[Go] Simplify phone number

simplify_phone.go

var validPhoneNumber = regexp.MustCompile(`^(\+7|8) \(\d{3}\) \d{7}$`)

// SimplifyPhoneNumber transform "+7 (909) 9316826" (or "8 (909) 9316826") to "9099316826"
func SimplifyPhoneNumber(number string) (string, error) {
	if !validPhoneNumber.MatchString(number) {
		return "", fmt.Errorf("invalid phone number format: %s", number)
	}
	if len(number) == 15 {
		return number[3:6] + number[8:], nil
	}
	return number[4:7] + number[9:], nil
}