACT <foo>(3) 457 3: END(0) 458 459As you can see, even though we parsed out a branch and a piece, it was ultimately 460only an atom. The final program shows us how things work. We have an C<EXACT> regop, 461followed by an C<END> regop. The number in parens indicates where the C<regnext> of 462the node goes. The C<regnext> of an C<END> regop is unused, as C<END> regops mean 463we have successfully matched. The number on the left indicates the position of 464the regop in the regnode array. 465 466Now let's try a harder pattern. We will add a quantifier, so now we have the pattern 467C</foo+/>. We will see that C<regbranch()> calls C<regpiece()> twice. 468 469 >foo+< 1 reg 470 brnc 471 piec 472 atom 473 >o+< 3 piec 474 atom 475 >< 6 tail~ EXACT <fo> (1) 476 7 tsdy~ EXACT <fo> (EXACT) (1) 477 ~ PLUS (END) (3) 478 ~ attach to END (6) offset to 3 479 480And we end up with the program: 481 482 1: EXACT <fo>(3) 483 3: PLUS(6) 484 4: EXACT <o>(0) 485 6: END(0) 486 487Now we have a special case. The C<EXACT> regop has a C<regnext> of 0. This is 488because if it matches it should try to match itself again. The C<PLUS> regop 489handles the actual failure of the C<EXACT> regop and acts appropriately (going 490to regnode 6 if the C<EXACT> matched at least once, or failing if it didn't). 491 492Now for something much more complex: C</x(?:foo*|b[a][rR])(foo|bar)$/> 493 494 >x(?:foo*|b... 1 reg 495 brnc 496 piec 497 atom 498 >(?:foo*|b[... 3 piec 499 atom 500 >?:foo*|b[a... reg 501 >foo*|b[a][... brnc 502 piec 503 atom 504 >o*|b[a][rR... 5 piec 505 atom 506 >|b[a][rR])... 8 tail~ EXACT <fo> (3) 507 >b[a][rR])(... 9 brnc 508 10 piec 509 atom 510 >[a][rR])(f... 12 piec 511 atom 512 >a][rR])(fo... clas 513 >[rR])(foo|... 14 tail~ EXACT <b> (10) 514 piec 515 atom 516 >rR])(foo|b... clas 517 >)(foo|bar)... 25 tail~ EXACT <a> (12) 518 tail~ BRANCH (3) 519 26 tsdy~ BRANCH (END) (9) 520 ~ attach to TAIL (25) offset to 16 521 tsdy~ EXACT <fo> (EXACT) (4) 522 ~ STAR (END) (6) 523 ~ attach to TAIL (25) offset to 19 524 tsdy~ EXACT <b> (EXACT) (10) 525 ~ EXACT <a> (EXACT) (12) 526 ~ ANYOF[Rr] (END) (14) 527 ~ attach to TAIL (25) offset to 11 528 >(foo|bar)$< tail~ EXACT <x> (1) 529 piec 530 atom 531 >foo|bar)$< reg 532 28 brnc 533 piec 534 atom 535 >|bar)$< 31 tail~ OPEN1 (26) 536 >bar)$< brnc 537 32 piec 538 atom 539 >)$< 34 tail~ BRANCH (28) 540 36 tsdy~ BRANCH (END) (31) 541 ~ attach to CLOSE1 (34) offset to 3 542 tsdy~ EXACT <foo> (EXACT) (29) 543 ~ attach to CLOSE1 (34) offset to 5 544 tsdy~ EXACT <bar> (EXACT) (32) 545 ~ attach to CLOSE1 (34) offset to 2 546 >$< tail~ BRANCH (3) 547 ~ BRANCH (9) 548 ~ TAIL (25) 549 piec 550 atom 551 >< 37 tail~ OPEN1 (26) 552 ~ BRANCH (28) 553 ~ BRANCH (31) 554 ~ CLOSE1 (34) 555 38 tsdy~ EXACT <x> (EXACT) (1) 556 ~ BRANCH (END) (3) 557 ~ BRANCH (END) (9) 558 ~ TAIL (END) (25) 559 ~ OPEN1 (END) (26) 560 ~ BRANCH (END) (28) 561 ~ BRANCH (END) (31) 562 ~ CLOSE1 (END) (34) 563 ~ EOL (END) (36) 564 ~ attach to END (37) offset to 1 565 566Resulting in the program 567 568 1: EXACT <x>(3) 569 3: BRANCH(9) 570 4: EXACT <fo>(6) 571 6: STAR(26) 572 7: EXACT <o>(0) 573 9: BRANCH(25) 574 10: EXACT <ba>(14) 575 12: OPTIMIZED (2 nodes) 576 14: ANYOF[Rr](26) 577 25: TAIL(26) 578 26: OPEN1(28) 579 28: TRIE-EXACT(34) 580 [StS:1 Wds:2 Cs:6 Uq:5 #Sts:7 Mn:3 Mx:3 Stcls:bf] 581 <foo> 582 <bar> 583 30: OPTIMIZED (4 nodes) 584 34: CLOSE1(36) 585 36: EOL(37) 586 37: END(0) 587 588Here we can see a much more complex program, with various optimisations in 589play. At regnode 10 we see an example where a character class with only 590one character in it was turned into an C<EXACT> node. We can also see where 591an entire alternation was turned into a C<TRIE-EXACT> node. As a consequence, 592some of the regnodes have been marked as optimised away. We can see that 593the C<$> symbol has been converted into an C<EOL> regop, a special piece of 594code that looks for C<\n> or the end of the string. 595 596The next pointer for C<BRANCH>es is interesting in that it points at where 597execution should go if the branch fails. When executing, if the engine 598tries to traverse from a branch to a C<regnext> that isn't a branch then 599the engine will know that the entire set of branches has failed. 600 601=head3 Peep-hole Optimisation and Analysis 602 603The regular expression engine can be a weighty tool to wield. On long 604strings and complex patterns it can end up having to do a lot of work 605to find a match, and even more to decide that no match is possible. 606Consider a situation like the following pattern. 