my $c = qr/(?>--[^\n]*(?:\n|\z))/; # VHDL comment regex

##</code><code>##

    # position 01234567 89
    my $str = "a --b x\n x";

##</code><code>##

    $str =~ m/$c* x/;

##</code><code>##

    $str =~ m/$c*x/;