int menu_counter = 0;
    cout << " ENTER THE BOOK ISBN:   ";
    // Let the User enter in Book ISBN...
    getline(cin, book_isbn);
    //cout << "book length is:  " << book_isbn.length() << endl;
    // Let's loop through the ISBN string to check if there are any normal letters instead of just numbers.
    cout << "The string is " << book_isbn.empty() << endl;
    for (int i = 0; menu_counter < book_isbn.size(); i++)
    {
        //cout << isdigit(book_isbn[menu_counter]) << endl;
        int result_test = isdigit(book_isbn[menu_counter]); // The Result of if the Position is a Digit or not
        // If the Digit is 0, then that means we came across a letter, and the user must start over.
        if (result_test == 0)
        {
            if (book_isbn[menu_counter] == '-')
            {
                menu_counter++; // Let's increase the menu counter to check more of the string...
            }
            else
            {
                //cout << endl;
                cout << "Please re-enter the ISBN Number:  ";
                getline(cin, book_isbn);
            }
        }
        if (book_isbn.empty())
        {
            //cout << endl;
        cout << "Please re-enter the ISBN Number:  ";
            getline(cin, book_isbn);
        }
        if(result_test != 0 && book_isbn.empty() != 1)
        {
            menu_counter++; // Let's increase the menu counter to check more of the string...
        }
    }