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 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 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 parens 358 regbranch() # parse a single branch of an alternation 359 regpiece() # parse a pattern followed by a quantifier 360 regatom() # parse a simple pattern 361 regclass() # used to handle a class 362 reg() # used to handle a parenthesised subpattern 363 .... 364 ... 365 regtail() # finish off the branch 366 ... 367 regtail() # finish off the branch sequence. Tie each 368 # branch's tail to the tail of the sequence 369 # (NEW) In Debug mode this is 370 # regtail_study(). 371 372A grammar form might be something like this: 373 374 atom : constant | class 375 quant : '*' | '+' | '?' | '{min,max}' 376 _branch: piece 377 | piece _branch 378 | nothing 379 branch: _branch 380 | _branch '|' branch 381 group : '(' branch ')' 382 _piece: atom | group 383 piece : _piece 384 | _piece quant 385 386=head3 Debug Output 387 388In the 5.9.x development version of perl you can C<< use re Debug => 'PARSE' >> 389to see some trace information about the parse process. We will start with some 390simple patterns and build up to more complex patterns. 391 392So when we parse C</foo/> we see something like the following table. The 393left shows what is being parsed, and the number indicates where the next regop 394would go. The stuff on the right is the trace output of the graph. The 395names are chosen to be short to make it less dense on the screen. 'tsdy' 396is a special form of C<regtail()> which does some extra analysis. 397 398 >foo< 1 reg 399 brnc 400 piec 401 atom 402 >< 4 tsdy~ EXACT <foo> (EXACT) (1) 403 ~ attach to END (3) offset to 2 404 405The resulting program then looks like: 406 407 1: EXACT <foo>(3) 408 3: END(0) 409 410As you can see, even though we parsed out a branch and a piece, it was ultimately 411only an atom. The final program shows us how things work. We have an C<EXACT> regop, 412followed by an C<END> regop. The number in parens indicates where the C<regnext> of 413the node goes. The C<regnext> of an C<END> regop is unused, as C<END> regops mean 414we have successfully matched. The number on the left indicates the position of 415the regop in the regnode array. 416 417Now let's try a harder pattern. We will add a quantifier, so now we have the pattern 418C</foo+/>. We will see that C<regbranch()> calls C<regpiece()> twice. 419 420 >foo+< 1 reg 421 brnc 422 piec 423 atom 424 >o+< 3 piec 425 atom 426 >< 6 tail~ EXACT <fo> (1) 427 7 tsdy~ EXACT <fo> (EXACT) (1) 428 ~ PLUS (END) (3) 429 ~ attach to END (6) offset to 3 430 431And we end up with the program: 432 433 1: EXACT <fo>(3) 434 3: PLUS(6) 435 4: EXACT <o>(0) 436 6: END(0) 437 438Now we have a special case. The C<EXACT> regop has a C<regnext> of 0. This is 439because if it matches it should try to match itself again. The C<PLUS> regop 440handles the actual failure of the C<EXACT> regop and acts appropriately (going 441to regnode 6 if the C<EXACT> matched at least once, or failing if it didn't). 442 443Now for something much more complex: C</x(?:foo*|b[a][rR])(foo|bar)$/> 444 445 >x(?:foo*|b... 1 reg 446 brnc 447 piec 448 atom 449 >(?:foo*|b[... 3 piec 450 atom 451 >?:foo*|b[a... reg 452 >foo*|b[a][... brnc 453 piec 454 atom 455 >o*|b[a][rR... 5 piec 456 atom 457 >|b[a][rR])... 8 tail~ EXACT <fo> (3) 458 >b[a][rR])(... 9 brnc 459 10 piec 460 atom 461 >[a][rR])(f... 12 piec 462 atom 463 >a][rR])(fo... clas 464 >[rR])(foo|... 14 tail~ EXACT <b> (10) 465 piec 466 atom 467 >rR])(foo|b... clas 468 >)(foo|bar)... 