As of Perl 5.9.5 there is a new interface for plugging and using
       regular expression engines other than the default one.

       Each engine is supposed to provide access to a constant structure of
       the following format:

           typedef struct regexp_engine {
               REGEXP* (*comp) (pTHX_
                                const SV * const pattern, const U32 flags);
               I32     (*exec) (pTHX_
                                REGEXP * const rx,
                                char* stringarg,
                                char* strend, char* strbeg,
                                I32 minend, SV* screamer,
                                void* data, U32 flags);
               char*   (*intuit) (pTHX_
                                  REGEXP * const rx, SV *sv,
                                  char *strpos, char *strend, U32 flags,
                                  struct re_scream_pos_data_s *data);
               SV*     (*checkstr) (pTHX_ REGEXP * const rx);
               void    (*free) (pTHX_ REGEXP * const rx);
               void    (*numbered_buff_FETCH) (pTHX_
                                               REGEXP * const rx,
                                               const I32 paren,
                                               SV * const sv);
               void    (*numbered_buff_STORE) (pTHX_
                                               REGEXP * const rx,
                                               const I32 paren,
                                               SV const * const value);
               I32     (*numbered_buff_LENGTH) (pTHX_
                                                REGEXP * const rx,
                                                const SV * const sv,
                                                const I32 paren);
               SV*     (*named_buff) (pTHX_
                                      REGEXP * const rx,
                                      SV * const key,
                                      SV * const value,
                                      U32 flags);
               SV*     (*named_buff_iter) (pTHX_
                                           REGEXP * const rx,
                                           const SV * const lastkey,
                                           const U32 flags);
               SV*     (*qr_package)(pTHX_ REGEXP * const rx);
           #ifdef USE_ITHREADS
               void*   (*dupe) (pTHX_ REGEXP * const rx, CLONE_PARAMS *param);
               REGEXP* (*op_comp) (...);

       When a regexp is compiled, its "engine" field is then set to point at
       the appropriate structure, so that when it needs to be used Perl can
       find the right routines to do so.

           REGEXP* comp(pTHX_ const SV * const pattern, const U32 flags);

       Compile the pattern stored in "pattern" using the given "flags" and
       return a pointer to a prepared "REGEXP" structure that can perform the
       match.  See "The REGEXP structure" below for an explanation of the
       individual fields in the REGEXP struct.

       The "pattern" parameter is the scalar that was used as the pattern.
       Previous versions of Perl would pass two "char*" indicating the start
       and end of the stringified pattern; the following snippet can be used
       to get the old parameters:

           STRLEN plen;
           char*  exp = SvPV(pattern, plen);
           char* xend = exp + plen;

       Since any scalar can be passed as a pattern, it's possible to implement
       an engine that does something with an array (""ook" =~ [ qw/ eek hlagh
       / ]") or with the non-stringified form of a compiled regular expression
       (""ook" =~ qr/eek/").  Perl's own engine will always stringify
       everything using the snippet above, but that doesn't mean other engines
       have to.

       The "flags" parameter is a bitfield which indicates which of the
       "msixp" flags the regex was compiled with.  It also contains additional
       info, such as if "use locale" is in effect.

       The "eogc" flags are stripped out before being passed to the comp
       routine.  The regex engine does not need to know if any of these are
       set, as those flags should only affect what Perl does with the pattern
       and its match variables, not how it gets compiled and executed.

       By the time the comp callback is called, some of these flags have
       already had effect (noted below where applicable).  However most of
       their effect occurs after the comp callback has run, in routines that
       read the "rx->extflags" field which it populates.

       In general the flags should be preserved in "rx->extflags" after
       compilation, although the regex engine might want to add or delete some
       of them to invoke or disable some special behavior in Perl.  The flags
       along with any special behavior they cause are documented below:

       The pattern modifiers:

       "/m" - RXf_PMf_MULTILINE
           If this is in "rx->extflags" it will be passed to "Perl_fbm_instr"
           by "pp_split" which will treat the subject string as a multi-line

       "/s" - RXf_PMf_SINGLELINE
       "/i" - RXf_PMf_FOLD
       "/x" - RXf_PMf_EXTENDED
           If present on a regex, "#" comments will be handled differently by
           currently documented value returned from it is
           REGEX_LOCALE_CHARSET, which is set if "use locale" is in effect. If
           present in "rx->extflags", "split" will use the locale dependent
           definition of whitespace when RXf_SKIPWHITE or RXf_WHITE is in
           effect.  ASCII whitespace is defined as per isSPACE, and by the
           internal macros "is_utf8_space" under UTF-8, and "isSPACE_LC" under
           "use locale".

