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NAME
       perlreguts - Description of the Perl regular expression
       engine.

DESCRIPTION
       This document is an attempt to shine some light on the
       guts of the regex engine and how it works. The regex
       engine represents a significant chunk of the perl
       codebase, but is relatively poorly understood. This
       document is a meagre attempt at addressing this situation.
       It is derived from the author's experience, comments in
       the source code, other papers on the regex engine,
       feedback on the perl5-porters mail list, and no doubt
       other places as well.

       NOTICE! It should be clearly understood that the behavior
       and structures discussed in this represents the state of
       the engine as the author understood it at the time of
       writing. It is NOT an API definition, it is purely an
       internals guide for those who want to hack the regex
       engine, or understand how the regex engine works. Readers
       of this document are expected to understand perl's regex
       syntax and its usage in detail. If you want to learn about
       the basics of Perl's regular expressions, see perlre. And
       if you want to replace the regex engine with your own see
       see perlreapi.

OVERVIEW
   A quick note on terms
       There is some debate as to whether to say "regexp" or
       "regex". In this document we will use the term "regex"
       unless there is a special reason not to, in which case we
       will explain why.

       When speaking about regexes we need to distinguish between
       their source code form and their internal form. In this
       document we will use the term "pattern" when we speak of
       their textual, source code form, and the term "program"
       when we speak of their internal representation. These
       correspond to the terms S-regex and B-regex that Mark
       Jason Dominus employs in his paper on "Rx" ([1] in
       "REFERENCES").

   What is a regular expression engine?
       A regular expression engine is a program that takes a set
       of constraints specified in a mini-language, and then
       applies those constraints to a target string, and
       determines whether or not the string satisfies the
       constraints. See perlre for a full definition of the
       language.

       In less grandiose terms, the first part of the job is to
       turn a pattern into something the computer can efficiently
       use to find the matching point in the string, and the



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       second part is performing the search itself.

       To do this we need to produce a program by parsing the
       text. We then need to execute the program to find the
       point in the string that matches. And we need to do the
       whole thing efficiently.

   Structure of a Regexp Program
       High Level

       Although it is a bit confusing and some people object to
       the terminology, it is worth taking a look at a comment
       that has been in regexp.h for years:

       This is essentially a linear encoding of a
       nondeterministic finite-state machine (aka syntax charts
       or "railroad normal form" in parsing technology).

       The term "railroad normal form" is a bit esoteric, with
       "syntax diagram/charts", or "railroad diagram/charts"
       being more common terms.  Nevertheless it provides a
       useful mental image of a regex program: each node can be
       thought of as a unit of track, with a single entry and in
       most cases a single exit point (there are pieces of track
       that fork, but statistically not many), and the whole
       forms a layout with a single entry and single exit point.
       The matching process can be thought of as a car that moves
       along the track, with the particular route through the
       system being determined by the character read at each
       possible connector point. A car can fall off the track at
       any point but it may only proceed as long as it matches
       the track.

       Thus the pattern "/foo(?:\w+|\d+|\s+)bar/" can be thought
       of as the following chart:

                             [start]
                                |
                              <foo>
                                |
                          +-----+-----+
                          |     |     |
                        <\w+> <\d+> <\s+>
                          |     |     |
                          +-----+-----+
                                |
                              <bar>
                                |
                              [end]

       The truth of the matter is that perl's regular expressions
       these days are much more complex than this kind of
       structure, but visualising it this way can help when
       trying to get your bearings, and it matches the current



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       implementation pretty closely.

       To be more precise, we will say that a regex program is an
       encoding of a graph. Each node in the graph corresponds to
       part of the original regex pattern, such as a literal
       string or a branch, and has a pointer to the nodes
       representing the next component to be matched. Since
       "node" and "opcode" already have other meanings in the
       perl source, we will call the nodes in a regex program
       "regops".

       The program is represented by an array of "regnode"
       structures, one or more of which represent a single regop
       of the program. Struct "regnode" is the smallest struct
       needed, and has a field structure which is shared with all
       the other larger structures.

