How to verify letters only or the $ character are used in a string?

public class Order {
private String itemName;
private int priceInCents;
private String discountCode;

public Order(String itemName, int priceInCents) {
this.itemName = itemName;
this.priceInCents = priceInCents;
}

              private  String normalizeDiscountCode (String code){
              code = code.toUpperCase();
              if (! Character.isLetter(code)){ // Can't convert char to string.
              throw new IllegalArgumentExceotion ("Invalid discount code.");
              }
              return code; 
              }


public String getItemName() {
return itemName;
}

public int getPriceInCents() {
return priceInCents;
}

public String getDiscountCode() {
return discountCode;
}

            public void applyDiscountCode(String discountCode) {
            this.discountCode = normalizeDiscountCode(discountCode);
            }
            }

public class Example {

  public static void main(String[] args) {
    // This is here just for example use cases.

    Order order = new Order(
            "Yoda PEZ Dispenser",
            600);

    // These are valid.  They are letters and the $ character only
    order.applyDiscountCode("abc");
    order.getDiscountCode(); // ABC

    order.applyDiscountCode("$ale");
    order.getDiscountCode(); // $ALE


    try {
      // This will throw an exception because it contains numbers
      order.applyDiscountCode("ABC123");
    } catch (IllegalArgumentException iae) {
      System.out.println(iae.getMessage());  // Prints "Invalid discount code"
    }
    try {
      // This will throw as well, because it contains a symbol.
      order.applyDiscountCode("w@w");
    }catch (IllegalArgumentException iae) {
      System.out.println(iae.getMessage());  // Prints "Invalid discount code"
    }

  }
}