[kh99]

In includes/class_dm.php, function verify_username() around line 456 is this:

// check for regex compliance
if (!preg_match('#' . str_replace('#', '\#', $this->registry->options['usernameregex']) . '#siU', $username))
{

I think that's where you'd want to remove it. Another approach might be to use a plugin on the registration page to change the username that's entered, or even build it from first name/last name fields.