code: public static boolean isHostPortCharacter(final char ch) {
          return isAlphaNum(ch) || ch == DASH || ch == PERIOD || ch == COLON;
      }
docs: Checks whether the character could be part of the host portion of a SIP URI.
  
  This does not perform an entirely robust validation as it does not operate
  with the full context of the hostname.
  
  @param ch
  @return true if the character is valid in hostnames, false otherwise