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PERLREAPI(1)	       Perl Programmers Reference Guide		  PERLREAPI(1)

NAME
       perlreapi - perl regular expression plugin interface

DESCRIPTION
       As of Perl 5.9.5 there is a new interface for plugging and using other
       regular expression engines 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);
	   #endif

       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.

       In order to install a new regexp handler, $^H{regcomp} is set to an
       integer which (when casted appropriately) resolves to one of these
       structures. When compiling, the "comp" method is executed, and the
       resulting regexp structure's engine field is expected to point back at
       the same structure.

       The pTHX_ symbol in the definition is a macro used by perl under
       threading to provide an extra argument to the routine holding a pointer
       back to the interpreter that is executing the regexp. So under
       threading all routines get an extra argument.

Callbacks
   comp
	   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 whether "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 whether 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
	   string.

       "/s" - RXf_PMf_SINGLELINE
       "/i" - RXf_PMf_FOLD
       "/x" - RXf_PMf_EXTENDED
	   If present on a regex "#" comments will be handled differently by
	   the tokenizer in some cases.

	   TODO: Document those cases.

       "/p" - RXf_PMf_KEEPCOPY

       Additional flags:

       RXf_PMf_LOCALE
	   Set if "use locale" is in effect. If present in "rx->extflags"
	   "split" will use the locale dependent definition of whitespace
	   under when RXf_SKIPWHITE or RXf_WHITE are in effect. Under ASCII
	   whitespace is defined as per isSPACE, and by the internal macros
	   "is_utf8_space" under UTF-8 and "isSPACE_LC" under "use locale".

       RXf_UTF8
	   Set if the pattern is SvUTF8(), set by Perl_pmruntime.

	   A regex engine may want to set or disable this flag during
	   compilation. The perl engine for instance may upgrade non-UTF-8
	   strings to UTF-8 if the pattern includes constructs such as
	   "\x{...}" that can only match Unicode values.

       RXf_SPLIT
	   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.

       RXf_SKIPWHITE
	   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 whether the
	   subject is a UTF-8 string and whether 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.

       RXf_START_ONLY
	   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.

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

	   Perl's engine sets this flag if the pattern is "\s+".

       RXf_NULL
	   Tells the split operator to split the target string on characters.
	   The definition of character varies depending on whether the target
	   string is a UTF-8 string.

	   Perl's engine sets this flag on empty patterns, this optimization
	   makes "split //" much faster than it would otherwise be. It's even
	   faster than "unpack".

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

       Execute a regexp.

   intuit
	   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 whether 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.

   checkstr
	   SV* checkstr(pTHX_ REGEXP * const rx);

       Return a SV containing a string that must appear in the pattern. Used
       by "split" for optimising matches.

   free
	   void free(pTHX_ REGEXP * const rx);

       Called by perl when it is freeing a regexp pattern so that the engine
       can release any resources pointed to by the "pprivate" member of the
       regexp structure. This is only responsible for freeing private data;
       perl will handle releasing anything else contained in the regexp
       structure.

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

       The "paren" parameter will be "-2" for "$`", "-1" for "$'", 0 for $&, 1
       for $1 and so forth.

       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
       magic.

       numbered_buff_FETCH

	   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 perlapi.

       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.

       numbered_buff_STORE

	   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).

       Example:

	   if ("ook" =~ /(o*)/) {
	       # `paren' will be `1' and `value' will be `ee'
	       $1 =~ tr/o/e/;
	   }

       Perl's own engine will croak on any attempt to modify the capture
       variables, to do this in another engine use the following callback
       (copied from "Perl_reg_numbered_buff_store"):

	   void
	   Example_reg_numbered_buff_store(pTHX_ REGEXP * const rx, const I32 paren,
								   SV const * const value)
	   {
	       PERL_UNUSED_ARG(rx);
	       PERL_UNUSED_ARG(paren);
	       PERL_UNUSED_ARG(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 => "CatptureVar";
	   $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.

       numbered_buff_LENGTH

	   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;

       This is a little bit more complex in the case of UTF-8, see what
       "Perl_reg_numbered_buff_length" does with is_utf8_string_loclen.

   Named capture callbacks
       Called to get/set the value of "%+" and "%-" as well as by some utility
       functions in re.