       The "next" pointers of all regops except "BRANCH"
       implement concatenation; a "next" pointer with a "BRANCH"
       on both ends of it is connecting two alternatives.  [Here
       we have one of the subtle syntax dependencies: an
       individual "BRANCH" (as opposed to a collection of them)
       is never concatenated with anything because of operator
       precedence.]

       The operand of some types of regop is a literal string;
       for others, it is a regop leading into a sub-program.  In
       particular, the operand of a "BRANCH" node is the first
       regop of the branch.

       NOTE: As the railroad metaphor suggests, this is not a
       tree structure:  the tail of the branch connects to the
       thing following the set of "BRANCH"es.  It is a like a
       single line of railway track that splits as it goes into a
       station or railway yard and rejoins as it comes out the
       other side.

       Regops

       The base structure of a regop is defined in regexp.h as
       follows:

           struct regnode {
               U8  flags;    /* Various purposes, sometimes overridden */
               U8  type;     /* Opcode value as specified by regnodes.h */
               U16 next_off; /* Offset in size regnode */
           };

       Other larger "regnode"-like structures are defined in
       regcomp.h. They are almost like subclasses in that they
       have the same fields as "regnode", with possibly
       additional fields following in the structure, and in some
       cases the specific meaning (and name) of some of base
       fields are overridden. The following is a more complete



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       description.

       "regnode_1"
       "regnode_2"
           "regnode_1" structures have the same header, followed
           by a single four-byte argument; "regnode_2" structures
           contain two two-byte arguments instead:

               regnode_1                U32 arg1;
               regnode_2                U16 arg1;  U16 arg2;

       "regnode_string"
           "regnode_string" structures, used for literal strings,
           follow the header with a one-byte length and then the
           string data. Strings are padded on the end with zero
           bytes so that the total length of the node is a
           multiple of four bytes:

               regnode_string           char string[1];
                                        U8 str_len; /* overrides flags */

       "regnode_charclass"
           Character classes are represented by
           "regnode_charclass" structures, which have a four-byte
           argument and then a 32-byte (256-bit) bitmap
           indicating which characters are included in the class.

               regnode_charclass        U32 arg1;
                                        char bitmap[ANYOF_BITMAP_SIZE];

       "regnode_charclass_class"
           There is also a larger form of a char class structure
           used to represent POSIX char classes called
           "regnode_charclass_class" which has an additional
           4-byte (32-bit) bitmap indicating which POSIX char
           classes have been included.

               regnode_charclass_class  U32 arg1;
                                        char bitmap[ANYOF_BITMAP_SIZE];
                                        char classflags[ANYOF_CLASSBITMAP_SIZE];

       regnodes.h defines an array called "regarglen[]" which
       gives the size of each opcode in units of "size regnode"
       (4-byte). A macro is used to calculate the size of an
       "EXACT" node based on its "str_len" field.

       The regops are defined in regnodes.h which is generated
       from regcomp.sym by regcomp.pl. Currently the maximum
       possible number of distinct regops is restricted to 256,
       with about a quarter already used.

       A set of macros makes accessing the fields easier and more
       consistent. These include "OP()", which is used to
       determine the type of a "regnode"-like structure;



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       "NEXT_OFF()", which is the offset to the next node (more
       on this later); "ARG()", "ARG1()", "ARG2()", "ARG_SET()",
       and equivalents for reading and setting the arguments; and
       "STR_LEN()", "STRING()" and "OPERAND()" for manipulating
       strings and regop bearing types.

       What regop is next?

       There are three distinct concepts of "next" in the regex
       engine, and it is important to keep them clear.

       o   There is the "next regnode" from a given regnode, a
           value which is rarely useful except that sometimes it
           matches up in terms of value with one of the others,
           and that sometimes the code assumes this to always be
           so.

       o   There is the "next regop" from a given regop/regnode.
           This is the regop physically located after the the
           current one, as determined by the size of the current
           regop. This is often useful, such as when dumping the
           structure we use this order to traverse. Sometimes the
           code assumes that the "next regnode" is the same as
           the "next regop", or in other words assumes that the
           sizeof a given regop type is always going to be one
           regnode large.

       o   There is the "regnext" from a given regop. This is the
           regop which is reached by jumping forward by the value
           of "NEXT_OFF()", or in a few cases for longer jumps by
           the "arg1" field of the "regnode_1" structure. The
           subroutine "regnext()" handles this transparently.
           This is the logical successor of the node, which in
           some cases, like that of the "BRANCH" regop, has
           special meaning.

