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   1  =head1 NAME
   2  
   3  perlreguts - Description of the Perl regular expression engine.
   4  
   5  =head1 DESCRIPTION
   6  
   7  This document is an attempt to shine some light on the guts of the regex
   8  engine and how it works. The regex engine represents a significant chunk
   9  of the perl codebase, but is relatively poorly understood. This document
  10  is a meagre attempt at addressing this situation. It is derived from the
  11  author's experience, comments in the source code, other papers on the
  12  regex engine, feedback on the perl5-porters mail list, and no doubt other
  13  places as well.
  14  
  15  B<NOTICE!> It should be clearly understood that the behavior and
  16  structures discussed in this represents the state of the engine as the
  17  author understood it at the time of writing. It is B<NOT> an API
  18  definition, it is purely an internals guide for those who want to hack
  19  the regex engine, or understand how the regex engine works. Readers of
  20  this document are expected to understand perl's regex syntax and its
  21  usage in detail. If you want to learn about the basics of Perl's
  22  regular expressions, see L<perlre>. And if you want to replace the
  23  regex engine with your own see see L<perlreapi>.
  24  
  25  =head1 OVERVIEW
  26  
  27  =head2 A quick note on terms
  28  
  29  There is some debate as to whether to say "regexp" or "regex". In this
  30  document we will use the term "regex" unless there is a special reason
  31  not to, in which case we will explain why.
  32  
  33  When speaking about regexes we need to distinguish between their source
  34  code form and their internal form. In this document we will use the term
  35  "pattern" when we speak of their textual, source code form, and the term
  36  "program" when we speak of their internal representation. These
  37  correspond to the terms I<S-regex> and I<B-regex> that Mark Jason
  38  Dominus employs in his paper on "Rx" ([1] in L</REFERENCES>).
  39  
  40  =head2 What is a regular expression engine?
  41  
  42  A regular expression engine is a program that takes a set of constraints
  43  specified in a mini-language, and then applies those constraints to a
  44  target string, and determines whether or not the string satisfies the
  45  constraints. See L<perlre> for a full definition of the language.
  46  
  47  In less grandiose terms, the first part of the job is to turn a pattern into
  48  something the computer can efficiently use to find the matching point in
  49  the string, and the second part is performing the search itself.
  50  
  51  To do this we need to produce a program by parsing the text. We then
  52  need to execute the program to find the point in the string that
  53  matches. And we need to do the whole thing efficiently.
  54  
  55  =head2 Structure of a Regexp Program
  56  
  57  =head3 High Level
  58  
  59  Although it is a bit confusing and some people object to the terminology, it
  60  is worth taking a look at a comment that has
  61  been in F<regexp.h> for years:
  62  
  63  I<This is essentially a linear encoding of a nondeterministic
  64  finite-state machine (aka syntax charts or "railroad normal form" in
  65  parsing technology).>
  66  
  67  The term "railroad normal form" is a bit esoteric, with "syntax
  68  diagram/charts", or "railroad diagram/charts" being more common terms.
  69  Nevertheless it provides a useful mental image of a regex program: each
  70  node can be thought of as a unit of track, with a single entry and in
  71  most cases a single exit point (there are pieces of track that fork, but
  72  statistically not many), and the whole forms a layout with a
  73  single entry and single exit point. The matching process can be thought
  74  of as a car that moves along the track, with the particular route through
  75  the system being determined by the character read at each possible
  76  connector point. A car can fall off the track at any point but it may
  77  only proceed as long as it matches the track.
  78  
  79  Thus the pattern C</foo(?:\w+|\d+|\s+)bar/> can be thought of as the
  80  following chart:
  81  
  82                        [start]
  83                           |
  84                         <foo>
  85                           |
  86                     +-----+-----+
  87                     |     |     |
  88                   <\w+> <\d+> <\s+>
  89                     |     |     |
  90                     +-----+-----+
  91                           |
  92                         <bar>
  93                           |
  94                         [end]
  95  
  96  The truth of the matter is that perl's regular expressions these days are
  97  much more complex than this kind of structure, but visualising it this way
  98  can help when trying to get your bearings, and it matches the
  99  current implementation pretty closely.
