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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
        REGEXP* (*op_comp) (...);


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

TODO: Document this

=item Character set

The character set semantics are determined by an enum that is contained
in this field.  This is still experimental and subject to change, but
the current interface returns the rules by use of the in-line function
C<get_regex_charset(const U32 flags)>.  The only currently documented
value returned from it is REGEX_LOCALE_CHARSET, which is set if
C<use locale> is in effect. If present in C<< rx->extflags >>,
C<split> will use the locale dependent definition of whitespace
when RXf_SKIPWHITE or RXf_WHITE is in effect. 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>.

=back

Additional flags:

=over 4

=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. The arguments are

=over 4

=item rx

The regular expression to execute.

=item screamer

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, C<pos()> etc.

=item strbeg

Pointer to the physical start of the string.

=item strend

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

=item stringarg

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

=item minend

Minimum length of string (measured in bytes from C<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.

=item data

Optimisation data; subject to change.

=item flags

Optimisation flags; subject to change.

=back

=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 groups (C<$1>, C<$2>, ...).

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

    ${^PREMATCH}  RX_BUFF_IDX_CARET_PREMATCH
    ${^POSTMATCH} RX_BUFF_IDX_CARET_POSTMATCH
    ${^MATCH}     RX_BUFF_IDX_CARET_FULLMATCH
    $`            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 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 => "CaptureVar";
    $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.

=head2 op_comp

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

=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 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:

=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<lastparen>, 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 group 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 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.

=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> C<suboffset> C<subcoffset>

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

In the presence of the C<REXEC_COPY_STR> flag, but with the addition of
the C<REXEC_COPY_SKIP_PRE> or C<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 C<RXf_PMf_KEEPCOPY> or the relevant bits being set in
C<PL_sawampersand>). In this case, it may set C<suboffset> to indicate the
number of bytes from the logical start of the buffer to the physical start
(i.e. C<subbeg>). It should also set C<subcoffset>, the number of
characters in the offset. The latter is needed to support C<@-> and C<@+>
which work in characters, not bytes.

=head2 C<wrapped> C<wraplen>

Stores the string C<qr//> stringifies to. The perl engine for example
stores C<(?^: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