example of matching a format and returning an error in go from JSON parsing

zipcode.go

type ZipP4 string

func (z *ZipP4) MarshalJSON() ([]byte, error) {
	return []byte(fmt.Sprintf(`"%s"`, *z)), nil
}

func (z *ZipP4) UnmarshalJSON(b []byte) (err error) {
	if z == nil {
		return errors.New("ZipP4: UnmarshalJSON on nil pointer")
	}
	if matched, _ := regexp.MatchString(`^"[0-9]{5}[0-9]{4}"$`, string(b)); matched == false {
		return errors.New("zipcode+4 format must be zzzzzpppp")
	}
	*z = ZipP4(string(b[1:10]))
	return nil
}