Perl regex vs. Raku regex, differences in the engine?

Question

I am trying to convert a regex based solution for the knapsack problem from Perl to raku. Details on Perlmonks

The Perl solution creates this regex:

(?<P>(?:vvvvvvvvvv)?)
(?<B>(?:vv)?)
(?<Y>(?:vvvv)?)
(?<G>(?:vv)?)
(?<R>(?:v)?)
0
(?=
(?(?{ $1 })wwww|)
(?(?{ $2 })w|)
(?(?{ $3 })wwwwwwwwwwww|)
(?(?{ $4 })ww|)
(?(?{ $5 })w|)
)

which gets matched against vvvvvvvvvvvvvvvvvvv0wwwwwwwwwwwwwww. After that the match hash %+ contains the items to put in the sack.

My raku conversion is:

$<B> = [ [ vv ]? ]
$<P> = [ [ vvvvvvvvvv ]? ]
$<R> = [ [ v ]? ]
$<Y> = [ [ vvvv ]? ]
$<G> = [ [ vv ]? ]
0
<?before
[ { say "B"; say $/<B>; say $0; say $1; $1 } w || { "" } ]
[ { say "P"; say $/<P>; say $0; say $1; $2 } wwww || { "" } ]
[ { say "R"; say $/<R>; say $0; say $1; $3 } w || { "" } ]
[ { say "Y"; say $/<Y>; say $0; say $1; $4 } wwwwwwwwwwww || { "" } ]
[ { say "G"; say $/<G>; say $0; say $1; $5 } ww || { "" } ]

which also matches vvvvvvvvvvvvvvvvvvv0wwwwwwwwwwwwwww. But the match object, $/ does not contain anything useful. Also, my debug says all say Nil, so at that point the backreference does not seem to work?

Here's my test script:

my $max-weight = 15;
my %items      =
    'R' => { w =>  1, v =>  1 },
    'B' => { w =>  1, v =>  2 },
    'G' => { w =>  2, v =>  2 },
    'Y' => { w => 12, v =>  4 },
    'P' => { w =>  4, v => 10 }
;

my $str = 'v' x  %items.map(*.value<v>).sum ~
          '0' ~
          'w' x  $max-weight;

say $str;

my $i = 0;
my $left = my $right = '';

for %items.keys -> $item-name
{
    my $v = 'v' x %items{ $item-name }<v>;
    my $w = 'w' x %items{ $item-name }<w>;

     $left  ~= sprintf( '$<%s> = [ [ %s ]? ] ' ~"\n", $item-name, $v );
     $right ~= sprintf( '[ { say "%s"; say $/<%s>; say $0; say $1; $%d } %s || { "" } ]' ~ "\n", $item-name, $item-name, ++$i, $w );
}
use MONKEY-SEE-NO-EVAL;

my $re = sprintf( '%s0' ~ "\n" ~ '<?before ' ~ "\n" ~ '%s>' ~ "\n", $left, $right );

say $re;
dd $/ if $str ~~ m:g/<$re>/;

Answer 1

This answer only covers what's going wrong. It does not address a solution. I have not filed corresponding bugs. I have not yet even searched bug queues to see if I can find reports corresponding to either or both the two issues I've surfaced.

my $lex-var;

sub debug { .say for ++$, :$<rex-var>, :$lex-var }

my $regex = / $<rex-var> = (.) { $lex-var = $<rex-var> } <?before . { debug }> / ;

'xx' ~~   $regex;     say $/;
'xx' ~~ / $regex /;   say $/;

displays:

1
rex-var => Nil
lex-var => ｢x｣
｢x｣
 rex-var => ｢x｣
2
rex-var => Nil
lex-var => ｢x｣
｢x｣

Focusing first on the first call of debug (the lines starting with 1 and ending at rex-var => ｢x｣), we can see that:

• Something's gone awry during the call to debug: $<rex-var> is reported as having the value Nil.

• When the regex match is complete and we return to the mainline, the say $/ reports a full and correctly populated result that includes the rex-var named match.

To begin to get a sense of what's gone wrong, please consider reading the bulk of my answer to another SO question. You can safely skip the Using ~. Footnotes 1,2, and 6 are also probably completely irrelevant to your scenario.

For the second match, we see that not only is $<rex-var> reported as being Nil during the debug call, the final match variable, as reported back in the mainline with the second say $/, is also missing the rex-var match. And the only difference is that the regex $regex is called from within an outer regex.