       Additional flags:

           This flag was removed in perl 5.18.0.  "split ' '" is now special-
           cased solely in the parser.  RXf_SPLIT is still #defined, so you
           can test for it.  This is how it used to work:

           If "split" is invoked as "split ' '" or with no arguments (which
           really means "split(' ', $_)", see split), Perl will set this flag.
           The regex engine can then check for it and set the SKIPWHITE and
           WHITE extflags.  To do this, the Perl engine does:

               if (flags & RXf_SPLIT && r->prelen == 1 && r->precomp[0] == ' ')
                   r->extflags |= (RXf_SKIPWHITE|RXf_WHITE);

       These flags can be set during compilation to enable optimizations in
       the "split" operator.

           This flag was removed in perl 5.18.0.  It is still #defined, so you
           can set it, but doing so will have no effect.  This is how it used
           to work:

           If the flag is present in "rx->extflags" "split" will delete
           whitespace from the start of the subject string before it's
           operated on.  What is considered whitespace depends on if the
           subject is a UTF-8 string and if the "RXf_PMf_LOCALE" flag is set.

           If RXf_WHITE is set in addition to this flag, "split" will behave
           like "split " "" under the Perl engine.

           Tells the split operator to split the target string on newlines
           ("\n") without invoking the regex engine.

           Perl's engine sets this if the pattern is "/^/" ("plen == 1 && *exp
           == '^'"), even under "/^/s"; see split.  Of course a different
           regex engine might want to use the same optimizations with a
           different syntax.

           Tells the split operator to split the target string on whitespace
           without invoking the regex engine.  The definition of whitespace
           varies depending on if the target string is a UTF-8 string and on
           if RXf_PMf_LOCALE is set.

           Added in perl 5.18.0, this flag indicates that a regular expression
           might perform an operation that would interfere with inplace
           substituion. For instance it might contain lookbehind, or assign to
           non-magical variables (such as $REGMARK and $REGERROR) during
           matching.  "s///" will skip certain optimisations when this is set.

           I32 exec(pTHX_ REGEXP * const rx,
                    char *stringarg, char* strend, char* strbeg,
                    I32 minend, SV* screamer,
                    void* data, U32 flags);

       Execute a regexp. The arguments are

       rx  The regular expression to execute.

           This strangely-named arg is the SV to be matched against.  Note
           that the actual char array to be matched against is supplied by the
           arguments described below; the SV is just used to determine
           UTF8ness, "pos()" etc.

           Pointer to the physical start of the string.

           Pointer to the character following the physical end of the string
           (i.e.  the "\0").

           Pointer to the position in the string where matching should start;
           it might not be equal to "strbeg" (for example in a later iteration
           of "/.../g").

           Minimum length of string (measured in bytes from "stringarg") that
           must match; if the engine reaches the end of the match but hasn't
           reached this position in the string, it should fail.

           Optimisation data; subject to change.

           Optimisation flags; subject to change.

           char* intuit(pTHX_ REGEXP * const rx,
                         SV *sv, char *strpos, char *strend,
                         const U32 flags, struct re_scream_pos_data_s *data);

       Find the start position where a regex match should be attempted, or
       possibly if the regex engine should not be run because the pattern
       can't match.  This is called, as appropriate, by the core, depending on
       the values of the "extflags" member of the "regexp" structure.
       "regexp" structure.  This is only responsible for freeing private data;
       Perl will handle releasing anything else contained in the "regexp"

   Numbered capture callbacks
       Called to get/set the value of "$`", "$'", $& and their named
       equivalents, ${^PREMATCH}, ${^POSTMATCH} and $^{MATCH}, as well as the
       numbered capture groups ($1, $2, ...).