 100  
 101  To be more precise, we will say that a regex program is an encoding
 102  of a graph. Each node in the graph corresponds to part of
 103  the original regex pattern, such as a literal string or a branch,
 104  and has a pointer to the nodes representing the next component
 105  to be matched. Since "node" and "opcode" already have other meanings in the
 106  perl source, we will call the nodes in a regex program "regops".
 107  
 108  The program is represented by an array of C<regnode> structures, one or
 109  more of which represent a single regop of the program. Struct
 110  C<regnode> is the smallest struct needed, and has a field structure which is
 111  shared with all the other larger structures.
 112  
 113  The "next" pointers of all regops except C<BRANCH> implement concatenation;
 114  a "next" pointer with a C<BRANCH> on both ends of it is connecting two
 115  alternatives.  [Here we have one of the subtle syntax dependencies: an
 116  individual C<BRANCH> (as opposed to a collection of them) is never
 117  concatenated with anything because of operator precedence.]
 118  
 119  The operand of some types of regop is a literal string; for others,
 120  it is a regop leading into a sub-program.  In particular, the operand
 121  of a C<BRANCH> node is the first regop of the branch.
 122  
 123  B<NOTE>: As the railroad metaphor suggests, this is B<not> a tree
 124  structure:  the tail of the branch connects to the thing following the
 125  set of C<BRANCH>es.  It is a like a single line of railway track that
 126  splits as it goes into a station or railway yard and rejoins as it comes
 127  out the other side.
 128  
 129  =head3 Regops
 130  
 131  The 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  
 139  Other larger C<regnode>-like structures are defined in F<regcomp.h>. They
 140  are almost like subclasses in that they have the same fields as
 141  C<regnode>, with possibly additional fields following in
 142  the structure, and in some cases the specific meaning (and name)
 143  of some of base fields are overridden. The following is a more
 144  complete description.
 145  
 146  =over 4
 147  
 148  =item C<regnode_1>
 149  
 150  =item C<regnode_2>
 151  
 152  C<regnode_1> structures have the same header, followed by a single
 153  four-byte argument; C<regnode_2> structures contain two two-byte
 154  arguments instead:
 155  
 156      regnode_1                U32 arg1;
 157      regnode_2                U16 arg1;  U16 arg2;
 158  
 159  =item C<regnode_string>
 160  
 161  C<regnode_string> structures, used for literal strings, follow the header
 162  with a one-byte length and then the string data. Strings are padded on
 163  the end with zero bytes so that the total length of the node is a
 164  multiple of four bytes:
 165  
 166      regnode_string           char string[1];
 167                               U8 str_len; /* overrides flags */
 168  
 169  =item C<regnode_charclass>
 170  
 171  Character classes are represented by C<regnode_charclass> structures,
 172  which have a four-byte argument and then a 32-byte (256-bit) bitmap
 173  indicating which characters are included in the class.
 174  
 175      regnode_charclass        U32 arg1;
 176                               char bitmap[ANYOF_BITMAP_SIZE];
 177  
 178  =item C<regnode_charclass_class>
 179  
 180  There is also a larger form of a char class structure used to represent
 181  POSIX char classes called C<regnode_charclass_class> which has an
 182  additional 4-byte (32-bit) bitmap indicating which POSIX char classes
 183  have 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  
 191  F<regnodes.h> defines an array called C<regarglen[]> which gives the size
 192  of each opcode in units of C<size regnode> (4-byte). A macro is used
 193  to calculate the size of an C<EXACT> node based on its C<str_len> field.
 194  
 195  The regops are defined in F<regnodes.h> which is generated from
 196  F<regcomp.sym> by F<regcomp.pl>. Currently the maximum possible number
 197  of distinct regops is restricted to 256, with about a quarter already
 198  used.
