Ack. I guess spending two days trying to grok yacc and Parse::Yapp led me to choose to do it the hard way. Thanks for a healthy does of simplicity.

Unfortunately, the following regexp break your very good idea:

$regex = qr/foo/; # should return 0
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That only means your code needs at least one capture:

sub captures {
    my $re = shift;
    return scalar ( @_ = '' =~ /(?:($re))?/ ) - 1;
}
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This passes my whole test suite, expect for this regexp:

 
$regex = qr/(?x)  # (comment)
            (?-x) # (capture) (?x) # (comment)/;
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Which dies horribly with the message:

Unmatched ( in regex; marked by <-- HERE in m/(?:(( <-- HERE ?-xism:(?x)  # (comment)
            (?-x) # (capture) (?x) # (comment))))?/

So, $regex compiles, but qr/(?:($regex))?/ doesn't? I'm lost.