       The "paren" parameter will be 1 for $1, 2 for $2 and so forth, and have
       these symbolic values for the special variables:

           $`            RX_BUFF_IDX_PREMATCH
           $'            RX_BUFF_IDX_POSTMATCH
           $&            RX_BUFF_IDX_FULLMATCH

       Note that in Perl 5.17.3 and earlier, the last three constants were
       also used for the caret variants of the variables.

       The names have been chosen by analogy with Tie::Scalar methods names
       with an additional LENGTH callback for efficiency.  However named
       capture variables are currently not tied internally but implemented via


           void numbered_buff_FETCH(pTHX_ REGEXP * const rx, const I32 paren,
                                    SV * const sv);

       Fetch a specified numbered capture.  "sv" should be set to the scalar
       to return, the scalar is passed as an argument rather than being
       returned from the function because when it's called Perl already has a
       scalar to store the value, creating another one would be redundant.
       The scalar can be set with "sv_setsv", "sv_setpvn" and friends, see

       This callback is where Perl untaints its own capture variables under
       taint mode (see perlsec).  See the "Perl_reg_numbered_buff_fetch"
       function in regcomp.c for how to untaint capture variables if that's
       something you'd like your engine to do as well.


           void    (*numbered_buff_STORE) (pTHX_
                                           REGEXP * const rx,
                                           const I32 paren,
                                           SV const * const value);

       Set the value of a numbered capture variable.  "value" is the scalar
       that is to be used as the new value.  It's up to the engine to make
       sure this is used as the new value (or reject it).
                                           REGEXP * const rx,
                                           const I32 paren,
                                           SV const * const value)

               if (!PL_localizing)
                   Perl_croak(aTHX_ PL_no_modify);

       Actually Perl will not always croak in a statement that looks like it
       would modify a numbered capture variable.  This is because the STORE
       callback will not be called if Perl can determine that it doesn't have
       to modify the value.  This is exactly how tied variables behave in the
       same situation:

           package CaptureVar;
           use base 'Tie::Scalar';

           sub TIESCALAR { bless [] }
           sub FETCH { undef }
           sub STORE { die "This doesn't get called" }

           package main;

           tie my $sv => "CaptureVar";
           $sv =~ y/a/b/;

       Because $sv is "undef" when the "y///" operator is applied to it, the
       transliteration won't actually execute and the program won't "die".
       This is different to how 5.8 and earlier versions behaved since the
       capture variables were READONLY variables then; now they'll just die
       when assigned to in the default engine.


           I32 numbered_buff_LENGTH (pTHX_
                                     REGEXP * const rx,
                                     const SV * const sv,
                                     const I32 paren);

       Get the "length" of a capture variable.  There's a special callback for
       this so that Perl doesn't have to do a FETCH and run "length" on the
       result, since the length is (in Perl's case) known from an offset
       stored in "rx->offs", this is much more efficient:

           I32 s1  = rx->offs[paren].start;
           I32 s2  = rx->offs[paren].end;
           I32 len = t1 - s1;

       The "flags" parameter can be used to determine which of these
       operations the callbacks should respond to.  The following flags are
       currently defined:

       Which Tie::Hash operation is being performed from the Perl level on
       "%+" or "%+", if any:


       If "%+" or "%-" is being operated on, if any.

           RXapif_ONE /* %+ */
           RXapif_ALL /* %- */

       If this is being called as "re::regname", "re::regnames" or
       "re::regnames_count", if any.  The first two will be combined with
       "RXapif_ONE" or "RXapif_ALL".


       Internally "%+" and "%-" are implemented with a real tied interface via
       Tie::Hash::NamedCapture.  The methods in that package will call back
       into these functions.  However the usage of Tie::Hash::NamedCapture for
       this purpose might change in future releases.  For instance this might
       be implemented by magic instead (would need an extension to mgvtbl).