 199  
 200  A set of macros makes accessing the fields
 201  easier and more consistent. These include C<OP()>, which is used to determine
 202  the type of a C<regnode>-like structure; C<NEXT_OFF()>, which is the offset to
 203  the next node (more on this later); C<ARG()>, C<ARG1()>, C<ARG2()>, C<ARG_SET()>,
 204  and equivalents for reading and setting the arguments; and C<STR_LEN()>,
 205  C<STRING()> and C<OPERAND()> for manipulating strings and regop bearing
 206  types.
 207  
 208  =head3 What regop is next?
 209  
 210  There are three distinct concepts of "next" in the regex engine, and
 211  it is important to keep them clear.
 212  
 213  =over 4
 214  
 215  =item *
 216  
 217  There is the "next regnode" from a given regnode, a value which is
 218  rarely useful except that sometimes it matches up in terms of value
 219  with one of the others, and that sometimes the code assumes this to
 220  always be so.
 221  
 222  =item *
 223  
 224  There is the "next regop" from a given regop/regnode. This is the
 225  regop physically located after the the current one, as determined by
 226  the size of the current regop. This is often useful, such as when
 227  dumping the structure we use this order to traverse. Sometimes the code
 228  assumes that the "next regnode" is the same as the "next regop", or in
 229  other words assumes that the sizeof a given regop type is always going
 230  to be one regnode large.
 231  
 232  =item *
 233  
 234  There is the "regnext" from a given regop. This is the regop which
 235  is reached by jumping forward by the value of C<NEXT_OFF()>,
 236  or in a few cases for longer jumps by the C<arg1> field of the C<regnode_1>
 237  structure. The subroutine C<regnext()> handles this transparently.
 238  This is the logical successor of the node, which in some cases, like
 239  that of the C<BRANCH> regop, has special meaning.
 240  
 241  =back
 242  
 243  =head1 Process Overview
 244  
 245  Broadly speaking, performing a match of a string against a pattern
 246  involves 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  
 275  Where these steps occur in the actual execution of a perl program is
 276  determined by whether the pattern involves interpolating any string
 277  variables. If interpolation occurs, then compilation happens at run time. If it
 278  does not, then compilation is performed at compile time. (The C</o> modifier changes this,
 279  as does C<qr//> to a certain extent.) The engine doesn't really care that
 280  much.
 281  
 282  =head2 Compilation
 283  
 284  This code resides primarily in F<regcomp.c>, along with the header files
 285  F<regcomp.h>, F<regexp.h> and F<regnodes.h>.
 286  
 287  Compilation starts with C<pregcomp()>, which is mostly an initialisation
 288  wrapper which farms work out to two other routines for the heavy lifting: the
 289  first is C<reg()>, which is the start point for parsing; the second,
 290  C<study_chunk()>, is responsible for optimisation.
 291  
 292  Initialisation in C<pregcomp()> mostly involves the creation and data-filling
 293  of a special structure, C<RExC_state_t> (defined in F<regcomp.c>).
 294  Almost all internally-used routines in F<regcomp.h> take a pointer to one
 295  of these structures as their first argument, with the name C<pRExC_state>.
 296  This structure is used to store the compilation state and contains many
 297  fields. Likewise there are many macros which operate on this
 298  variable: anything that looks like C<RExC_xxxx> is a macro that operates on
 299  this pointer/structure.
 300  
 301  =head3 Parsing for size
 302  
 303  In this pass the input pattern is parsed in order to calculate how much
 304  space is needed for each regop we would need to emit. The size is also
 305  used to determine whether long jumps will be required in the program.
 306  
 307  This stage is controlled by the macro C<SIZE_ONLY> being set.
 308  
 309  The parse proceeds pretty much exactly as it does during the
 310  construction phase, except that most routines are short-circuited to
 311  change the size field C<RExC_size> and not do anything else.
 312  
 313  =head3 Parsing for construction
 314  
 315  Once the size of the program has been determined, the pattern is parsed
 316  again, but this time for real. Now C<SIZE_ONLY> will be false, and the
 317  actual construction can occur.
