[MUG]

In register.php, find:

PHP Code:

$username = trim($username); 


Under it, add:

PHP Code:

$username = str_replace(' ', '_', $username);

if(!preg_match('#^([A-Za-z0-9\_]+)$#', $username)) {
  eval("standarderror(\"".gettemplate("error_badusername")."\");"); 
  exit;
} 


Then create a template named error_badusername

The username you have chosen is invalid. It can only contain alphanumeric characters and underscores.

Untested, but it should work