25 tail~ EXACT <a> (12) 469 tail~ BRANCH (3) 470 26 tsdy~ BRANCH (END) (9) 471 ~ attach to TAIL (25) offset to 16 472 tsdy~ EXACT <fo> (EXACT) (4) 473 ~ STAR (END) (6) 474 ~ attach to TAIL (25) offset to 19 475 tsdy~ EXACT <b> (EXACT) (10) 476 ~ EXACT <a> (EXACT) (12) 477 ~ ANYOF[Rr] (END) (14) 478 ~ attach to TAIL (25) offset to 11 479 >(foo|bar)$< tail~ EXACT <x> (1) 480 piec 481 atom 482 >foo|bar)$< reg 483 28 brnc 484 piec 485 atom 486 >|bar)$< 31 tail~ OPEN1 (26) 487 >bar)$< brnc 488 32 piec 489 atom 490 >)$< 34 tail~ BRANCH (28) 491 36 tsdy~ BRANCH (END) (31) 492 ~ attach to CLOSE1 (34) offset to 3 493 tsdy~ EXACT <foo> (EXACT) (29) 494 ~ attach to CLOSE1 (34) offset to 5 495 tsdy~ EXACT <bar> (EXACT) (32) 496 ~ attach to CLOSE1 (34) offset to 2 497 >$< tail~ BRANCH (3) 498 ~ BRANCH (9) 499 ~ TAIL (25) 500 piec 501 atom 502 >< 37 tail~ OPEN1 (26) 503 ~ BRANCH (28) 504 ~ BRANCH (31) 505 ~ CLOSE1 (34) 506 38 tsdy~ EXACT <x> (EXACT) (1) 507 ~ BRANCH (END) (3) 508 ~ BRANCH (END) (9) 509 ~ TAIL (END) (25) 510 ~ OPEN1 (END) (26) 511 ~ BRANCH (END) (28) 512 ~ BRANCH (END) (31) 513 ~ CLOSE1 (END) (34) 514 ~ EOL (END) (36) 515 ~ attach to END (37) offset to 1 516 517Resulting in the program 518 519 1: EXACT <x>(3) 520 3: BRANCH(9) 521 4: EXACT <fo>(6) 522 6: STAR(26) 523 7: EXACT <o>(0) 524 9: BRANCH(25) 525 10: EXACT <ba>(14) 526 12: OPTIMIZED (2 nodes) 527 14: ANYOF[Rr](26) 528 25: TAIL(26) 529 26: OPEN1(28) 530 28: TRIE-EXACT(34) 531 [StS:1 Wds:2 Cs:6 Uq:5 #Sts:7 Mn:3 Mx:3 Stcls:bf] 532 <foo> 533 <bar> 534 30: OPTIMIZED (4 nodes) 535 34: CLOSE1(36) 536 36: EOL(37) 537 37: END(0) 538 539Here we can see a much more complex program, with various optimisations in 540play. At regnode 10 we see an example where a character class with only 541one character in it was turned into an C<EXACT> node. We can also see where 542an entire alternation was turned into a C<TRIE-EXACT> node. As a consequence, 543some of the regnodes have been marked as optimised away. We can see that 544the C<$> symbol has been converted into an C<EOL> regop, a special piece of 545code that looks for C<\n> or the end of the string. 546 547The next pointer for C<BRANCH>es is interesting in that it points at where 548execution should go if the branch fails. When executing, if the engine 549tries to traverse from a branch to a C<regnext> that isn't a branch then 550the engine will know that the entire set of branches has failed. 551 552=head3 Peep-hole Optimisation and Analysis 553 554The regular expression engine can be a weighty tool to wield. On long 555strings and complex patterns it can end up having to do a lot of work 556to find a match, and even more to decide that no match is possible. 557Consider a situation like the following pattern. 558 559 'ababababababababababab' =~ /(a|b)*z/ 560 561The C<(a|b)*> part can match at every char in the string, and then fail 562every time because there is no C<z> in the string. So obviously we can 563avoid using the regex engine unless there is a C<z> in the string. 564Likewise in a pattern like: 565 566 /foo(\w+)bar/ 567 568In this case we know that the string must contain a C<foo> which must be 569followed by C<bar>. We can use Fast Boyer-Moore matching as implemented 570in C<fbm_instr()> to find the location of these strings. If they don't exist 571then we don't need to resort to the much more expensive regex engine. 572Even better, if they do exist then we can use their positions to 573reduce the search space that the regex engine needs to cover to determine 574if the entire pattern matches. 575 576There are various aspects of the pattern that can be used to facilitate 577optimisations along these lines: 578 579=over 5 580 581=item * anchored fixed strings 582 583=item * floating fixed strings 584 585=item * minimum and maximum length requirements 586 587=item * start class 588 589=item * Beginning/End of line positions 590 591=back 592 593Another form of optimisation that can occur is the post-parse "peep-hole" 594optimisation, where inefficient constructs are replaced by more efficient 595constructs. The C<TAIL> regops which are used during parsing to mark the end 596of branches and the end of groups are examples of this. These regops are used 597as place-holders during construction and "always match" so they can be 598"optimised away" by making the things that point to the C<TAIL> point to the 599thing that C<TAIL> points to, thus "skipping" the node. 600 601Another optimisation that can occur is that of "C<EXACT> merging" which is 602where two consecutive C<EXACT> nodes are merged into a single 603regop. An even more aggressive form of this is that a branch 604sequence of the form C<EXACT BRANCH ... EXACT> can be converted into a 605C<TRIE-EXACT> regop. 