 318  
 319  C<reg()> is the start of the parse process. It is responsible for
 320  parsing an arbitrary chunk of pattern up to either the end of the
 321  string, or the first closing parenthesis it encounters in the pattern.
 322  This means it can be used to parse the top-level regex, or any section
 323  inside of a grouping parenthesis. It also handles the "special parens"
 324  that perl's regexes have. For instance when parsing C</x(?:foo)y/> C<reg()>
 325  will at one point be called to parse from the "?" symbol up to and
 326  including the ")".
 327  
 328  Additionally, C<reg()> is responsible for parsing the one or more
 329  branches from the pattern, and for "finishing them off" by correctly
 330  setting their next pointers. In order to do the parsing, it repeatedly
 331  calls out to C<regbranch()>, which is responsible for handling up to the
 332  first C<|> symbol it sees.
 333  
 334  C<regbranch()> in turn calls C<regpiece()> which
 335  handles "things" followed by a quantifier. In order to parse the
 336  "things", C<regatom()> is called. This is the lowest level routine, which
 337  parses out constant strings, character classes, and the
 338  various special symbols like C<$>. If C<regatom()> encounters a "("
 339  character it in turn calls C<reg()>.
 340  
 341  The routine C<regtail()> is called by both C<reg()> and C<regbranch()>
 342  in order to "set the tail pointer" correctly. When executing and
 343  we get to the end of a branch, we need to go to the node following the
 344  grouping parens. When parsing, however, we don't know where the end will
 345  be until we get there, so when we do we must go back and update the
 346  offsets as appropriate. C<regtail> is used to make this easier.
 347  
 348  A subtlety of the parsing process means that a regex like C</foo/> is
 349  originally parsed into an alternation with a single branch. It is only
 350  afterwards that the optimiser converts single branch alternations into the
 351  simpler form.
 352  
 353  =head3 Parse Call Graph and a Grammar
 354  
 355  The 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  
 372  A 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  
 388  In the 5.9.x development version of perl you can C<<use re Debug => 'PARSE'>>
 389  to see some trace information about the parse process. We will start with some
 390  simple patterns and build up to more complex patterns.
 391  
 392  So when we parse C</foo/> we see something like the following table. The
 393  left shows what is being parsed, and the number indicates where the next regop
 394  would go. The stuff on the right is the trace output of the graph. The
 395  names are chosen to be short to make it less dense on the screen. 'tsdy'
 396  is 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  
 405  The resulting program then looks like:
 406  
 407     1: EXACT <foo>(3)
 408     3: END(0)
 409  
 410  As you can see, even though we parsed out a branch and a piece, it was ultimately
 411  only an atom. The final program shows us how things work. We have an C<EXACT> regop,
 412  followed by an C<END> regop. The number in parens indicates where the C<regnext> of
 413  the node goes. The C<regnext> of an C<END> regop is unused, as C<END> regops mean
 414  we have successfully matched. The number on the left indicates the position of
 415  the regop in the regnode array.
 416  
 417  Now let's try a harder pattern. We will add a quantifier, so now we have the pattern
 418  C</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  
 431  And 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  
 438  Now we have a special case. The C<EXACT> regop has a C<regnext> of 0. This is
 439  because if it matches it should try to match itself again. The C<PLUS> regop
 440  handles the actual failure of the C<EXACT> regop and acts appropriately (going
 441  to regnode 6 if the C<EXACT> matched at least once, or failing if it didn't).
 442  
 443  Now 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  
 517  Resulting 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  
 539  Here we can see a much more complex program, with various optimisations in
 540  play. At regnode 10 we see an example where a character class with only
 541  one character in it was turned into an C<EXACT> node. We can also see where
 542  an entire alternation was turned into a C<TRIE-EXACT> node. As a consequence,
 543  some of the regnodes have been marked as optimised away. We can see that
 544  the C<$> symbol has been converted into an C<EOL> regop, a special piece of
 545  code that looks for C<\n> or the end of the string.
 546  
 547  The next pointer for C<BRANCH>es is interesting in that it points at where
 548  execution should go if the branch fails. When executing, if the engine
 549  tries to traverse from a branch to a C<regnext> that isn't a branch then
 550  the engine will know that the entire set of branches has failed.
