need stric regex for form

[Andrus]

hey i need something to restrict alot of funny chars in the username

right now i got this

// check the username for bad chars
if(!eregi("[A-Z]",$username)){
echo '<script language="Javascript">alert ("Bad characters in username")</script>';
include("index.html");
exit();
}


but it still allows weird looking characters like Ũ and stuff, how do i restrict it to only A-Z and a-z