code: public static boolean isThisASecureCallbackName(String callback) {
          return !Strings.isNullOrEmpty(callback)
                  && !callback.contains("..") 
                  && CALLBACK_SECURITY_VALIDATION_REGEXP.matcher(callback).matches();
      }
docs: Tests whether the given function name is a valid JSONP function
  name/path.
  
  @param callback Callback value to test.
  @return Whether the given function name is a valid JSONP function
  name/path.