How do I restrict the type of characters that can be used at registrations? I want to disallow the use of spaces and all symbols except "_".
Based on what you said in your comment, you should just use Pathauto. Pathauto allows you to remove special characters from user/node/taxonomy URLs; if someone's username is 'hey$3#@", you could make their user profile URL http://yourdomain.com/user/hey3.