Restrict the use of special characters in username

I would like to restrict the use of certain characters in the user name/displayname when people create an account.

Is this possible?

I guess I need to edit the following line in register.php:

<input name="user_login" type="text" id="user_login" size="30" maxlength="30" value="<?php echo $user_login; ?>" />

?

I thought that bbpress will filter these special characters, do you tried those characters during the registration?

To be more specific: I don't want them to use dots and hyphens

Than it's needed to use a validation function before you write the data to the database. I don't think that this is a standard function or plugin

Can it be done with javascript?

Sure that is possible but you need to include the code as well. I'm not sure if other client side script will break the current validation.

The whole registration process is currently not very good (think about what happens if the human test has failed, you need to re-enter all your details)

Than it's needed to use a validation function before you write the data to the database. I don't think that this is a standard function or plugin

How do get this, presuming I am not a programmer?

I will reply by mail.