Process Overview
       Broadly speaking, performing a match of a string against a
       pattern involves the following steps:

       A. Compilation
            1. Parsing for size
            2. Parsing for construction
            3. Peep-hole optimisation and analysis
       B. Execution
            4. Start position and no-match optimisations
            5. Program execution

       Where these steps occur in the actual execution of a perl
       program is determined by whether the pattern involves
       interpolating any string variables. If interpolation
       occurs, then compilation happens at run time. If it does
       not, then compilation is performed at compile time. (The
       "/o" modifier changes this, as does "qr//" to a certain



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       extent.) The engine doesn't really care that much.

   Compilation
       This code resides primarily in regcomp.c, along with the
       header files regcomp.h, regexp.h and regnodes.h.

       Compilation starts with "pregcomp()", which is mostly an
       initialisation wrapper which farms work out to two other
       routines for the heavy lifting: the first is "reg()",
       which is the start point for parsing; the second,
       "study_chunk()", is responsible for optimisation.

       Initialisation in "pregcomp()" mostly involves the
       creation and data-filling of a special structure,
       "RExC_state_t" (defined in regcomp.c).  Almost all
       internally-used routines in regcomp.h take a pointer to
       one of these structures as their first argument, with the
       name "pRExC_state".  This structure is used to store the
       compilation state and contains many fields. Likewise there
       are many macros which operate on this variable: anything
       that looks like "RExC_xxxx" is a macro that operates on
       this pointer/structure.

       Parsing for size

       In this pass the input pattern is parsed in order to
       calculate how much space is needed for each regop we would
       need to emit. The size is also used to determine whether
       long jumps will be required in the program.

       This stage is controlled by the macro "SIZE_ONLY" being
       set.

       The parse proceeds pretty much exactly as it does during
       the construction phase, except that most routines are
       short-circuited to change the size field "RExC_size" and
       not do anything else.

       Parsing for construction

       Once the size of the program has been determined, the
       pattern is parsed again, but this time for real. Now
       "SIZE_ONLY" will be false, and the actual construction can
       occur.

       "reg()" is the start of the parse process. It is
       responsible for parsing an arbitrary chunk of pattern up
       to either the end of the string, or the first closing
       parenthesis it encounters in the pattern.  This means it
       can be used to parse the top-level regex, or any section
       inside of a grouping parenthesis. It also handles the
       "special parens" that perl's regexes have. For instance
       when parsing "/x(?:foo)y/" "reg()" will at one point be
       called to parse from the "?" symbol up to and including



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       the ")".

       Additionally, "reg()" is responsible for parsing the one
       or more branches from the pattern, and for "finishing them
       off" by correctly setting their next pointers. In order to
       do the parsing, it repeatedly calls out to "regbranch()",
       which is responsible for handling up to the first "|"
       symbol it sees.

       "regbranch()" in turn calls "regpiece()" which handles
       "things" followed by a quantifier. In order to parse the
       "things", "regatom()" is called. This is the lowest level
       routine, which parses out constant strings, character
       classes, and the various special symbols like "$". If
       "regatom()" encounters a "(" character it in turn calls
       "reg()".

       The routine "regtail()" is called by both "reg()" and
       "regbranch()" in order to "set the tail pointer"
       correctly. When executing and we get to the end of a
       branch, we need to go to the node following the grouping
       parens. When parsing, however, we don't know where the end
       will be until we get there, so when we do we must go back
       and update the offsets as appropriate. "regtail" is used
       to make this easier.

       A subtlety of the parsing process means that a regex like
       "/foo/" is originally parsed into an alternation with a
       single branch. It is only afterwards that the optimiser
       converts single branch alternations into the simpler form.

