Email id validation through JavaScript (core coding)

Write this javascript code inside "HEAD" - "SCRIPT" tag

function echeck(str) {

var at="@"
var dot="."
var lat=str.indexOf(at)
var lstr=str.length
var ldot=str.indexOf(dot)
if (str.indexOf(at)==-1){
alert("Invalid E-mail ID")
return false
}

if (str.indexOf(at)==-1 || str.indexOf(at)==0 || str.indexOf(at)==lstr){
alert("Invalid E-mail ID")
return false
}

if (str.indexOf(dot)==-1 || str.indexOf(dot)==0 || str.indexOf(dot)==lstr){
alert("Invalid E-mail ID")
return false
}

if (str.indexOf(at,(lat+1))!=-1){
alert("Invalid E-mail ID")
return false
}

if (str.substring(lat-1,lat)==dot || str.substring(lat+1,lat+2)==dot){
alert("Invalid E-mail ID")
return false
}

if (str.indexOf(dot,(lat+2))==-1){
alert("Invalid E-mail ID")
return false
}

if (str.indexOf(" ")!=-1){
alert("Invalid E-mail ID")
return false
}

return true
}

function ValidateForm(){
var TextBox1=document.form1.TextBox1

if ((TextBox1.value==null)||(TextBox1.value=="")){
alert("Please Enter your Email ID")
TextBox1.focus()
return false
}
if (echeck(TextBox1.value)==false){
TextBox1.value=""
TextBox1.focus()
return false
}
return true
}

call this function in form's onsubmit event

form id="form1" runat="server" onsubmit="return ValidateForm()"