 551  
 552  =head3 Peep-hole Optimisation and Analysis
 553  
 554  The regular expression engine can be a weighty tool to wield. On long
 555  strings and complex patterns it can end up having to do a lot of work
 556  to find a match, and even more to decide that no match is possible.
 557  Consider a situation like the following pattern.
 558  
 559     'ababababababababababab' =~ /(a|b)*z/
 560  
 561  The C<(a|b)*> part can match at every char in the string, and then fail
 562  every time because there is no C<z> in the string. So obviously we can
 563  avoid using the regex engine unless there is a C<z> in the string.
 564  Likewise in a pattern like:
 565  
 566     /foo(\w+)bar/
 567  
 568  In this case we know that the string must contain a C<foo> which must be
 569  followed by C<bar>. We can use Fast Boyer-Moore matching as implemented
 570  in C<fbm_instr()> to find the location of these strings. If they don't exist
 571  then we don't need to resort to the much more expensive regex engine.
 572  Even better, if they do exist then we can use their positions to
 573  reduce the search space that the regex engine needs to cover to determine
 574  if the entire pattern matches.
 575  
 576  There are various aspects of the pattern that can be used to facilitate
 577  optimisations 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  
 593  Another form of optimisation that can occur is the post-parse "peep-hole"
 594  optimisation, where inefficient constructs are replaced by more efficient
 595  constructs. The C<TAIL> regops which are used during parsing to mark the end
 596  of branches and the end of groups are examples of this. These regops are used
 597  as 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
 599  thing that C<TAIL> points to, thus "skipping" the node.
 600  
 601  Another optimisation that can occur is that of "C<EXACT> merging" which is
 602  where two consecutive C<EXACT> nodes are merged into a single
 603  regop. An even more aggressive form of this is that a branch
 604  sequence of the form C<EXACT BRANCH ... EXACT> can be converted into a
 605  C<TRIE-EXACT> regop.
 606  
 607  All of this occurs in the routine C<study_chunk()> which uses a special
 608  structure C<scan_data_t> to store the analysis that it has performed, and
 609  does the "peep-hole" optimisations as it goes.
 610  
 611  The code involved in C<study_chunk()> is extremely cryptic. Be careful. :-)
 612  
 613  =head2 Execution
 614  
 615  Execution of a regex generally involves two phases, the first being
 616  finding the start point in the string where we should match from,
 617  and the second being running the regop interpreter.
 618  
 619  If we can tell that there is no valid start point then we don't bother running
 620  interpreter at all. Likewise, if we know from the analysis phase that we
 621  cannot detect a short-cut to the start position, we go straight to the
 622  interpreter.
 623  
 624  The two entry points are C<re_intuit_start()> and C<pregexec()>. These routines
 625  have a somewhat incestuous relationship with overlap between their functions,
 626  and C<pregexec()> may even call C<re_intuit_start()> on its own. Nevertheless
 627  other parts of the the perl source code may call into either, or both.
 628  
 629  Execution of the interpreter itself used to be recursive, but thanks to the
 630  efforts of Dave Mitchell in the 5.9.x development track, that has changed: now an
 631  internal stack is maintained on the heap and the routine is fully
 632  iterative. This can make it tricky as the code is quite conservative
 633  about what state it stores, with the result that that two consecutive lines in the
 634  code can actually be running in totally different contexts due to the
 635  simulated recursion.
 636  
 637  =head3 Start position and no-match optimisations
 638  
 639  C<re_intuit_start()> is responsible for handling start points and no-match
 640  optimisations as determined by the results of the analysis done by
 641  C<study_chunk()> (and described in L<Peep-hole Optimisation and Analysis>).
 642  
 643  The basic structure of this routine is to try to find the start- and/or
 644  end-points of where the pattern could match, and to ensure that the string
 645  is long enough to match the pattern. It tries to use more efficient
 646  methods over less efficient methods and may involve considerable
 647  cross-checking of constraints to find the place in the string that matches.