607 608 'ababababababababababab' =~ /(a|b)*z/ 609 610The C<(a|b)*> part can match at every char in the string, and then fail 611every time because there is no C<z> in the string. So obviously we can 612avoid using the regex engine unless there is a C<z> in the string. 613Likewise in a pattern like: 614 615 /foo(\w+)bar/ 616 617In this case we know that the string must contain a C<foo> which must be 618followed by C<bar>. We can use Fast Boyer-Moore matching as implemented 619in C<fbm_instr()> to find the location of these strings. If they don't exist 620then we don't need to resort to the much more expensive regex engine. 621Even better, if they do exist then we can use their positions to 622reduce the search space that the regex engine needs to cover to determine 623if the entire pattern matches. 624 625There are various aspects of the pattern that can be used to facilitate 626optimisations along these lines: 627 628=over 5 629 630=item * anchored fixed strings 631 632=item * floating fixed strings 633 634=item * minimum and maximum length requirements 635 636=item * start class 637 638=item * Beginning/End of line positions 639 640=back 641 642Another form of optimisation that can occur is the post-parse "peep-hole" 643optimisation, where inefficient constructs are replaced by more efficient 644constructs. The C<TAIL> regops which are used during parsing to mark the end 645of branches and the end of groups are examples of this. These regops are used 646as place-holders during construction and "always match" so they can be 647"optimised away" by making the things that point to the C<TAIL> point to the 648thing that C<TAIL> points to, thus "skipping" the node. 649 650Another optimisation that can occur is that of "C<EXACT> merging" which is 651where two consecutive C<EXACT> nodes are merged into a single 652regop. An even more aggressive form of this is that a branch 653sequence of the form C<EXACT BRANCH ... EXACT> can be converted into a 654C<TRIE-EXACT> regop. 655 656All of this occurs in the routine C<study_chunk()> which uses a special 657structure C<scan_data_t> to store the analysis that it has performed, and 658does the "peep-hole" optimisations as it goes. 659 660The code involved in C<study_chunk()> is extremely cryptic. Be careful. :-) 661 662=head2 Execution 663 664Execution of a regex generally involves two phases, the first being 665finding the start point in the string where we should match from, 666and the second being running the regop interpreter. 667 668If we can tell that there is no valid start point then we don't bother running 669interpreter at all. Likewise, if we know from the analysis phase that we 670cannot detect a short-cut to the start position, we go straight to the 671interpreter. 672 673The two entry points are C<re_intuit_start()> and C<pregexec()>. These routines 674have a somewhat incestuous relationship with overlap between their functions, 675and C<pregexec()> may even call C<re_intuit_start()> on its own. Nevertheless 676other parts of the perl source code may call into either, or both. 677 678Execution of the interpreter itself used to be recursive, but thanks to the 679efforts of Dave Mitchell in the 5.9.x development track, that has changed: now an 680internal stack is maintained on the heap and the routine is fully 681iterative. This can make it tricky as the code is quite conservative 682about what state it stores, with the result that two consecutive lines in the 683code can actually be running in totally different contexts due to the 684simulated recursion. 685 686=head3 Start position and no-match optimisations 687 688C<re_intuit_start()> is responsible for handling start points and no-match 689optimisations as determined by the results of the analysis done by 690C<study_chunk()> (and described in L<Peep-hole Optimisation and Analysis>). 691 692The basic structure of this routine is to try to find the start- and/or 693end-points of where the pattern could match, and to ensure that the string 694is long enough to match the pattern. It tries to use more efficient 695methods over less efficient methods and may involve considerable 696cross-checking of constraints to find the place in the string that matches. 697For instance it may try to determine that a given fixed string must be 698not only present but a certain number of chars before the end of the 699string, or whatever. 700 701It calls several other routines, such as C<fbm_instr()> which does 702Fast Boyer Moore matching and C<find_byclass()> which is responsible for 703finding the start using the first mandatory regop in the program. 704 705When the optimisation criteria have been satisfied, C<reg_try()> is called 706to perform the match. 