           SV*     (*named_buff) (pTHX_ REGEXP * const rx, SV * const key,
                                  SV * const value, U32 flags);


           SV*     (*named_buff_iter) (pTHX_
                                       REGEXP * const rx,
                                       const SV * const lastkey,
                                       const U32 flags);

           SV* qr_package(pTHX_ REGEXP * const rx);

       The package the qr// magic object is blessed into (as seen by "ref
       qr//").  It is recommended that engines change this to their package
       name for identification regardless of if they implement methods on the

       Any method calls on an object created with "qr//" will be dispatched to
       the package as a normal object.

           use re::engine::Example;
           my $re = qr//;
           $re->meth; # dispatched to re::engine::Example::meth()

       To retrieve the "REGEXP" object from the scalar in an XS function use
       the "SvRX" macro, see "REGEXP Functions" in perlapi.

           void meth(SV * rv)
               REGEXP * re = SvRX(sv);

           void* dupe(pTHX_ REGEXP * const rx, CLONE_PARAMS *param);

       On threaded builds a regexp may need to be duplicated so that the
       pattern can be used by multiple threads.  This routine is expected to
       handle the duplication of any private data pointed to by the "pprivate"
       member of the "regexp" structure.  It will be called with the
       preconstructed new "regexp" structure as an argument, the "pprivate"
       member will point at the old private structure, and it is this
       routine's responsibility to construct a copy and return a pointer to it
       (which Perl will then use to overwrite the field as passed to this

       This allows the engine to dupe its private data but also if necessary
       modify the final structure if it really must.

       On unthreaded builds this field doesn't exist.

       This is private to the Perl core and subject to change. Should be left

The REGEXP structure
       The REGEXP struct is defined in regexp.h.  All regex engines must be
       able to correctly build such a structure in their "comp" routine.

       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 if the pattern
       anchored in some way, or what flags were used during the compile, or if
       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.  "pprivate" is a void pointer to an
       arbitrary structure, whose use and management is the responsibility of
               I32 minlen;     /* mininum possible number of chars in */
                                  string to match */
               I32 minlenret;  /* mininum possible number of chars in $& */
               U32 gofs;       /* chars left of pos that we search from */

               /* substring data about strings that must appear
                  in the final match, used for optimisations */
               struct reg_substr_data *substrs;

               U32 nparens;  /* number of capture groups */

               /* private engine specific data */
               U32 intflags;   /* Engine Specific Internal flags */
               void *pprivate; /* Data private to the regex engine which
                                  created this object. */

               /* Data about the last/current match. These are modified during
                * matching*/
               U32 lastparen;            /* highest close paren matched ($+) */
               U32 lastcloseparen;       /* last close paren matched ($^N) */
               regexp_paren_pair *swap;  /* Swap copy of *offs */
               regexp_paren_pair *offs;  /* Array of offsets for (@-) and
                                            (@+) */

               char *subbeg;  /* saved or original string so \digit works
                                 forever. */
               SV_SAVED_COPY  /* If non-NULL, SV which is COW from original */
               I32 sublen;    /* Length of string pointed by subbeg */
               I32 suboffset;  /* byte offset of subbeg from logical start of
                                  str */
               I32 subcoffset; /* suboffset equiv, but in chars (for @-/@+) */

               /* Information about the match that isn't often used */
               I32 prelen;           /* length of precomp */
               const char *precomp;  /* pre-compilation regular expression */

               char *wrapped;  /* wrapped version of the pattern */
               I32 wraplen;    /* length of wrapped */

               I32 seen_evals;   /* number of eval groups in the pattern - for
                                    security checks */
               HV *paren_names;  /* Optional hash of paren names */

               /* Refcount of this regexp */
               I32 refcnt;             /* Refcount of this regexp */
           } regexp;

       The fields are discussed in more detail below:

       This field points at a "regexp_engine" structure which contains
       pointers to the subroutines that are to be used for performing a match.
       It is the compiling routine's responsibility to populate this field
       before returning the regexp object.
       the comp callback.  See the comp documentation for valid flags.

   "minlen" "minlenret"
       The minimum string length (in characters) required for the pattern to
       match.  This is used to prune the search space by not bothering to
       match any closer to the end of a string than would allow a match.  For
       instance there is no point in even starting the regex engine if the
       minlen is 10 but the string is only 5 characters long.  There is no way
       that the pattern can match.

       "minlenret" is the minimum length (in characters) of the string that
       would be found in $& after a match.

       The difference between "minlen" and "minlenret" can be seen in the
       following pattern:


       where the "minlen" would be 3 but "minlenret" would only be 2 as the \d
       is required to match but is not actually included in the matched
       content.  This distinction is particularly important as the
       substitution logic uses the "minlenret" to tell if it can do in-place
       substitutions (these can result in considerable speed-up).

