Internationalization and Validation

My app has a registration page for new users and I'm trying to carefully support as many languages and locales as possible. One of the rules for their desired username is that it must be alphanumeric, but when I type Japanese characters into that field it never passes validation. 开发者_开发技巧 Do you know of any good resources on best-practices for this type of issue?

How are you doing your validation? +[NSCharacterSet alphanumericCharacterSet] includes Japanese letter characters.