code: @Size(2)
      @NonNull
      static String[] separateDateStringParts(@NonNull @Size(max = 4) String expiryInput) {
          String[] parts = new String[2];
          if (expiryInput.length() >= 2) {
              parts[0] = expiryInput.substring(0, 2);
              parts[1] = expiryInput.substring(2);
          } else {
              parts[0] = expiryInput;
              parts[1] = "";
          }
          return parts;
      }
docs: Separates raw string input of the format MMYY into a "month" group and a "year" group.
  Either or both of these may be incomplete. This method does not check to see if the input
  is valid.
  
  @param expiryInput up to four characters of user input
  @return a length-2 array containing the first two characters in the 0 index, and the last
  two characters in the 1 index. "123" gets split into {"12" , "3"}, and "1" becomes {"1", ""}.