       Parse Call Graph and a Grammar

       The call graph looks like this:

           reg()                        # parse a top level regex, or inside of parens
               regbranch()              # parse a single branch of an alternation
                   regpiece()           # parse a pattern followed by a quantifier
                       regatom()        # parse a simple pattern
                           regclass()   #   used to handle a class
                           reg()        #   used to handle a parenthesised subpattern
                           ....
                   ...
                   regtail()            # finish off the branch
               ...
               regtail()                # finish off the branch sequence. Tie each
                                        # branch's tail to the tail of the sequence
                                        # (NEW) In Debug mode this is
                                        # regtail_study().

       A grammar form might be something like this:






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           atom  : constant | class
           quant : '*' | '+' | '?' | '{min,max}'
           _branch: piece
                  | piece _branch
                  | nothing
           branch: _branch
                 | _branch '|' branch
           group : '(' branch ')'
           _piece: atom | group
           piece : _piece
                 | _piece quant

       Debug Output

       In the 5.9.x development version of perl you can "use re
       Debug => 'PARSE'" to see some trace information about the
       parse process. We will start with some simple patterns and
       build up to more complex patterns.

       So when we parse "/foo/" we see something like the
       following table. The left shows what is being parsed, and
       the number indicates where the next regop would go. The
       stuff on the right is the trace output of the graph. The
       names are chosen to be short to make it less dense on the
       screen. 'tsdy' is a special form of "regtail()" which does
       some extra analysis.

        >foo<             1    reg
                                 brnc
                                   piec
                                     atom
        ><                4      tsdy~ EXACT <foo> (EXACT) (1)
                                     ~ attach to END (3) offset to 2

       The resulting program then looks like:

          1: EXACT <foo>(3)
          3: END(0)

       As you can see, even though we parsed out a branch and a
       piece, it was ultimately only an atom. The final program
       shows us how things work. We have an "EXACT" regop,
       followed by an "END" regop. The number in parens indicates
       where the "regnext" of the node goes. The "regnext" of an
       "END" regop is unused, as "END" regops mean we have
       successfully matched. The number on the left indicates the
       position of the regop in the regnode array.

       Now let's try a harder pattern. We will add a quantifier,
       so now we have the pattern "/foo+/". We will see that
       "regbranch()" calls "regpiece()" twice.






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        >foo+<            1    reg
                                 brnc
                                   piec
                                     atom
        >o+<              3        piec
                                     atom
        ><                6        tail~ EXACT <fo> (1)
                          7      tsdy~ EXACT <fo> (EXACT) (1)
                                     ~ PLUS (END) (3)
                                     ~ attach to END (6) offset to 3

       And we end up with the program:

          1: EXACT <fo>(3)
          3: PLUS(6)
          4:   EXACT <o>(0)
          6: END(0)

       Now we have a special case. The "EXACT" regop has a
       "regnext" of 0. This is because if it matches it should
       try to match itself again. The "PLUS" regop handles the
       actual failure of the "EXACT" regop and acts appropriately
       (going to regnode 6 if the "EXACT" matched at least once,
       or failing if it didn't).

       Now for something much more complex:
       "/x(?:foo*|b[a][rR])(foo|bar)$/"

        >x(?:foo*|b...    1    reg
                                 brnc
                                   piec
                                     atom
        >(?:foo*|b[...    3        piec
                                     atom
        >?:foo*|b[a...                 reg
        >foo*|b[a][...                   brnc
                                           piec
                                             atom
        >o*|b[a][rR...    5                piec
                                             atom
        >|b[a][rR])...    8                tail~ EXACT <fo> (3)
        >b[a][rR])(...    9              brnc
                         10                piec
                                             atom
        >[a][rR])(f...   12                piec
                                             atom
        >a][rR])(fo...                         clas
        >[rR])(foo|...   14                tail~ EXACT <b> (10)
                                           piec
                                             atom
        >rR])(foo|b...                         clas
        >)(foo|bar)...   25                tail~ EXACT <a> (12)
                                         tail~ BRANCH (3)
                         26              tsdy~ BRANCH (END) (9)



