my $pat = qr/\w+ ((w)hit(e)|(b)lac(k))/;
    $str = 'mostly black but some white spaces';
    while ($str =~ /$pat/g) {
        my @m = get_captures($str);
        print "$m[1] starts with $m[2] and ends with $m[3]\n";
    }

    sub get_captures {
        map { defined $+[$_] 
                ? substr($_[0],$-[$_],$+[$_]-$-[$_]) 
                :()
            } 0..$#+;
    }