code: func validateDeviceCgroupRule(val string) (string, error) {
  	if deviceCgroupRuleRegexp.MatchString(val) {
  		return val, nil
  	}
  
  	return val, errors.Errorf("invalid device cgroup format '%s'", val)
  }
docs: // validateDeviceCgroupRule validates a device cgroup rule string format
  // It will make sure 'val' is in the form:
  //    'type major:minor mode'