nickname

Unique nickname vs one nickname for many users

In all my web-projects I've made 1 unique nickname for 1 user (i.e. in DB table User has primary key ID and unique varchar 'nickname' or 'login' or whatever else). At Stackoverflow we can see just the opposite solution and many users have the same nickname. Futhermore, user can change his nickname here whenever he wants. I wonder wha...

Function to prevent special characters in nickname

Hi, I've a website where users can register. I would like to prevent all special characters, accents, etc in the nickname (used to login). I use PHP. How can I do that ? Edit: Another question, can you give me a regular expression (in PHP) who allow ONLY the 26 letters : abcdefghijklmnopqrstuvwxyz and no more ? Thanks ...