How to use regex for alphanumeric expressions

public void AlphaNumericString()
       {

           //string value;
           string s = BacBox.Text.ToString();
           int.TryParse(s, out int result);


           Regex r = new Regex("^[a-zA-Z0-9]*$");
           if (r.IsMatch(s) && (s.Length > 3  && s.Length< 5 ))
           {

                   BacBox.Text = s;

           }
          else
           {
               MessageBox.Show("Enter 4 alphanumeric characters only");
           }


       }