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+=head1 NAME
+
+perlreapi - perl regular expression plugin interface
+
+=head1 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 C<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, C<$^H{regcomp}> is set
+to an integer which (when casted appropriately) resolves to one of these
+structures. When compiling, the C<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.
+
+=head1 Callbacks
+
+=head2 comp
+
+ REGEXP* comp(pTHX_ const SV * const pattern, const U32 flags);
+
+Compile the pattern stored in C<pattern> using the given C<flags> and
+return a pointer to a prepared C<REGEXP> structure that can perform
+the match. See L</The REGEXP structure> below for an explanation of
+the individual fields in the REGEXP struct.
+
+The C<pattern> parameter is the scalar that was used as the
+pattern. previous versions of perl would pass two C<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 (C<< "ook" =~ [ qw/ eek
+hlagh / ] >>) or with the non-stringified form of a compiled regular
+expression (C<< "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 C<flags> parameter is a bitfield which indicates which of the
+C<msixp> flags the regex was compiled with. It also contains
+additional info such as whether C<use locale> is in effect.
+
+The C<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 C<< rx->extflags >> field which it populates.
+
+In general the flags should be preserved in C<< 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:
+
+=over 4
+
+=item C</m> - RXf_PMf_MULTILINE
+
+If this is in C<< rx->extflags >> it will be passed to
+C<Perl_fbm_instr> by C<pp_split> which will treat the subject string
+as a multi-line string.
+
+=item C</s> - RXf_PMf_SINGLELINE
+
+=item C</i> - RXf_PMf_FOLD
+
+=item C</x> - RXf_PMf_EXTENDED
+
+If present on a regex C<#> comments will be handled differently by the
+tokenizer in some cases.
+
+TODO: Document those cases.
+
+=item C</p> - RXf_PMf_KEEPCOPY
+
+=back
+
+Additional flags:
+
+=over 4
+
+=item RXf_PMf_LOCALE
+
+Set if C<use locale> is in effect. If present in C<< rx->extflags >>
+C<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 L<isSPACE|perlapi/ISSPACE>, and by the internal
+macros C<is_utf8_space> under UTF-8 and C<isSPACE_LC> under C<use
+locale>.
+
+=item RXf_UTF8
+
+Set if the pattern is L<SvUTF8()|perlapi/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 C<\x{...}>
+that can only match Unicode values.
+
+=item RXf_SPLIT
+
+If C<split> is invoked as C<split ' '> or with no arguments (which
+really means C<split(' ', $_)>, see L<split|perlfunc/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);
+
+=back
+
+These flags can be set during compilation to enable optimizations in
+the C<split> operator.
+
+=over 4
+
+=item RXf_SKIPWHITE
+
+If the flag is present in C<< rx->extflags >> C<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 C<RXf_PMf_LOCALE> flag is set.
+
+If RXf_WHITE is set in addition to this flag C<split> will behave like
+C<split " "> under the perl engine.
+
+=item RXf_START_ONLY
+
+Tells the split operator to split the target string on newlines
+(C<\n>) without invoking the regex engine.
+
+Perl's engine sets this if the pattern is C</^/> (C<plen == 1 && *exp
+== '^'>), even under C</^/s>, see L<split|perlfunc>. Of course a
+different regex engine might want to use the same optimizations
+with a different syntax.
+
+=item 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 C<\s+>.
+
+=item 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 C<split //> much faster than it would otherwise be. It's even
+faster than C<unpack>.
+
+=back
+
+=head2 exec
+
+ I32 exec(pTHX_ REGEXP * const rx,
+ char *stringarg, char* strend, char* strbeg,
+ I32 minend, SV* screamer,
+ void* data, U32 flags);
+
+Execute a regexp.
+
+=head2 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.
+
+=head2 checkstr
+
+ SV* checkstr(pTHX_ REGEXP * const rx);
+
+Return a SV containing a string that must appear in the pattern. Used
+by C<split> for optimising matches.
+
+=head2 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 C<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.
+
+=head2 Numbered capture callbacks
+
+Called to get/set the value of C<$`>, C<$'>, C<$&> and their named
+equivalents, ${^PREMATCH}, ${^POSTMATCH} and $^{MATCH}, as well as the
+numbered capture buffers (C<$1>, C<$2>, ...).
+
+The C<paren> parameter will be C<-2> for C<$`>, C<-1> for C<$'>, C<0>
+for C<$&>, C<1> for C<$1> and so forth.
+
+The names have been chosen by analogy with L<Tie::Scalar> methods
+names with an additional B<LENGTH> callback for efficiency. However
+named capture variables are currently not tied internally but
+implemented via magic.
+
+=head3 numbered_buff_FETCH
+
+ void numbered_buff_FETCH(pTHX_ REGEXP * const rx, const I32 paren,
+ SV * const sv);
+
+Fetch a specified numbered capture. C<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 C<sv_setsv>, C<sv_setpvn> and
+friends, see L<perlapi>.
