Hi Tooey,
I could possibly add some web.config setting that would allow you to specify a regex expression and text to display if the regex fails validation. But, the problem is in many cases the userid and the display name are generated. Specifically it is generated for anyone registering with OpenID/RPX/Windows Live and also it is generated if you use this setting:
<add key="AutoGenerateAndHideUserNamesWhenUsingEmailForLogin" value="true" />
This setting reduces registration friction because there is less for the user to fill out to register.
I don't have an easy way to let you control how it is generated so making it match the expression would not be an easy thing to accomplish. Basically it tries to use the first part of the email before the @ sign as the userid and display name, but since it must be unique it checks against the existing users and if needed appends numbers until it finds one that is not in use.
Best,
Joe