       Left offset from pos() to start match at.

       Substring data about strings that must appear in the final match.  This
       is currently only used internally by Perl's engine, but might be used
       in the future for all engines for optimisations.

   "nparens", "lastparen", and "lastcloseparen"
       These fields are used to keep track of how many paren groups could be
       matched in the pattern, which was the last open paren to be entered,
       and which was the last close paren to be entered.

       The engine's private copy of the flags the pattern was compiled with.
       Usually this is the same as "extflags" unless the engine chose to
       modify one of them.

       A void* pointing to an engine-defined data structure.  The Perl engine
       uses the "regexp_internal" structure (see "Base Structures" in
       perlreguts) but a custom engine should use something else.

       Unused.  Left in for compatibility with Perl 5.10.0.

       A "regexp_paren_pair" structure which defines offsets into the string
       being matched which correspond to the $& and $1, $2 etc. captures, the
       "regexp_paren_pair" struct is defined as follows:
       Used for optimisations.  "precomp" holds a copy of the pattern that was
       compiled and "prelen" its length.  When a new pattern is to be compiled
       (such as inside a loop) the internal "regcomp" operator checks if the
       last compiled "REGEXP"'s "precomp" and "prelen" are equivalent to the
       new one, and if so uses the old pattern instead of compiling a new one.

       The relevant snippet from "Perl_pp_regcomp":

               if (!re || !re->precomp || re->prelen != (I32)len ||
                   memNE(re->precomp, t, len))
               /* Compile a new pattern */

       This is a hash used internally to track named capture groups and their
       offsets.  The keys are the names of the buffers the values are
       dualvars, with the IV slot holding the number of buffers with the given
       name and the pv being an embedded array of I32.  The values may also be
       contained independently in the data array in cases where named
       backreferences are used.

       Holds information on the longest string that must occur at a fixed
       offset from the start of the pattern, and the longest string that must
       occur at a floating offset from the start of the pattern.  Used to do
       Fast-Boyer-Moore searches on the string to find out if its worth using
       the regex engine at all, and if so where in the string to search.

   "subbeg" "sublen" "saved_copy" "suboffset" "subcoffset"
       Used during the execution phase for managing search and replace
       patterns, and for providing the text for $&, $1 etc. "subbeg" points to
       a buffer (either the original string, or a copy in the case of
       "RX_MATCH_COPIED(rx)"), and "sublen" is the length of the buffer.  The
       "RX_OFFS" start and end indices index into this buffer.

       In the presence of the "REXEC_COPY_STR" flag, but with the addition of
       the "REXEC_COPY_SKIP_PRE" or "REXEC_COPY_SKIP_POST" flags, an engine
       can choose not to copy the full buffer (although it must still do so in
       the presence of "RXf_PMf_KEEPCOPY" or the relevant bits being set in
       "PL_sawampersand").  In this case, it may set "suboffset" to indicate
       the number of bytes from the logical start of the buffer to the
       physical start (i.e. "subbeg").  It should also set "subcoffset", the
       number of characters in the offset. The latter is needed to support
       "@-" and "@+" which work in characters, not bytes.

   "wrapped" "wraplen"
       Stores the string "qr//" stringifies to. The Perl engine for example
       stores "(?^:eek)" in the case of "qr/eek/".

       When using a custom engine that doesn't support the "(?:)" construct
       for inline modifiers, it's probably best to have "qr//" stringify to
       the supplied pattern, note that this will create undesired patterns in
       cases such as:

           my $x = qr/a|b/;  # "a|b"

       The number of times the structure is referenced.  When this falls to 0,
       the regexp is automatically freed by a call to pregfree.  This should
       be set to 1 in each engine's "comp" routine.

       Originally part of perlreguts.

       Originally written by Yves Orton, expanded by AEvar Arnfjoer`

       Copyright 2006 Yves Orton and 2007 AEvar Arnfjoer` Bjarmason.

       This program is free software; you can redistribute it and/or modify it
       under the same terms as Perl itself.

perl v5.18.2                      2014-01-06                      PERLREAPI(1)
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