It is not difficult to fix. I dont wish to touch the UserValidator class though since its way too messy and likely to be replaced in future. For now, just go to register.php file and find this line:
PHP Code:
elseif($mysidia->input->action() == "register"){
// First of all, lets examine if the user register session is set
$mysidia->session->validate("register");
add below:
PHP Code:
$pattern = '#^[A-Z0-9 ]+$#i';
if(!preg_match ($pattern, $mysidia->input->post("username"))){
throw new Exception("Error: Invalid Username Specified, it can only contain alphanumeric characters and blank space!");
}