606 607All of this occurs in the routine C<study_chunk()> which uses a special 608structure C<scan_data_t> to store the analysis that it has performed, and 609does the "peep-hole" optimisations as it goes. 610 611The code involved in C<study_chunk()> is extremely cryptic. Be careful. :-) 612 613=head2 Execution 614 615Execution of a regex generally involves two phases, the first being 616finding the start point in the string where we should match from, 617and the second being running the regop interpreter. 618 619If we can tell that there is no valid start point then we don't bother running 620interpreter at all. Likewise, if we know from the analysis phase that we 621cannot detect a short-cut to the start position, we go straight to the 622interpreter. 623 624The two entry points are C<re_intuit_start()> and C<pregexec()>. These routines 625have a somewhat incestuous relationship with overlap between their functions, 626and C<pregexec()> may even call C<re_intuit_start()> on its own. Nevertheless 627other parts of the the perl source code may call into either, or both. 628 629Execution of the interpreter itself used to be recursive, but thanks to the 630efforts of Dave Mitchell in the 5.9.x development track, that has changed: now an 631internal stack is maintained on the heap and the routine is fully 632iterative. This can make it tricky as the code is quite conservative 633about what state it stores, with the result that that two consecutive lines in the 634code can actually be running in totally different contexts due to the 635simulated recursion. 636 637=head3 Start position and no-match optimisations 638 639C<re_intuit_start()> is responsible for handling start points and no-match 640optimisations as determined by the results of the analysis done by 641C<study_chunk()> (and described in L<Peep-hole Optimisation and Analysis>). 642 643The basic structure of this routine is to try to find the start- and/or 644end-points of where the pattern could match, and to ensure that the string 645is long enough to match the pattern. It tries to use more efficient 646methods over less efficient methods and may involve considerable 647cross-checking of constraints to find the place in the string that matches. 648For instance it may try to determine that a given fixed string must be 649not only present but a certain number of chars before the end of the 650string, or whatever. 651 652It calls several other routines, such as C<fbm_instr()> which does 653Fast Boyer Moore matching and C<find_byclass()> which is responsible for 654finding the start using the first mandatory regop in the program. 655 656When the optimisation criteria have been satisfied, C<reg_try()> is called 657to perform the match. 658 659=head3 Program execution 660 661C<pregexec()> is the main entry point for running a regex. It contains 662support for initialising the regex interpreter's state, running 663C<re_intuit_start()> if needed, and running the interpreter on the string 664from various start positions as needed. When it is necessary to use 665the regex interpreter C<pregexec()> calls C<regtry()>. 666 667C<regtry()> is the entry point into the regex interpreter. It expects 668as arguments a pointer to a C<regmatch_info> structure and a pointer to 669a string. It returns an integer 1 for success and a 0 for failure. 670It is basically a set-up wrapper around C<regmatch()>. 671 672C<regmatch> is the main "recursive loop" of the interpreter. It is 673basically a giant switch statement that implements a state machine, where 674the possible states are the regops themselves, plus a number of additional 675intermediate and failure states. A few of the states are implemented as 676subroutines but the bulk are inline code. 677 678=head1 MISCELLANEOUS 679 680=head2 Unicode and Localisation Support 681 682When dealing with strings containing characters that cannot be represented 683using an eight-bit character set, perl uses an internal representation 684that is a permissive version of Unicode's UTF-8 encoding[2]. This uses single 685bytes to represent characters from the ASCII character set, and sequences 686of two or more bytes for all other characters. (See L<perlunitut> 687for more information about the relationship between UTF-8 and perl's 688encoding, utf8 -- the difference isn't important for this discussion.) 