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PERLREGUTS(1pm)        Perl Programmers Reference Guide        PERLREGUTS(1pm)




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 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 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 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 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"
           Bracketed 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 in the Latin1 range
           are included in the class.

               regnode_charclass        U32 arg1;
                                        char bitmap[ANYOF_BITMAP_SIZE];

           Various flags whose names begin with "ANYOF_" are used for special
           situations.  Above Latin1 matches and things not known until run-
           time are stored in "Perl's pprivate structure".

       "regnode_charclass_posixl"
           There is also a larger form of a char class structure used to
           represent POSIX char classes under "/l" matching, called
           "regnode_charclass_posixl" which has an additional 32-bit bitmap
           indicating which POSIX char classes have been included.

              regnode_charclass_posixl U32 arg1;
                                       char bitmap[ANYOF_BITMAP_SIZE];
                                       U32 classflags;

       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; "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 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 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 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:

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

       Parsing complications

       The implication of the above description is that a pattern containing
       nested parentheses will result in a call graph which cycles through
       "reg()", "regbranch()", "regpiece()", "regatom()", "reg()",
       "regbranch()" etc multiple times, until the deepest level of nesting is
       reached. All the above routines return a pointer to a "regnode", which
       is usually the last regnode added to the program. However, one
       complication is that reg() returns NULL for parsing "(?:)" syntax for
       embedded modifiers, setting the flag "TRYAGAIN". The "TRYAGAIN"
       propagates upwards until it is captured, in some cases by "regatom()",
       but otherwise unconditionally by "regbranch()". Hence it will never be
       returned by "regbranch()" to "reg()". This flag permits patterns such
       as "(?i)+" to be detected as errors (Quantifier follows nothing in
       regex; marked by <-- HERE in m/(?i)+ <-- HERE /).

       Another complication is that the representation used for the program
       differs if it needs to store Unicode, but it's not always possible to
       know for sure whether it does until midway through parsing. The Unicode
       representation for the program is larger, and cannot be matched as
       efficiently. (See "Unicode and Localisation Support" below for more
       details as to why.)  If the pattern contains literal Unicode, it's
       obvious that the program needs to store Unicode. Otherwise, the parser
       optimistically assumes that the more efficient representation can be
       used, and starts sizing on this basis.  However, if it then encounters
       something in the pattern which must be stored as Unicode, such as an
       "\x{...}" escape sequence representing a character literal, then this
       means that all previously calculated sizes need to be redone, using
       values appropriate for the Unicode representation. Currently, all
       regular expression constructions which can trigger this are parsed by
       code in "regatom()".

       To avoid wasted work when a restart is needed, the sizing pass is
       abandoned - "regatom()" immediately returns NULL, setting the flag
       "RESTART_UTF8".  (This action is encapsulated using the macro
       "REQUIRE_UTF8".) This restart request is propagated up the call chain
       in a similar fashion, until it is "caught" in "Perl_re_op_compile()",
       which marks the pattern as containing Unicode, and restarts the sizing
       pass. It is also possible for constructions within run-time code blocks
       to turn out to need Unicode representation., which is signalled by
       "S_compile_runtime_code()" returning false to "Perl_re_op_compile()".

       The restart was previously implemented using a "longjmp" in "regatom()"
       back to a "setjmp" in "Perl_re_op_compile()", but this proved to be
       problematic as the latter is a large function containing many automatic
       variables, which interact badly with the emergent control flow of
       "setjmp".

       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.

        >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)
                                             ~ 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

          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/

       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.

       If we can tell that there is no valid start point then we don't bother
       running the 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 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 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 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.

       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.

   Base Structures
       The "regexp" structure described in perlreapi is common to all regex
       engines. Two of its fields 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//".

       The other structure is pointed to by 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 {
                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;

       "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(1)

       perlre(1)

       perlunitut(1)


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>



perl v5.24.0                      2016-02-05                   PERLREGUTS(1pm)

perl 5.24 - Generated Sun Dec 4 13:01:43 CST 2016
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