public static function isSeparator($str)
    {
        // NOTES:   Only non alphanumeric ASCII characters may be separators.
        //          `~` is the last printable ASCII character.
        return ($str <= '~') && ($str !== '_')
            && (($str < '0') || ($str > '9'))
            && (($str < 'a') || ($str > 'z'))
            && (($str < 'A') || ($str > 'Z'));
    }