Prevent special characters when registering an account
 

I only want to allow the following characters for nicknames when someone is registering an account at register.php:

a ... z
A ... Z
0 ... 9
underscore "_"

How can I achieve this?

Use REGEX ^[a-zA-Z0-9??????]*$ read about it here http://www.regular-expressions.info/unicode.html

Could you please explain what the bold snippet means? I guess that it will allow stuff like ? ? ? ? ? ? ? ? ? ? ? ?? If so, why don't add vowels "e" "E" and "i" "I" to your regex?

Also I'd like to allow white spaces " " in nicknames (yes, the character you make when you press the spacebar button in your keyboard).

Did you read the link? the fact that a-z are allowed means every character between a AND z including a AND z are allowed.

The bold snippet is for foriegn characters.