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