Search bar in orders page

Based on admin site section, lesson 9 from The Ultimate Django Series: Part 1 I want to add the search bar on Orders page and search from an order id. I succeeded to add it, but I can type anything there - both numeric and alphabetical values. Because the id field is a numeric field, I want to be able to type only numeric values in the search bar. Can somebody help me and explain to me how can I do it? Thanks!