 648  For instance it may try to determine that a given fixed string must be
 649  not only present but a certain number of chars before the end of the
 650  string, or whatever.
 651  
 652  It calls several other routines, such as C<fbm_instr()> which does
 653  Fast Boyer Moore matching and C<find_byclass()> which is responsible for
 654  finding the start using the first mandatory regop in the program.
 655  
 656  When the optimisation criteria have been satisfied, C<reg_try()> is called
 657  to perform the match.
 658  
 659  =head3 Program execution
 660  
 661  C<pregexec()> is the main entry point for running a regex. It contains
 662  support for initialising the regex interpreter's state, running
 663  C<re_intuit_start()> if needed, and running the interpreter on the string
 664  from various start positions as needed. When it is necessary to use
 665  the regex interpreter C<pregexec()> calls C<regtry()>.
 666  
 667  C<regtry()> is the entry point into the regex interpreter. It expects
 668  as arguments a pointer to a C<regmatch_info> structure and a pointer to
 669  a string.  It returns an integer 1 for success and a 0 for failure.
 670  It is basically a set-up wrapper around C<regmatch()>.
 671  
 672  C<regmatch> is the main "recursive loop" of the interpreter. It is
 673  basically a giant switch statement that implements a state machine, where
 674  the possible states are the regops themselves, plus a number of additional
 675  intermediate and failure states. A few of the states are implemented as
 676  subroutines but the bulk are inline code.
 677  
 678  =head1 MISCELLANEOUS
 679  
 680  =head2 Unicode and Localisation Support
 681  
 682  When dealing with strings containing characters that cannot be represented
 683  using an eight-bit character set, perl uses an internal representation
 684  that is a permissive version of Unicode's UTF-8 encoding[2]. This uses single
 685  bytes to represent characters from the ASCII character set, and sequences
 686  of two or more bytes for all other characters. (See L<perlunitut>
 687  for more information about the relationship between UTF-8 and perl's
 688  encoding, utf8 -- the difference isn't important for this discussion.)
 689  
 690  No matter how you look at it, Unicode support is going to be a pain in a
 691  regex engine. Tricks that might be fine when you have 256 possible
 692  characters often won't scale to handle the size of the UTF-8 character
 693  set.  Things you can take for granted with ASCII may not be true with
 694  Unicode. For instance, in ASCII, it is safe to assume that
 695  C<sizeof(char1) == sizeof(char2)>, but in UTF-8 it isn't. Unicode case folding is
 696  vastly more complex than the simple rules of ASCII, and even when not
 697  using Unicode but only localised single byte encodings, things can get
 698  tricky (for example, B<LATIN SMALL LETTER SHARP S> (U+00DF, E<szlig>)
 699  should match 'SS' in localised case-insensitive matching).
 700  
 701  Making things worse is that UTF-8 support was a later addition to the
 702  regex engine (as it was to perl) and this necessarily  made things a lot
 703  more complicated. Obviously it is easier to design a regex engine with
 704  Unicode support in mind from the beginning than it is to retrofit it to
 705  one that wasn't.
 706  
 707  Nearly all regops that involve looking at the input string have
 708  two cases, one for UTF-8, and one not. In fact, it's often more complex
 709  than that, as the pattern may be UTF-8 as well.
 710  
 711  Care must be taken when making changes to make sure that you handle
 712  UTF-8 properly, both at compile time and at execution time, including
 713  when the string and pattern are mismatched.
 714  
 715  The following comment in F<regcomp.h> gives an example of exactly how
 716  tricky 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  
 747  The C<regexp> structure described in L<perlreapi> is common to all
 748  regex engines. Two of its fields that are intended for the private use
 749  of the regex engine that compiled the pattern. These are the
 750  C<intflags> and pprivate members. The C<pprivate> is a void pointer to
 751  an arbitrary structure whose use and management is the responsibility
 752  of the compiling engine. perl will never modify either of these
 753  values. In the case of the stock engine the structure pointed to by
 754  C<pprivate> is called C<regexp_internal>.
 755  
 756  Its C<pprivate> and C<intflags> fields contain data
 757  specific to each engine.
