<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def reminder_validator(input_str):
 """ Allows a string that matches utils.REMINDER_REGEX. Raises ValidationError otherwise. """

    match = re.match(REMINDER_REGEX, input_str)
    if match or input_str == '.':
        return input_str
    else:
        raise ValidationError('Expected format: <number><w|d|h|m> '
                              '<popup|email|sms>. (Ctrl-C to exit)\n')