code: def validate_regex(regex, sub_service):
          
          # The regex generated by stackinabox starts with ^
          # and ends with $. Enforce that the provided regex does the same.
  
          if regex.pattern.startswith('^') is False:
              logger.debug('StackInABoxService: Pattern must start with ^')
              raise InvalidRouteRegexError('Pattern must start with ^')
  
          # Note: pattern may end with $ even if sub_service is True
          if regex.pattern.endswith('$') is False and sub_service is False:
              logger.debug('StackInABoxService: Pattern must end with $')
              raise InvalidRouteRegexError('Pattern must end with $')
  
          # Enforce that if the pattern does not end with $ that it is a service
          if regex.pattern.endswith('$') is True and sub_service is True:
              logger.debug(
                  'StackInABoxService: Sub-Service RegEx Pattern must not '
                  'end with $')
              raise InvalidRouteRegexError('Pattern must end with $')
docs: Is the regex valid for StackInABox routing?
  
          :param regex: Python regex object to match the URI
          :param sub_service: boolean for whether or not the regex is for
                              a sub-service
  
          :raises: InvalidRouteRegexError if the regex does not meet the
                   requirements.