正则表达式，用于检查字符数

I have to write a .NET Regular Expression which validates if a string is alpha-numeric and has 4 or 8 characters (nothing less, nothing more). How can I do that? I tried with ([a-zA-Z0-9]{4})|([a-zA-Z0-9]{8}) but it doesn't work.I have to write a .NET Regular Expression which