function check_email(str){
if (str.value == ""){
	alert ("Inserire un indirizzo email corretto");
	str.focus();
	return false;
	}
else{
		 if (isEmail (str.value) == false){
		 	alert("Indirizzo e-mail non corretto!");
   			str.focus();
		 return false;
		}
 }

}

function isEmail(str){
  // are regular expressions supported?
  var supported = 0;
  if (window.RegExp) {
    var tempStr = "a";
    var tempReg = new RegExp(tempStr);
    if (tempReg.test(tempStr)) supported = 1;
	}
  if (!supported) 
    return (str.indexOf(".") > 2) && (str.indexOf("@") > 0);
  var r1 = new RegExp("(@.*@)|(\\.\\.)|(@\\.)|(^\\.)");
  var r2 = new RegExp("^.+\\@(\\[?)[a-zA-Z0-9\\-\\.]+\\.([a-zA-Z]{2,4}|[0-9]{1,3})(\\]?)$");
  return (!r1.test(str) && r2.test(str));
}