code: func isUrnRFC2141(fl FieldLevel) bool {
  	field := fl.Field()
  
  	switch field.Kind() {
  
  	case reflect.String:
  
  		str := field.String()
  
  		_, match := urn.Parse([]byte(str))
  
  		return match
  	}
  
  	panic(fmt.Sprintf("Bad field type %T", field.Interface()))
  }
docs: // isUrnRFC2141 is the validation function for validating if the current field's value is a valid URN as per RFC 2141.