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                                             ~ attach to TAIL (25) offset to 16
                                         tsdy~ EXACT <fo> (EXACT) (4)
                                             ~ STAR (END) (6)
                                             ~ attach to TAIL (25) offset to 19
                                         tsdy~ EXACT <b> (EXACT) (10)
                                             ~ EXACT <a> (EXACT) (12)
                                             ~ ANYOF[Rr] (END) (14)
                                             ~ attach to TAIL (25) offset to 11
        >(foo|bar)$<               tail~ EXACT <x> (1)
                                   piec
                                     atom
        >foo|bar)$<                    reg
                         28              brnc
                                           piec
                                             atom
        >|bar)$<         31              tail~ OPEN1 (26)
        >bar)$<                          brnc
                         32                piec
                                             atom
        >)$<             34              tail~ BRANCH (28)
                         36              tsdy~ BRANCH (END) (31)
                                             ~ attach to CLOSE1 (34) offset to 3
                                         tsdy~ EXACT <foo> (EXACT) (29)
                                             ~ attach to CLOSE1 (34) offset to 5
                                         tsdy~ EXACT <bar> (EXACT) (32)
                                             ~ attach to CLOSE1 (34) offset to 2
        >$<                        tail~ BRANCH (3)
                                       ~ BRANCH (9)
                                       ~ TAIL (25)
                                   piec
                                     atom
        ><               37        tail~ OPEN1 (26)
                                       ~ BRANCH (28)
                                       ~ BRANCH (31)
                                       ~ CLOSE1 (34)
                         38      tsdy~ EXACT <x> (EXACT) (1)
                                     ~ BRANCH (END) (3)
                                     ~ BRANCH (END) (9)
                                     ~ TAIL (END) (25)
                                     ~ OPEN1 (END) (26)
                                     ~ BRANCH (END) (28)
                                     ~ BRANCH (END) (31)
                                     ~ CLOSE1 (END) (34)
                                     ~ EOL (END) (36)
                                     ~ attach to END (37) offset to 1

       Resulting in the program










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          1: EXACT <x>(3)
          3: BRANCH(9)
          4:   EXACT <fo>(6)
          6:   STAR(26)
          7:     EXACT <o>(0)
          9: BRANCH(25)
         10:   EXACT <ba>(14)
         12:   OPTIMIZED (2 nodes)
         14:   ANYOF[Rr](26)
         25: TAIL(26)
         26: OPEN1(28)
         28:   TRIE-EXACT(34)
               [StS:1 Wds:2 Cs:6 Uq:5 #Sts:7 Mn:3 Mx:3 Stcls:bf]
                 <foo>
                 <bar>
         30:   OPTIMIZED (4 nodes)
         34: CLOSE1(36)
         36: EOL(37)
         37: END(0)

       Here we can see a much more complex program, with various
       optimisations in play. At regnode 10 we see an example
       where a character class with only one character in it was
       turned into an "EXACT" node. We can also see where an
       entire alternation was turned into a "TRIE-EXACT" node. As
       a consequence, some of the regnodes have been marked as
       optimised away. We can see that the "$" symbol has been
       converted into an "EOL" regop, a special piece of code
       that looks for "\n" or the end of the string.

       The next pointer for "BRANCH"es is interesting in that it
       points at where execution should go if the branch fails.
       When executing, if the engine tries to traverse from a
       branch to a "regnext" that isn't a branch then the engine
       will know that the entire set of branches has failed.

       Peep-hole Optimisation and Analysis

       The regular expression engine can be a weighty tool to
       wield. On long strings and complex patterns it can end up
       having to do a lot of work to find a match, and even more
       to decide that no match is possible.  Consider a situation
       like the following pattern.