 758  
 759  There are two structures used to store a compiled regular expression.
 760  One, the C<regexp> structure described in L<perlreapi> is populated by
 761  the engine currently being. used and some of its fields read by perl to
 762  implement things such as the stringification of C<qr//>.
 763  
 764  
 765  The other structure is pointed to be the C<regexp> struct's
 766  C<pprivate> and is in addition to C<intflags> in the same struct
 767  considered to be the property of the regex engine which compiled the
 768  regular expression; 
 769  
 770  The regexp structure contains all the data that perl needs to be aware of
 771  to properly work with the regular expression. It includes data about
 772  optimisations that perl can use to determine if the regex engine should
 773  really be used, and various other control info that is needed to properly
 774  execute patterns in various contexts such as is the pattern anchored in
 775  some way, or what flags were used during the compile, or whether the
 776  program contains special constructs that perl needs to be aware of.
 777  
 778  In addition it contains two fields that are intended for the private use
 779  of the regex engine that compiled the pattern. These are the C<intflags>
 780  and pprivate members. The C<pprivate> is a void pointer to an arbitrary
 781  structure whose use and management is the responsibility of the compiling
 782  engine. perl will never modify either of these values.
 783  
 784  As mentioned earlier, in the case of the default engines, the C<pprivate>
 785  will be a pointer to a regexp_internal structure which holds the compiled
 786  program and any additional data that is private to the regex engine
 787  implementation.
 788  
 789  =head3 Perl's C<pprivate> structure
 790  
 791  The following structure is used as the C<pprivate> struct by perl's
 792  regex engine. Since it is specific to perl it is only of curiosity
 793  value 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  
 813  C<swap> is an extra set of startp/endp stored in a C<regexp_paren_ofs>
 814  struct. This is used when the last successful match was from the same pattern
 815  as the current pattern, so that a partial match doesn't overwrite the
 816  previous match's results. When this field is data filled the matching
 817  engine will swap buffers before every match attempt. If the match fails,
 818  then it swaps them back. If it's successful it leaves them. This field
 819  is populated on demand and is by default null.
 820  
 821  =item C<offsets>
 822  
 823  Offsets holds a mapping of offset in the C<program>
 824  to offset in the C<precomp> string. This is only used by ActiveState's
 825  visual regex debugger.
 826  
 827  =item C<regstclass>
 828  
 829  Special regop that is used by C<re_intuit_start()> to check if a pattern
 830  can match at a certain position. For instance if the regex engine knows
 831  that the pattern must start with a 'Z' then it can scan the string until
 832  it finds one and then launch the regex engine from there. The routine
 833  that handles this is called C<find_by_class()>. Sometimes this field
 834  points at a regop embedded in the program, and sometimes it points at
 835  an independent synthetic regop that has been constructed by the optimiser.
 836  
 837  =item C<data>
 838  
 839  This 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  
 847  This structure is used for handling data structures that the regex engine
 848  needs to handle specially during a clone or free operation on the compiled
 849  product. Each element in the data array has a corresponding element in the
 850  what array. During compilation regops that need special structures stored
 851  will add an element to each array using the add_data() routine and then store
 852  the index in the regop.
 853  
 854  =item C<program>
 855  
 856  Compiled program. Inlined into the structure so the entire struct can be
 857  treated as a single blob.
 858  
 859  =back
 860  
 861  =head1 SEE ALSO
 862  
 863  L<perlreapi>
 864  
 865  L<perlre>
 866  
 867  L<perlunitut>
 868  
 869  =head1 AUTHOR
 870  
 871  by Yves Orton, 2006.
 872  
 873  With excerpts from Perl, and contributions and suggestions from
 874  Ronald J. Kimball, Dave Mitchell, Dominic Dunlop, Mark Jason Dominus,
 875  Stephen McCamant, and David Landgren.
 876  
 877  =head1 LICENCE
 878  
 879  Same 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