


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.




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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:





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       "/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.



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           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



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       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:



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           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



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       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



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       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);





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   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 */



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               /* 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>





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   "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



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       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.




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   "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                2009-10-12                         13