689 690No matter how you look at it, Unicode support is going to be a pain in a 691regex engine. Tricks that might be fine when you have 256 possible 692characters often won't scale to handle the size of the UTF-8 character 693set. Things you can take for granted with ASCII may not be true with 694Unicode. For instance, in ASCII, it is safe to assume that 695C<sizeof(char1) == sizeof(char2)>, but in UTF-8 it isn't. Unicode case folding is 696vastly more complex than the simple rules of ASCII, and even when not 697using Unicode but only localised single byte encodings, things can get 698tricky (for example, B<LATIN SMALL LETTER SHARP S> (U+00DF, E<szlig>) 699should match 'SS' in localised case-insensitive matching). 700 701Making things worse is that UTF-8 support was a later addition to the 702regex engine (as it was to perl) and this necessarily made things a lot 703more complicated. Obviously it is easier to design a regex engine with 704Unicode support in mind from the beginning than it is to retrofit it to 705one that wasn't. 706 707Nearly all regops that involve looking at the input string have 708two cases, one for UTF-8, and one not. In fact, it's often more complex 709than that, as the pattern may be UTF-8 as well. 710 711Care must be taken when making changes to make sure that you handle 712UTF-8 properly, both at compile time and at execution time, including 713when the string and pattern are mismatched. 714 715The following comment in F<regcomp.h> gives an example of exactly how 716tricky this can be: 717 718 Two problematic code points in Unicode casefolding of EXACT nodes: 719 720 U+0390 - GREEK SMALL LETTER IOTA WITH DIALYTIKA AND TONOS 721 U+03B0 - GREEK SMALL LETTER UPSILON WITH DIALYTIKA AND TONOS 722 723 which casefold to 724 725 Unicode UTF-8 726 727 U+03B9 U+0308 U+0301 0xCE 0xB9 0xCC 0x88 0xCC 0x81 728 U+03C5 U+0308 U+0301 0xCF 0x85 0xCC 0x88 0xCC 0x81 729 730 This means that in case-insensitive matching (or "loose matching", 731 as Unicode calls it), an EXACTF of length six (the UTF-8 encoded 732 byte length of the above casefolded versions) can match a target 733 string of length two (the byte length of UTF-8 encoded U+0390 or 734 U+03B0). This would rather mess up the minimum length computation. 735 736 What we'll do is to look for the tail four bytes, and then peek 737 at the preceding two bytes to see whether we need to decrease 738 the minimum length by four (six minus two). 739 740 Thanks to the design of UTF-8, there cannot be false matches: 741 A sequence of valid UTF-8 bytes cannot be a subsequence of 742 another valid sequence of UTF-8 bytes. 743 744 745=head2 Base Structures 746 747The C<regexp> structure described in L<perlreapi> is common to all 748regex engines. Two of its fields that are intended for the private use 749of the regex engine that compiled the pattern. These are the 750C<intflags> and pprivate members. The C<pprivate> is a void pointer to 751an arbitrary structure whose use and management is the responsibility 752of the compiling engine. perl will never modify either of these 753values. In the case of the stock engine the structure pointed to by 754C<pprivate> is called C<regexp_internal>. 755 756Its C<pprivate> and C<intflags> fields contain data 757specific to each engine. 758 759There are two structures used to store a compiled regular expression. 760One, the C<regexp> structure described in L<perlreapi> is populated by 761the engine currently being. used and some of its fields read by perl to 762implement things such as the stringification of C<qr//>. 