Block new Symbols in new registrations?

[pagekeeper]

hey,

i thought it would be useful to have a hack to block symbols in new registrations with edits to registration pages to warn users under the username field and stop page displaying this field cannot accept symbols.

you would be able to go more sopisicated if you add a function to remove current users with symbols and send emails to those users with their new username etc. useful for the bigger forums.

i only need this because it will decrease the length of some of my younger users, and to prevent this from hapening from registration. underscore would likely to be the only ok one i suspect.

!"£$%^&*()_+-=:~# etc