       There are two callbacks, "named_buff" is called in all the cases the
       FETCH, STORE, DELETE, CLEAR, EXISTS and SCALAR Tie::Hash callbacks
       would be on changes to "%+" and "%-" and "named_buff_iter" in the same
       cases as FIRSTKEY and NEXTKEY.

       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:

	   RXapif_FETCH
	   RXapif_STORE
	   RXapif_DELETE
	   RXapif_CLEAR
	   RXapif_EXISTS
	   RXapif_SCALAR
	   RXapif_FIRSTKEY
	   RXapif_NEXTKEY

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

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

       Whether 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".

	   RXapif_REGNAME
	   RXapif_REGNAMES
	   RXapif_REGNAMES_COUNT

       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).

       named_buff

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

       named_buff_iter

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

   qr_package
	   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 whether they implement methods on
       the object.

       The package this method returns should also have the internal "Regexp"
       package in its @ISA. "qr//-"isa("Regexp")> should always be true
       regardless of what engine is being used.

       Example implementation might be:

	   SV*
	   Example_qr_package(pTHX_ REGEXP * const rx)
	   {
	       PERL_UNUSED_ARG(rx);
	       return newSVpvs("re::engine::Example");
	   }

       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)
	   PPCODE:
	       REGEXP * re = SvRX(sv);

   dupe
	   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
       routine.)

       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.

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 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. "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.

	   typedef struct regexp {
	       /* what engine created this regexp? */
	       const struct regexp_engine* engine;

	       /* what re is this a lightweight copy of? */
	       struct regexp* mother_re;

	       /* Information about the match that the perl core uses to manage things */
	       U32 extflags;   /* Flags used both externally and internally */
	       I32 minlen;     /* mininum possible length of string to match */
	       I32 minlenret;  /* mininum possible length of $& */
	       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 buffers */

	       /* 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;		 /* last open paren matched */
	       U32 lastcloseparen;	 /* last close paren matched */
	       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 */

	       /* 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:

   "engine"
       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.

       Internally this is set to "NULL" unless a custom engine is specified in
       $^H{regcomp}, perl's own set of callbacks can be accessed in the struct
       pointed to by "RE_ENGINE_PTR".

   "mother_re"
       TODO, see
       <http://www.mail-archive.com/perl5-changes@perl.org/msg17328.html>

   "extflags"
       This will be used by perl to see what flags the regexp was compiled
       with, this will normally be set to the value of the flags parameter by
       the comp callback. See the comp documentation for valid flags.

   "minlen" "minlenret"
       The minimum string length 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 of the string that would be found in
       $& after a match.

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

	   /ns(?=\d)/

       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 whether it can do in-place
       substitution which can result in considerable speedup.

   "gofs"
       Left offset from pos() to start match at.

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

   "nparens", "lasparen", 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.

   "intflags"
       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.

   "pprivate"
       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.

   "swap"
       TODO: document

   "offs"
       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:

	   typedef struct regexp_paren_pair {
	       I32 start;
	       I32 end;
	   } regexp_paren_pair;

       If "->offs[num].start" or "->offs[num].end" is "-1" then that capture
       buffer did not match. "->offs[0].start/end" represents $& (or
       "${^MATCH" under "//p") and "->offs[paren].end" matches $$paren where
       $paren = 1>.

   "precomp" "prelen"
       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 whether
       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 */

   "paren_names"
       This is a hash used internally to track named capture buffers 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.

   "substrs"
       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"
       Used during execution phase for managing search and replace patterns.

   "wrapped" "wraplen"
       Stores the string "qr//" stringifies to. The perl engine for example
       stores "(?-xism: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"
	   my $y = qr/c/i;   # "c"
	   my $z = qr/$x$y/; # "a|bc"

       There's no solution for this problem other than making the custom
       engine understand a construct like "(?:)".

   "seen_evals"
       This stores the number of eval groups in the pattern. This is used for
       security purposes when embedding compiled regexes into larger patterns
       with "qr//".

   "refcnt"
       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.

HISTORY
       Originally part of perlreguts.

AUTHORS
       Originally written by Yves Orton, expanded by var Arnfjoer` Bjarmason.

LICENSE
       Copyright 2006 Yves Orton and 2007 var Arnfjoer` Bjarmason.

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

perl v5.10.1			  2009-07-27			  PERLREAPI(1)
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