763 764 765The other structure is pointed to be the C<regexp> struct's 766C<pprivate> and is in addition to C<intflags> in the same struct 767considered to be the property of the regex engine which compiled the 768regular expression; 769 770The regexp structure contains all the data that perl needs to be aware of 771to properly work with the regular expression. It includes data about 772optimisations that perl can use to determine if the regex engine should 773really be used, and various other control info that is needed to properly 774execute patterns in various contexts such as is the pattern anchored in 775some way, or what flags were used during the compile, or whether the 776program contains special constructs that perl needs to be aware of. 777 778In addition it contains two fields that are intended for the private use 779of the regex engine that compiled the pattern. These are the C<intflags> 780and pprivate members. The C<pprivate> is a void pointer to an arbitrary 781structure whose use and management is the responsibility of the compiling 782engine. perl will never modify either of these values. 783 784As mentioned earlier, in the case of the default engines, the C<pprivate> 785will be a pointer to a regexp_internal structure which holds the compiled 786program and any additional data that is private to the regex engine 787implementation. 788 789=head3 Perl's C<pprivate> structure 790 791The following structure is used as the C<pprivate> struct by perl's 792regex engine. Since it is specific to perl it is only of curiosity 793value to other engine implementations. 794 795 typedef struct regexp_internal { 796 regexp_paren_ofs *swap; /* Swap copy of *startp / *endp */ 797 U32 *offsets; /* offset annotations 20001228 MJD 798 data about mapping the program to the 799 string*/ 800 regnode *regstclass; /* Optional startclass as identified or constructed 801 by the optimiser */ 802 struct reg_data *data; /* Additional miscellaneous data used by the program. 803 Used to make it easier to clone and free arbitrary 804 data that the regops need. Often the ARG field of 805 a regop is an index into this structure */ 806 regnode program[1]; /* Unwarranted chumminess with compiler. */ 807 } regexp_internal; 808 809=over 5 810 811=item C<swap> 812 813C<swap> is an extra set of startp/endp stored in a C<regexp_paren_ofs> 814struct. This is used when the last successful match was from the same pattern 815as the current pattern, so that a partial match doesn't overwrite the 816previous match's results. When this field is data filled the matching 817engine will swap buffers before every match attempt. If the match fails, 818then it swaps them back. If it's successful it leaves them. This field 819is populated on demand and is by default null. 820 821=item C<offsets> 822 823Offsets holds a mapping of offset in the C<program> 824to offset in the C<precomp> string. This is only used by ActiveState's 825visual regex debugger. 826 827=item C<regstclass> 828 829Special regop that is used by C<re_intuit_start()> to check if a pattern 830can match at a certain position. For instance if the regex engine knows 831that the pattern must start with a 'Z' then it can scan the string until 832it finds one and then launch the regex engine from there. The routine 833that handles this is called C<find_by_class()>. Sometimes this field 834points at a regop embedded in the program, and sometimes it points at 835an independent synthetic regop that has been constructed by the optimiser. 836 837=item C<data> 838 839This field points at a reg_data structure, which is defined as follows 840 841 struct reg_data { 842 U32 count; 843 U8 *what; 844 void* data[1]; 845 }; 846 847This structure is used for handling data structures that the regex engine 848needs to handle specially during a clone or free operation on the compiled 849product. Each element in the data array has a corresponding element in the 850what array. During compilation regops that need special structures stored 851will add an element to each array using the add_data() routine and then store 852the index in the regop. 853 854=item C<program> 855 856Compiled program. Inlined into the structure so the entire struct can be 857treated as a single blob. 858 859=back 860 861=head1 SEE ALSO 862 863L<perlreapi> 864 865L<perlre> 866 867L<perlunitut> 868 869=head1 AUTHOR 870 871by Yves Orton, 2006. 872 873With excerpts from Perl, and contributions and suggestions from 874Ronald J. Kimball, Dave Mitchell, Dominic Dunlop, Mark Jason Dominus, 875Stephen McCamant, and David Landgren. 876 877=head1 LICENCE 878 879Same terms as Perl. 880 881=head1 REFERENCES 882 883[1] L<http://perl.plover.com/Rx/paper/> 884 885[2] L<http://www.unicode.org> 886 887=cut 888