+
+This callback is where perl untaints its own capture variables under
+taint mode (see L<perlsec>). See the C<Perl_reg_numbered_buff_fetch>
+function in F<regcomp.c> for how to untaint capture variables if
+that's something you'd like your engine to do as well.
+
+=head3 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. C<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 C<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 I<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";
+ $sv =~ y/a/b/;
+
+Because C<$sv> is C<undef> when the C<y///> operator is applied to it
+the transliteration won't actually execute and the program won't
+C<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.
+
+=head3 numbered_buff_LENGTH
+
+ I32 numbered_buff_LENGTH (pTHX_ REGEXP * const rx, const SV * const sv,
+ const I32 paren);
+
+Get the C<length> of a capture variable. There's a special callback
+for this so that perl doesn't have to do a FETCH and run C<length> on
+the result, since the length is (in perl's case) known from an offset
+stored in C<< 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
+C<Perl_reg_numbered_buff_length> does with
+L<is_utf8_string_loclen|perlapi/is_utf8_string_loclen>.
+
+=head2 Named capture callbacks
+
+Called to get/set the value of C<%+> and C<%-> as well as by some
+utility functions in L<re>.
+
+There are two callbacks, C<named_buff> is called in all the cases the
+FETCH, STORE, DELETE, CLEAR, EXISTS and SCALAR L<Tie::Hash> callbacks
+would be on changes to C<%+> and C<%-> and C<named_buff_iter> in the
+same cases as FIRSTKEY and NEXTKEY.
+
+The C<flags> parameter can be used to determine which of these
+operations the callbacks should respond to, the following flags are
+currently defined:
+
+Which L<Tie::Hash> operation is being performed from the Perl level on
+C<%+> or C<%+>, if any:
+
+ RXapif_FETCH
+ RXapif_STORE
+ RXapif_DELETE
+ RXapif_CLEAR
+ RXapif_EXISTS
+ RXapif_SCALAR
+ RXapif_FIRSTKEY
+ RXapif_NEXTKEY
+
+Whether C<%+> or C<%-> is being operated on, if any.
+
+ RXapif_ONE /* %+ */
+ RXapif_ALL /* %- */
+
+Whether this is being called as C<re::regname>, C<re::regnames> or
+C<re::regnames_count>, if any. The first two will be combined with
+C<RXapif_ONE> or C<RXapif_ALL>.
+
+ RXapif_REGNAME
+ RXapif_REGNAMES
+ RXapif_REGNAMES_COUNT
+
+Internally C<%+> and C<%-> are implemented with a real tied interface
+via L<Tie::Hash::NamedCapture>. The methods in that package will call
+back into these functions. However the usage of
+L<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).
+
+=head3 named_buff
+
+ SV* (*named_buff) (pTHX_ REGEXP * const rx, SV * const key,
+ SV * const value, U32 flags);
+
+=head3 named_buff_iter
+
+ SV* (*named_buff_iter) (pTHX_ REGEXP * const rx, const SV * const lastkey,
+ const U32 flags);
+
+=head2 qr_package
+
+ SV* qr_package(pTHX_ REGEXP * const rx);
+
+The package the qr// magic object is blessed into (as seen by C<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
+C<Regexp> package in its C<@ISA>. C<< 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 C<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 C<REGEXP> object from the scalar in an XS function use
+the C<SvRX> macro, see L<"REGEXP Functions" in perlapi|perlapi/REGEXP
+Functions>.
+
+ void meth(SV * rv)
+ PPCODE:
+ REGEXP * re = SvRX(sv);
+
+=head2 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 C<pprivate> member of
+the regexp structure. It will be called with the preconstructed new
+regexp structure as an argument, the C<pprivate> member will point at
+the B<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.
+
+=head1 The REGEXP structure
+
+The REGEXP struct is defined in F<regexp.h>. All regex engines must be able to
+correctly build such a structure in their L</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
+C<intflags> and C<pprivate> members. C<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 $& */
+ U32 gofs; /* chars left of pos that we search from */
+
+ /* substring data about strings that must appear
+ in the final match, used for optimisations */
+ struct reg_substr_data *substrs;
+
+ U32 nparens; /* number of capture buffers */
+
+ /* private engine specific data */
+ U32 intflags; /* Engine Specific Internal flags */
+ void *pprivate; /* Data private to the regex engine which
+ created this object. */
+
+ /* Data about the last/current match. These are modified during matching*/
+ U32 lastparen; /* last open paren matched */
+ U32 lastcloseparen; /* last close paren matched */
+ regexp_paren_pair *swap; /* Swap copy of *offs */
+ regexp_paren_pair *offs; /* Array of offsets for (@-) and (@+) */
+
+ char *subbeg; /* saved or original string so \digit works forever. */
+ SV_SAVED_COPY /* If non-NULL, SV which is COW from original */
+ I32 sublen; /* Length of string pointed by subbeg */
+
+ /* Information about the match that isn't often used */
+ I32 prelen; /* length of precomp */
+ const char *precomp; /* pre-compilation regular expression */
+
+ char *wrapped; /* wrapped version of the pattern */
+ I32 wraplen; /* length of wrapped */
+
+ I32 seen_evals; /* number of eval groups in the pattern - for security checks */
+ HV *paren_names; /* Optional hash of paren names */
+
+ /* Refcount of this regexp */
+ I32 refcnt; /* Refcount of this regexp */
+ } regexp;
+
+The fields are discussed in more detail below:
+
+=head2 C<engine>
+
+This field points at a regexp_engine structure which contains pointers
+to the subroutines that are to be used for performing a match. It
+is the compiling routine's responsibility to populate this field before
+returning the regexp object.
