Use the Server Validations from validation toolkit on the registration form.
You can user Alpha Numeric validation to force the password to contains numbers and letters. And use Entry Length validation to force a minimum number of characters.
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.