How to verify US postal code by regular expression?

        public static bool IsValid (string postcode)

        {

            if (postcode == "") return true;

            System.Text.RegularExpressions.Regex r = new System.Text.RegularExpressions.Regex(@"^(\d{5}-\d{4}|\d{5}|\d{9})$|^([a-zA-Z]\d[a-zA-Z] \d[a-zA-Z]\d)$");

            return r.IsMatch(postcode.ToUpper());

        }