public static function isValid($url, $absolute = FALSE) {
    if ($absolute) {
      return (bool) preg_match("
        /^                                                      # Start at the beginning of the text
        (?:ftp|https?|feed):\/\/                                # Look for ftp, http, https or feed schemes
        (?:                                                     # Userinfo (optional) which is typically
          (?:(?:[\w\.\-\+!$&'\(\)*\+,;=]|%[0-9a-f]{2})+:)*      # a username or a username and password
          (?:[\w\.\-\+%!$&'\(\)*\+,;=]|%[0-9a-f]{2})+@          # combination
        )?
        (?:
          (?:[a-z0-9\-\.]|%[0-9a-f]{2})+                        # A domain name or a IPv4 address
          |(?:\[(?:[0-9a-f]{0,4}:)*(?:[0-9a-f]{0,4})\])         # or a well formed IPv6 address
        )
        (?::[0-9]+)?                                            # Server port number (optional)
        (?:[\/|\?]
          (?:[\w#!:\.\?\+=&@$'~*,;\/\(\)\[\]\-]|%[0-9a-f]{2})   # The path and query (optional)
        *)?
      $/xi", $url);
    }
    else {
      return (bool) preg_match("/^(?:[\w#!:\.\?\+=&@$'~*,;\/\(\)\[\]\-]|%[0-9a-f]{2})+$/i", $url);
    }
  }