checking for special characters in a login /register script

[supermanjnk]

I have a script to register in order to access a certain part of my site, however I have no clue how to go about stopping people from using!@#$%^&*() ect. in their usernames (passwords are alright) Also i'm not quite sure how to set a minimum limit to the number of characters they have to enter in a field.

[dmmh]

edited...I totally misunderstood your question..brb :)

[RonPK]

if (!preg_match("/^[a-z0-9]{5,10}$/i", $_POST['username']))  
 echo 'some nice error message'; 

This should only allow strings with ordinary letters and digits, length between 5 and 10 characters.