code: public ValidateFieldResult<String> validateAndConvert (String string) {
          ValidateFieldResult<String> result = cleanString(string);
          try {
              new URL(result.clean); // TODO call this to validate, but we can't default to zero
              return result;
          } catch (Exception ex) {
              result.errors.add(NewGTFSError.forFeed(NewGTFSErrorType.URL_FORMAT, string));
              return result;
          }
      }
docs: Check that a string can be properly parsed and is in range.