function checkMail(mail)
{
    if(mail.length==0)return;
	var filter  = /^([a-zA-Z0-9_\.\-])+\@(([a-zA-Z0-9\-])+\.)+([a-zA-Z0-9]{2,4})+$/;
	if(!filter.test(mail)){
	    alert("Ulovlig epostadresse. Eksempel: olanormann@domene.no");
	}
}