检查 E-mail 是否已被注册
function CheckExists()
{
var e = document.getElementById("mailaddress").value; 
if(e != "") {
if(!/(\S)+[@](\S)+[.](\w)+/.test(e)) 
{ 
alert("请输入格式正确的 e-mail 地址！"); 
var email = document.getElementById ( "mailaddress" ); 
email.value = ""; 
email.focus ();
}
else 
{ 
QuickRegisterBox.IsEMailExists(e, CheckExists_Callback); 
} 
}
}