=head1 NAME

re::engine::Plugin - API to write custom regex engines

=head1 DESCRIPTION

As of perl 5.9.5 it's possible to lexically replace perl's built-in
regular expression engine with your own (see L<perlreapi> and
L<perlpragma>). This module provides a glue interface to the relevant
parts of the perl C API enabling you to write an engine in Perl
instead of the C/XS interface provided by the core.

=head2 The gory details

Each regex in perl is compiled into an internal C<REGEXP> structure
(see L<perlreapi|perlreapi/The REGEXP structure>), this can happen
either during compile time in the case of patterns in the format
C</pattern/> or runtime for C<qr//> patterns, or something inbetween
depending on variable interpolation etc.

When this module is loaded into a scope it inserts a hook into
C<$^H{regcomp}> (as described in L<perlreapi>) to have each regexp
constructed in its lexical scope handled by this engine, but it
differs from other engines in that it also inserts other hooks into
C<%^H> in the same scope that point to user-defined subroutines to use
during compilation, execution etc, these are described in
L</CALLBACKS> below.

The callbacks (e.g. L</comp>) then get called with a
L<re::engine::Plugin> object as their first argument. This object
provies access to perl's internal REGEXP struct in addition to its own
state (e.g. a L<stash|/stash>). The L<methods|/METHODS> on this object
allow for altering the C<REGEXP> struct's internal state, adding new
callbacks, etc.

=head1 CALLBACKS

Callbacks are specified in the C<re::engine::Plugin> import list as
key-value pairs of names and subroutine references:

    use re::engine::Plugin (
        comp => sub {},
        exec => sub {},
    );

To write a custom engine which imports your functions into the
caller's scope use use the following snippet:

    package re::engine::Example;
    use re::engine::Plugin ();

    sub import
    {
        # Populates the caller's %^H with our callbacks
        re::engine::Plugin->import(
            comp => \&comp,
            exec => \&exec,
        );
    }

   *unimport = \&re::engine::Plugin::unimport;

    # Implementation of the engine
    sub comp { ... }
    sub exec { ... }

    1;

=head2 comp

    comp => sub {
        my ($rx) = @_;

        # return value discarded
    }

Called when a regex is compiled by perl, this is always the first
callback to be called and may be called multiple times or not at all
depending on what perl sees fit at the time.

The first argument will be a freshly constructed C<re::engine::Plugin>
object (think of it as C<$self>) which you can interact with using the
L<methods|/METHODS> below, this object will be passed around the other
L<callbacks|/CALLBACKS> and L<methods|/METHODS> for the lifetime of
the regex.

Calling C<die> or anything that uses it (such as C<carp>) here will
not be trapped by an C<eval> block that the pattern is in, i.e.

   use Carp 'croak';
   use re::engine::Plugin(
       comp => sub {
           my $rx = shift;
           croak "Your pattern is invalid"
               unless $rx->pattern ~~ /pony/;
       }
   );

   # Ignores the eval block
   eval { /you die in C<eval>, you die for real/ };

This happens because the real subroutine call happens indirectly at
compile time and not in the scope of the C<eval> block. This is how
perl's own engine would behave in the same situation if given an
invalid pattern such as C</(/>.

=head2 exec

    exec => sub {
        my ($rx, $str) = @_;

        # We always like ponies!
        return 1 if $str ~~ /pony/;

        # Failed to match
        return;
    }

Called when a regex is being executed, i.e. when it's being matched
against something. The scalar being matched against the pattern is
available as the second argument (C<$str>) and through the L<str|/str>
method. The routine should return a true value if the match was
successful, and a false one if it wasn't.

=head1 METHODS

=head2 str

    "str" ~~ /pattern/;
    # in comp/exec/methods:
    my $str = $rx->str;

The last scalar to be matched against the L<pattern|/pattern> or
C<undef> if there hasn't been a match yet.

perl's own engine always stringifies the scalar being matched against
a given pattern, however a custom engine need not have such
restrictions. One could write a engine that matched a file handle
against a pattern or any other complex data structure.

=head2 pattern

The pattern that the engine was asked to compile, this can be either a
classic Perl pattern with modifiers like C</pat/ix> or C<qr/pat/ix> or
an arbitary scalar. The latter allows for passing anything that
doesn't fit in a string and five L<modifier|/mod> characters, such as
hashrefs, objects, etc.

=head2 mod

    my %mod = $rx->mod;
    say "has /ix" if %mod ~~ 'i' and %mod ~~ 'x';

A key-value pair list of the modifiers the pattern was compiled with.
The keys will zero or more of C<imsxp> and the values will be true
values (so that you don't have to write C<exists>).

You don't get to know if the C<eogc> modifiers were attached to the
pattern since these are internal to perl and shouldn't matter to
regexp engines.

=head2 stash

    comp => sub { shift->stash( [ 1 .. 5 ) },
    exec => sub { shift->stash }, # Get [ 1 .. 5 ]

Returns or sets a user defined stash that's passed around as part of
the C<$rx> object, useful for passing around all sorts of data between
the callback routines and methods.

=head2 minlen

    $rx->minlen($num);
    my $minlen = $rx->minlen // "not set";

The minimum C<length> a string must be to match the pattern, perl will
use this internally during matching to check whether the stringified
form of the string (or other object) being matched is at least this
long, if not the regexp engine in effect (that means you!) will not be
called at all.