707 708=head3 Program execution 709 710C<pregexec()> is the main entry point for running a regex. It contains 711support for initialising the regex interpreter's state, running 712C<re_intuit_start()> if needed, and running the interpreter on the string 713from various start positions as needed. When it is necessary to use 714the regex interpreter C<pregexec()> calls C<regtry()>. 715 716C<regtry()> is the entry point into the regex interpreter. It expects 717as arguments a pointer to a C<regmatch_info> structure and a pointer to 718a string. It returns an integer 1 for success and a 0 for failure. 719It is basically a set-up wrapper around C<regmatch()>. 720 721C<regmatch> is the main "recursive loop" of the interpreter. It is 722basically a giant switch statement that implements a state machine, where 723the possible states are the regops themselves, plus a number of additional 724intermediate and failure states. A few of the states are implemented as 725subroutines but the bulk are inline code. 726 727=head1 MISCELLANEOUS 728 729=head2 Unicode and Localisation Support 730 731When dealing with strings containing characters that cannot be represented 732using an eight-bit character set, perl uses an internal representation 733that is a permissive version of Unicode's UTF-8 encoding[2]. This uses single 734bytes to represent characters from the ASCII character set, and sequences 735of two or more bytes for all other characters. (See L<perlunitut> 736for more information about the relationship between UTF-8 and perl's 737encoding, utf8. The difference isn't important for this discussion.) 738 739No matter how you look at it, Unicode support is going to be a pain in a 740regex engine. Tricks that might be fine when you have 256 possible 741characters often won't scale to handle the size of the UTF-8 character 742set. Things you can take for granted with ASCII may not be true with 743Unicode. For instance, in ASCII, it is safe to assume that 744C<sizeof(char1) == sizeof(char2)>, but in UTF-8 it isn't. Unicode case folding is 745vastly more complex than the simple rules of ASCII, and even when not 746using Unicode but only localised single byte encodings, things can get 747tricky (for example, B<LATIN SMALL LETTER SHARP S> (U+00DF, E<szlig>) 748should match 'SS' in localised case-insensitive matching). 749 750Making things worse is that UTF-8 support was a later addition to the 751regex engine (as it was to perl) and this necessarily made things a lot 752more complicated. Obviously it is easier to design a regex engine with 753Unicode support in mind from the beginning than it is to retrofit it to 754one that wasn't. 755 756Nearly all regops that involve looking at the input string have 757two cases, one for UTF-8, and one not. In fact, it's often more complex 758than that, as the pattern may be UTF-8 as well. 759 760Care must be taken when making changes to make sure that you handle 761UTF-8 properly, both at compile time and at execution time, including 762when the string and pattern are mismatched. 763 764The following comment in F<regcomp.h> gives an example of exactly how 765tricky this can be: 766 767 Two problematic code points in Unicode casefolding of EXACT nodes: 768 769 U+0390 - GREEK SMALL LETTER IOTA WITH DIALYTIKA AND TONOS 770 U+03B0 - GREEK SMALL LETTER UPSILON WITH DIALYTIKA AND TONOS 771 772 which casefold to 773 774 Unicode UTF-8 775 776 U+03B9 U+0308 U+0301 0xCE 0xB9 0xCC 0x88 0xCC 0x81 777 U+03C5 U+0308 U+0301 0xCF 0x85 0xCC 0x88 0xCC 0x81 778 779 This means that in case-insensitive matching (or "loose matching", 780 as Unicode calls it), an EXACTF of length six (the UTF-8 encoded 781 byte length of the above casefolded versions) can match a target 782 string of length two (the byte length of UTF-8 encoded U+0390 or 783 U+03B0). This would rather mess up the minimum length computation. 784 785 What we'll do is to look for the tail four bytes, and then peek 786 at the preceding two bytes to see whether we need to decrease 787 the minimum length by four (six minus two). 788 789 Thanks to the design of UTF-8, there cannot be false matches: 790 A sequence of valid UTF-8 bytes cannot be a subsequence of 791 another valid sequence of UTF-8 bytes. 792 793 794=head2 Base Structures 795 796The C<regexp> structure described in L<perlreapi> is common to all 797regex engines. Two of its fields that are intended for the private use 798of the regex engine that compiled the pattern. These are the 799C<intflags> and pprivate members. The C<pprivate> is a void pointer to 800an arbitrary structure whose use and management is the responsibility 801of the compiling engine. perl will never modify either of these 802values. In the case of the stock engine the structure pointed to by 803C<pprivate> is called C<regexp_internal>. 