正则表达式验证mail格式

function checkEmail(mail){
if (mail == null || mail =="") {
return true;
} else {
var flag;
var reg = /^([a-zA-Z0-9_-])+@([a-zA-Z0-9_-])+(.[a-zA-Z0-9_-])+/; 
eMail = mail.trim();
if(eMail != null && eMail != ""){
var mailOk = reg.test(eMail); 
if(mailOk){
flag = true;
} else {
flag=false;
}
}
return flag;
}
}