gistfile1.txt

use v6;

#    Match       : X aX baX
#    No match    : cX aaX

grammar Goo {
    my %seen;
        # Allow at most only one each of 'a' or 'b' per match.
    token TOP {
        <!{ %seen = () }> <ab>? <ab>? 'X'
    }
    token ab {
        ('a' | 'b') <?{
            sub ($a_or_b) {
                my $seen = $a_or_b ~~ %seen;
                %seen{$a_or_b} = True;
                ! $seen;
            }.($0)
        }>
    }
}

for ( :X, :aX, :baX, :!bbX, :!cX, ) {
    my ($str, $should-match) = $_.kv;
    my $match = Goo.parse($str);
    say ($match.Bool == $should-match)
        ?? "Okay: $str"
        !! ($should-match ?? "? " !! "! ") ~ "{$str}"
    ;
}