Jvascript Validation

Regular Expression for Username which allows alphabets and Spacebar:
var username=document.getElementById("").value;
var Expusername=/^[A-Za-z]+[\" "\A-Za-z]+$/;
if(!username.match(Expusername))
{
alert('Enter only alphabets,digits and spacebar ');
document.getElementById("").focus();
return false;
}