javascript電子郵件(email)驗證

javascript function checkemail()
{
if ("" == document.lyform.email.value) {
alert("請輸入郵件地址")
document.lyform.email.focus()
return false
}
var email = document.lyform.email.value
var is_error = false
var pn_0 = email.indexOf("@")
var pn_1 = email.indexOf(".",pn_0)
var pn_2 = email.length
if (pn_0<1 || pn_1<pn_0+2 || pn_1+2>pn_2) is_error=true
if (is_error) {
alert("請輸入正確的郵件地址")
document.lyform.email.focus()
return false
}
}