          'ababababababababababab' =~ /(a|b)*z/

       The "(a|b)*" part can match at every char in the string,
       and then fail every time because there is no "z" in the
       string. So obviously we can avoid using the regex engine
       unless there is a "z" in the string.  Likewise in a
       pattern like:

          /foo(\w+)bar/




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       In this case we know that the string must contain a "foo"
       which must be followed by "bar". We can use Fast Boyer-
       Moore matching as implemented in "fbm_instr()" to find the
       location of these strings. If they don't exist then we
       don't need to resort to the much more expensive regex
       engine.  Even better, if they do exist then we can use
       their positions to reduce the search space that the regex
       engine needs to cover to determine if the entire pattern
       matches.

       There are various aspects of the pattern that can be used
       to facilitate optimisations along these lines:

       o    anchored fixed strings

       o    floating fixed strings

       o    minimum and maximum length requirements

       o    start class

       o    Beginning/End of line positions

       Another form of optimisation that can occur is the post-
       parse "peep-hole" optimisation, where inefficient
       constructs are replaced by more efficient constructs. The
       "TAIL" regops which are used during parsing to mark the
       end of branches and the end of groups are examples of
       this. These regops are used as place-holders during
       construction and "always match" so they can be "optimised
       away" by making the things that point to the "TAIL" point
       to the thing that "TAIL" points to, thus "skipping" the
       node.

       Another optimisation that can occur is that of ""EXACT"
       merging" which is where two consecutive "EXACT" nodes are
       merged into a single regop. An even more aggressive form
       of this is that a branch sequence of the form "EXACT
       BRANCH ... EXACT" can be converted into a "TRIE-EXACT"
       regop.

       All of this occurs in the routine "study_chunk()" which
       uses a special structure "scan_data_t" to store the
       analysis that it has performed, and does the "peep-hole"
       optimisations as it goes.

       The code involved in "study_chunk()" is extremely cryptic.
       Be careful. :-)

   Execution
       Execution of a regex generally involves two phases, the
       first being finding the start point in the string where we
       should match from, and the second being running the regop
       interpreter.



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       If we can tell that there is no valid start point then we
       don't bother running interpreter at all. Likewise, if we
       know from the analysis phase that we cannot detect a
       short-cut to the start position, we go straight to the
       interpreter.

       The two entry points are "re_intuit_start()" and
       "pregexec()". These routines have a somewhat incestuous
       relationship with overlap between their functions, and
       "pregexec()" may even call "re_intuit_start()" on its own.
       Nevertheless other parts of the the perl source code may
       call into either, or both.

       Execution of the interpreter itself used to be recursive,
       but thanks to the efforts of Dave Mitchell in the 5.9.x
       development track, that has changed: now an internal stack
       is maintained on the heap and the routine is fully
       iterative. This can make it tricky as the code is quite
       conservative about what state it stores, with the result
       that that two consecutive lines in the code can actually
       be running in totally different contexts due to the
       simulated recursion.

       Start position and no-match optimisations

       "re_intuit_start()" is responsible for handling start
       points and no-match optimisations as determined by the
       results of the analysis done by "study_chunk()" (and
       described in "Peep-hole Optimisation and Analysis").

       The basic structure of this routine is to try to find the
       start- and/or end-points of where the pattern could match,
       and to ensure that the string is long enough to match the
       pattern. It tries to use more efficient methods over less
       efficient methods and may involve considerable cross-
       checking of constraints to find the place in the string
       that matches.  For instance it may try to determine that a
       given fixed string must be not only present but a certain
       number of chars before the end of the string, or whatever.

       It calls several other routines, such as "fbm_instr()"
       which does Fast Boyer Moore matching and "find_byclass()"
       which is responsible for finding the start using the first
       mandatory regop in the program.

       When the optimisation criteria have been satisfied,
       "reg_try()" is called to perform the match.

       Program execution

       "pregexec()" is the main entry point for running a regex.
       It contains support for initialising the regex
       interpreter's state, running "re_intuit_start()" if
       needed, and running the interpreter on the string from



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       various start positions as needed. When it is necessary to
       use the regex interpreter "pregexec()" calls "regtry()".

       "regtry()" is the entry point into the regex interpreter.
       It expects as arguments a pointer to a "regmatch_info"
       structure and a pointer to a string.  It returns an
       integer 1 for success and a 0 for failure.  It is
       basically a set-up wrapper around "regmatch()".

