src/main/java/com/conveyal/gtfs/loader/URLField.java
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;
        }
    }
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;
        }
    }
Check that a string can be properly parsed and is in range.