804 805Its C<pprivate> and C<intflags> fields contain data 806specific to each engine. 807 808There are two structures used to store a compiled regular expression. 809One, the C<regexp> structure described in L<perlreapi> is populated by 810the engine currently being. used and some of its fields read by perl to 811implement things such as the stringification of C<qr//>. 812 813 814The other structure is pointed to be the C<regexp> struct's 815C<pprivate> and is in addition to C<intflags> in the same struct 816considered to be the property of the regex engine which compiled the 817regular expression; 818 819The regexp structure contains all the data that perl needs to be aware of 820to properly work with the regular expression. It includes data about 821optimisations that perl can use to determine if the regex engine should 822really be used, and various other control info that is needed to properly 823execute patterns in various contexts such as is the pattern anchored in 824some way, or what flags were used during the compile, or whether the 825program contains special constructs that perl needs to be aware of. 826 827In addition it contains two fields that are intended for the private use 828of the regex engine that compiled the pattern. These are the C<intflags> 829and pprivate members. The C<pprivate> is a void pointer to an arbitrary 830structure whose use and management is the responsibility of the compiling 831engine. perl will never modify either of these values. 832 833As mentioned earlier, in the case of the default engines, the C<pprivate> 834will be a pointer to a regexp_internal structure which holds the compiled 835program and any additional data that is private to the regex engine 836implementation. 837 838=head3 Perl's C<pprivate> structure 839 840The following structure is used as the C<pprivate> struct by perl's 841regex engine. Since it is specific to perl it is only of curiosity 842value to other engine implementations. 843 844 typedef struct regexp_internal { 845 U32 *offsets; /* offset annotations 20001228 MJD 846 * data about mapping the program to 847 * the string*/ 848 regnode *regstclass; /* Optional startclass as identified or 849 * constructed by the optimiser */ 850 struct reg_data *data; /* Additional miscellaneous data used 851 * by the program. Used to make it 852 * easier to clone and free arbitrary 853 * data that the regops need. Often the 854 * ARG field of a regop is an index 855 * into this structure */ 856 regnode program[1]; /* Unwarranted chumminess with 857 * compiler. */ 858 } regexp_internal; 859 860=over 5 861 862=item C<offsets> 863 864Offsets holds a mapping of offset in the C<program> 865to offset in the C<precomp> string. This is only used by ActiveState's 866visual regex debugger. 867 868=item C<regstclass> 869 870Special regop that is used by C<re_intuit_start()> to check if a pattern 871can match at a certain position. For instance if the regex engine knows 872that the pattern must start with a 'Z' then it can scan the string until 873it finds one and then launch the regex engine from there. The routine 874that handles this is called C<find_by_class()>. Sometimes this field 875points at a regop embedded in the program, and sometimes it points at 876an independent synthetic regop that has been constructed by the optimiser. 877 878=item C<data> 879 880This field points at a reg_data structure, which is defined as follows 881 882 struct reg_data { 883 U32 count; 884 U8 *what; 885 void* data[1]; 886 }; 887 888This structure is used for handling data structures that the regex engine 889needs to handle specially during a clone or free operation on the compiled 890product. Each element in the data array has a corresponding element in the 891what array. During compilation regops that need special structures stored 892will add an element to each array using the add_data() routine and then store 893the index in the regop. 894 895=item C<program> 896 897Compiled program. Inlined into the structure so the entire struct can be 898treated as a single blob. 899 900=back 901 902=head1 SEE ALSO 903 904L<perlreapi> 905 906L<perlre> 907 908L<perlunitut> 909 910=head1 AUTHOR 911 912by Yves Orton, 2006. 913 914With excerpts from Perl, and contributions and suggestions from 915Ronald J. Kimball, Dave Mitchell, Dominic Dunlop, Mark Jason Dominus, 916Stephen McCamant, and David Landgren. 917 918=head1 LICENCE 919 920Same terms as Perl. 921 922=head1 REFERENCES 923 924[1] L<http://perl.plover.com/Rx/paper/> 925 926[2] L<http://www.unicode.org> 927 928=cut 929