Email Validation by Javascript

function EmailCheck()
{
var reg = /^([A-Za-z0-9_\-\.])+\@([A-Za-z0-9_\-\.])+\.([A-Za-z]{2,4})$/;
var str =document.getElementById('<%= TxtTo.ClientID %>').value;

if(reg.test(str) == false)
{
alert('Invalid Email Address');
document.getElementById('<%= TxtTo.ClientID %>').focus();
}
}