sub numbers {
    my ($class, $value, $len) = @_;
    if (!$value) {
        return (undef, { msg => 'cannot be blank.' });
    } elsif (length($value) > $len ) {
        return (undef, { msg => 'has too many characters.' });
    } elsif ($value !~ /^([0-9 \.-]*)$/) {
        return (undef, { msg => 'can only use numbers.' });
    } else {
        return ("$1", { msg => '' });
    }
}