sub match_re {
    my $self = shift;

    # retrieve from cache if it's there:
    my $re = $self->{match_re};
    return $re if defined $re;

    # otherwise compute it and store it:
    $re = $self->match_string;
    $re = qr/\Q$re\E/;
    return $self->{match_re} = $re;
}