+
+Internally this is set to C<NULL> unless a custom engine is specified in
+C<$^H{regcomp}>, perl's own set of callbacks can be accessed in the struct
+pointed to by C<RE_ENGINE_PTR>.
+
+=head2 C<mother_re>
+
+TODO, see L<http://www.mail-archive.com/perl5-changes@perl.org/msg17328.html>
+
+=head2 C<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 L<comp|/comp> callback. See the L<comp|/comp> documentation for
+valid flags.
+
+=head2 C<minlen> C<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.
+
+C<minlenret> is the minimum length of the string that would be found
+in $& after a match.
+
+The difference between C<minlen> and C<minlenret> can be seen in the
+following pattern:
+
+ /ns(?=\d)/
+
+where the C<minlen> would be 3 but C<minlenret> would only be 2 as the \d is
+required to match but is not actually included in the matched content. This
+distinction is particularly important as the substitution logic uses the
+C<minlenret> to tell whether it can do in-place substitution which can result in
+considerable speedup.
+
+=head2 C<gofs>
+
+Left offset from pos() to start match at.
+
+=head2 C<substrs>
+
+Substring data about strings that must appear in the final match. This
+is currently only used internally by perl's engine for but might be
+used in the future for all engines for optimisations.
+
+=head2 C<nparens>, C<lasparen>, and C<lastcloseparen>
+
+These fields are used to keep track of how many paren groups could be matched
+in the pattern, which was the last open paren to be entered, and which was
+the last close paren to be entered.
+
+=head2 C<intflags>
+
+The engine's private copy of the flags the pattern was compiled with. Usually
+this is the same as C<extflags> unless the engine chose to modify one of them.
+
+=head2 C<pprivate>
+
+A void* pointing to an engine-defined data structure. The perl engine uses the
+C<regexp_internal> structure (see L<perlreguts/Base Structures>) but a custom
+engine should use something else.
+
+=head2 C<swap>
+
+Unused. Left in for compatibility with perl 5.10.0.
+
+=head2 C<offs>
+
+A C<regexp_paren_pair> structure which defines offsets into the string being
+matched which correspond to the C<$&> and C<$1>, C<$2> etc. captures, the
+C<regexp_paren_pair> struct is defined as follows:
+
+ typedef struct regexp_paren_pair {
+ I32 start;
+ I32 end;
+ } regexp_paren_pair;
+
+If C<< ->offs[num].start >> or C<< ->offs[num].end >> is C<-1> then that
+capture buffer did not match. C<< ->offs[0].start/end >> represents C<$&> (or
+C<${^MATCH> under C<//p>) and C<< ->offs[paren].end >> matches C<$$paren> where
+C<$paren >= 1>.
+
+=head2 C<precomp> C<prelen>
+
+Used for optimisations. C<precomp> holds a copy of the pattern that
+was compiled and C<prelen> its length. When a new pattern is to be
+compiled (such as inside a loop) the internal C<regcomp> operator
+checks whether the last compiled C<REGEXP>'s C<precomp> and C<prelen>
+are equivalent to the new one, and if so uses the old pattern instead
+of compiling a new one.
+
+The relevant snippet from C<Perl_pp_regcomp>:
+
+ if (!re || !re->precomp || re->prelen != (I32)len ||
+ memNE(re->precomp, t, len))
+ /* Compile a new pattern */
+
+=head2 C<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.
+
+=head2 C<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.
+
+=head2 C<subbeg> C<sublen> C<saved_copy>
+
+Used during execution phase for managing search and replace patterns.
+
+=head2 C<wrapped> C<wraplen>
+
+Stores the string C<qr//> stringifies to. The perl engine for example
+stores C<(?-xism:eek)> in the case of C<qr/eek/>.
+
+When using a custom engine that doesn't support the C<(?:)> construct
+for inline modifiers, it's probably best to have C<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 C<(?:)>.
+
+=head2 C<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 C<qr//>.
+
+=head2 C<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 L</comp> routine.
+
+=head1 HISTORY
+
+Originally part of L<perlreguts>.
+
+=head1 AUTHORS
+
+Originally written by Yves Orton, expanded by E<AElig>var ArnfjE<ouml>rE<eth>
+Bjarmason.
+
+=head1 LICENSE
+
+Copyright 2006 Yves Orton and 2007 E<AElig>var ArnfjE<ouml>rE<eth> Bjarmason.
+
+This program is free software; you can redistribute it and/or modify it under
+the same terms as Perl itself.
+
+=cut