       "regmatch" is the main "recursive loop" of the
       interpreter. It is basically a giant switch statement that
       implements a state machine, where the possible states are
       the regops themselves, plus a number of additional
       intermediate and failure states. A few of the states are
       implemented as subroutines but the bulk are inline code.

MISCELLANEOUS
   Unicode and Localisation Support
       When dealing with strings containing characters that
       cannot be represented using an eight-bit character set,
       perl uses an internal representation that is a permissive
       version of Unicode's UTF-8 encoding[2]. This uses single
       bytes to represent characters from the ASCII character
       set, and sequences of two or more bytes for all other
       characters. (See perlunitut for more information about the
       relationship between UTF-8 and perl's encoding, utf8 --
       the difference isn't important for this discussion.)

       No matter how you look at it, Unicode support is going to
       be a pain in a regex engine. Tricks that might be fine
       when you have 256 possible characters often won't scale to
       handle the size of the UTF-8 character set.  Things you
       can take for granted with ASCII may not be true with
       Unicode. For instance, in ASCII, it is safe to assume that
       "sizeof(char1) == sizeof(char2)", but in UTF-8 it isn't.
       Unicode case folding is vastly more complex than the
       simple rules of ASCII, and even when not using Unicode but
       only localised single byte encodings, things can get
       tricky (for example, LATIN SMALL LETTER SHARP S (U+00DF,
       ss) should match 'SS' in localised case-insensitive
       matching).

       Making things worse is that UTF-8 support was a later
       addition to the regex engine (as it was to perl) and this
       necessarily  made things a lot more complicated. Obviously
       it is easier to design a regex engine with Unicode support
       in mind from the beginning than it is to retrofit it to
       one that wasn't.

       Nearly all regops that involve looking at the input string
       have two cases, one for UTF-8, and one not. In fact, it's
       often more complex than that, as the pattern may be UTF-8
       as well.




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       Care must be taken when making changes to make sure that
       you handle UTF-8 properly, both at compile time and at
       execution time, including when the string and pattern are
       mismatched.

       The following comment in regcomp.h gives an example of
       exactly how tricky this can be:

           Two problematic code points in Unicode casefolding of EXACT nodes:

           U+0390 - GREEK SMALL LETTER IOTA WITH DIALYTIKA AND TONOS
           U+03B0 - GREEK SMALL LETTER UPSILON WITH DIALYTIKA AND TONOS

           which casefold to

           Unicode                      UTF-8

           U+03B9 U+0308 U+0301         0xCE 0xB9 0xCC 0x88 0xCC 0x81
           U+03C5 U+0308 U+0301         0xCF 0x85 0xCC 0x88 0xCC 0x81

           This means that in case-insensitive matching (or "loose matching",
           as Unicode calls it), an EXACTF of length six (the UTF-8 encoded
           byte length of the above casefolded versions) can match a target
           string of length two (the byte length of UTF-8 encoded U+0390 or
           U+03B0). This would rather mess up the minimum length computation.

           What we'll do is to look for the tail four bytes, and then peek
           at the preceding two bytes to see whether we need to decrease
           the minimum length by four (six minus two).

           Thanks to the design of UTF-8, there cannot be false matches:
           A sequence of valid UTF-8 bytes cannot be a subsequence of
           another valid sequence of UTF-8 bytes.

   Base Structures
       The "regexp" structure described in perlreapi is common to
       all regex engines. Two of its fields that are intended for
       the private use of the regex engine that compiled the
       pattern. These are the "intflags" and pprivate members.
       The "pprivate" is a void pointer to an arbitrary structure
       whose use and management is the responsibility of the
       compiling engine. perl will never modify either of these
       values. In the case of the stock engine the structure
       pointed to by "pprivate" is called "regexp_internal".

       Its "pprivate" and "intflags" fields contain data specific
       to each engine.

