문자열 전화번호 포맷

public static String makePhoneNumber(String phoneNumber) {
  String regEx = "(\\d{3})(\\d{3,4})(\\d{4})";
  
  if (phoneNumber == null) {
    phoneNumber = "";
  }
  
  if(!Pattern.matches(regEx, phoneNumber)) {
    return phoneNumber;
  }
  
  return phoneNumber.replaceAll(regEx, "$1-$2-$3");
}