The length specified will be used as a a byte length (using
L<SvPV|perlapi/SvPV>), not a character length.

=head2 num_captures

    $re->num_captures(
        FETCH => sub {
            my ($re, $paren) = @_;

            return "value";
        },
        STORE => sub {
            my ($re, $paren, $rhs) = @_;

            # return value discarded
        },
        LENGTH => sub {
            my ($re, $paren) = @_;

            return 123;
        },
    );

Takes a list of key-value pairs of names and subroutines that
implement numbered capture variables. C<FETCH> will be called on value
retrieval (C<say $1>), C<STORE> on assignment (C<$1 = "ook">) and
C<LENGTH> on C<length $1>.

The second paramater of each routine is the paren number being
requested/stored, the following mapping applies for those numbers:

    -2 => $` or ${^PREMATCH}
    -1 => $' or ${^POSTMATCH}
     0 => $& or ${^MATCH}
     1 => $1
     # ...

Assignment to capture variables makes it possible to implement
something like Perl 6 C<:rw> semantics, and since it's possible to
make the capture variables return any scalar instead of just a string
it becomes possible to implement Perl 6 match object semantics (to
name an example).

=head2 named_captures

B<TODO>: implement

perl internals still needs to be changed to support this but when it's
done it'll allow the binding of C<%+> and C<%-> and support the
L<Tie::Hash> methods FETCH, STORE, DELETE, CLEAR, EXISTS, FIRSTKEY,
NEXTKEY and SCALAR.

=head1 Tainting

The only way to untaint an existing variable in Perl is to use it as a
hash key or referencing subpatterns from a regular expression match
(see L<perlsec|perlsec/Laundering and Detecting Tainted Data>), the
latter only works in perl's regex engine because it explicitly
untaints capture variables which a custom engine will also need to do
if it wants its capture variables to be untanted.

There are basically two ways to go about this, the first and obvious
one is to make use of Perl'l lexical scoping which enables the use of
its built-in regex engine in the scope of the overriding engine's
callbacks:

    use re::engine::Plugin (
        exec => sub {
            my ($re, $str) = @_; # $str is tainted

            $re->num_captures(
                FETCH => sub {
                    my ($re, $paren) = @_;

                    # This is perl's engine doing the match
                    $str ~~ /(.*)/;

                    # $1 has been untainted
                    return $1;
                },
            );
        },
    );

The second is to use something like L<Taint::Util> which flips the
taint flag on the scalar without invoking the perl's regex engine:

    use Taint::Util;
    use re::engine::Plugin (
        exec => sub {
            my ($re, $str) = @_; # $str is tainted

            $re->num_captures(
                FETCH => sub {
                    my ($re, $paren) = @_;

                    # Copy $str and untaint the copy
                    untaint(my $ret = $str);

                    # Return the untainted value
                    return $ret;
                },
            );
        },
    );

In either case a regex engine using perl's L<regex api|perlapi> or
this module is responsible for how and if it untaints its variables.

=head1 SEE ALSO

L<perlreapi>, L<Taint::Util>

=head1 TODO / CAVEATS

I<here be dragons>

=over

=item *

Engines implemented with this module don't support C<s///> and C<split
//>, the appropriate parts of the C<REGEXP> struct need to be wrapped
and documented.

=item *

Still not a complete wrapper for L<perlreapi> in other ways, needs
methods for some C<REGEXP> struct members, some callbacks aren't
implemented etc.

=item *

Support overloading operations on the C<qr//> object, this allow
control over the of C<qr//> objects in a manner that isn't limited by
C<wrapped>/C<wraplen>.

    $re->overload(
        '""'  => sub { ... },
        '@{}' => sub { ... },
        ...
    );

=item *

Support the dispatch of arbitary methods from the re::engine::Plugin
qr// object to user defined subroutines via AUTOLOAD;

    package re::engine::Plugin;
    sub AUTOLOAD
    {
        our $AUTOLOAD;
        my ($name) = $AUTOLOAD =~ /.*::(.*?)/;
        my $cv = getmeth($name); # or something like that
        goto &$cv;
    }

    package re::engine::SomeEngine;

    sub comp
    {
        my $re = shift;

        $re->add_method( # or something like that
            foshizzle => sub {
                my ($re, @arg) = @_; # re::engine::Plugin, 1..5
            },
        );
    }

    package main;
    use re::engine::SomeEngine;
    later:

    my $re = qr//;
    $re->foshizzle(1..5);

=item *

Implement the dupe callback, test this on a threaded perl (and learn
how to use threads and how they break the current model).

=item *

Allow the user to specify ->offs either as an array or a packed
string. Can pack() even pack I32? Only IV? int?

=item *

Add tests that check for different behavior when curpm is and is not
set.

=item *

Add tests that check the refcount of the stash and other things I'm
mucking with, run valgrind and make sure everything is destroyed when
it should.

=item *

Run the debugger on the testsuite and find cases when the intuit and
checkstr callbacks are called. Write wrappers around them and add
tests.

=back

=head1 BUGS

Please report any bugs that aren't already listed at
L<http://rt.cpan.org/Dist/Display.html?Queue=re-engine-Plugin> to
L<http://rt.cpan.org/Public/Bug/Report.html?Queue=re-engine-Plugin>

=head1 AUTHOR

E<AElig>var ArnfjE<ouml>rE<eth> Bjarmason <avar@cpan.org>

=head1 LICENSE

Copyright 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