       There are two structures used to store a compiled regular
       expression.  One, the "regexp" structure described in
       perlreapi is populated by the engine currently being. used
       and some of its fields read by perl to implement things
       such as the stringification of "qr//".




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       The other structure is pointed to be the "regexp" struct's
       "pprivate" and is in addition to "intflags" in the same
       struct considered to be the property of the regex engine
       which compiled the regular expression;

       The regexp structure contains all the data that perl needs
       to be aware of to properly work with the regular
       expression. It includes data about optimisations that perl
       can use to determine if the regex engine should really be
       used, and various other control info that is needed to
       properly execute patterns in various contexts such as is
       the pattern anchored in some way, or what flags were used
       during the compile, or whether the program contains
       special constructs that perl needs to be aware of.

       In addition it contains two fields that are intended for
       the private use of the regex engine that compiled the
       pattern. These are the "intflags" and pprivate members.
       The "pprivate" is a void pointer to an arbitrary structure
       whose use and management is the responsibility of the
       compiling engine. perl will never modify either of these
       values.

       As mentioned earlier, in the case of the default engines,
       the "pprivate" will be a pointer to a regexp_internal
       structure which holds the compiled program and any
       additional data that is private to the regex engine
       implementation.

       Perl's "pprivate" structure

       The following structure is used as the "pprivate" struct
       by perl's regex engine. Since it is specific to perl it is
       only of curiosity value to other engine implementations.

           typedef struct regexp_internal {
                   regexp_paren_ofs *swap; /* Swap copy of *startp / *endp */
                   U32 *offsets;           /* offset annotations 20001228 MJD
                                              data about mapping the program to the
                                              string*/
                   regnode *regstclass;    /* Optional startclass as identified or constructed
                                              by the optimiser */
                   struct reg_data *data;  /* Additional miscellaneous data used by the program.
                                              Used to make it easier to clone and free arbitrary
                                              data that the regops need. Often the ARG field of
                                              a regop is an index into this structure */
                   regnode program[1];     /* Unwarranted chumminess with compiler. */
           } regexp_internal;

       "swap"
            "swap" is an extra set of startp/endp stored in a
            "regexp_paren_ofs" struct. This is used when the last
            successful match was from the same pattern as the
            current pattern, so that a partial match doesn't



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            overwrite the previous match's results. When this
            field is data filled the matching engine will swap
            buffers before every match attempt. If the match
            fails, then it swaps them back. If it's successful it
            leaves them. This field is populated on demand and is
            by default null.

       "offsets"
            Offsets holds a mapping of offset in the "program" to
            offset in the "precomp" string. This is only used by
            ActiveState's visual regex debugger.

       "regstclass"
            Special regop that is used by "re_intuit_start()" to
            check if a pattern can match at a certain position.
            For instance if the regex engine knows that the
            pattern must start with a 'Z' then it can scan the
            string until it finds one and then launch the regex
            engine from there. The routine that handles this is
            called "find_by_class()". Sometimes this field points
            at a regop embedded in the program, and sometimes it
            points at an independent synthetic regop that has
            been constructed by the optimiser.

       "data"
            This field points at a reg_data structure, which is
            defined as follows

                struct reg_data {
                    U32 count;
                    U8 *what;
                    void* data[1];
                };

            This structure is used for handling data structures
            that the regex engine needs to handle specially
            during a clone or free operation on the compiled
            product. Each element in the data array has a
            corresponding element in the what array. During
            compilation regops that need special structures
            stored will add an element to each array using the
            add_data() routine and then store the index in the
            regop.

       "program"
            Compiled program. Inlined into the structure so the
            entire struct can be treated as a single blob.

SEE ALSO
       perlreapi

       perlre

       perlunitut



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AUTHOR
       by Yves Orton, 2006.

       With excerpts from Perl, and contributions and suggestions
       from Ronald J. Kimball, Dave Mitchell, Dominic Dunlop,
       Mark Jason Dominus, Stephen McCamant, and David Landgren.

LICENCE
       Same terms as Perl.

REFERENCES
       [1] <http://perl.plover.com/Rx/paper/>

       [2] <http://www.unicode.org>











































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