function test(obj,msg)
{
var regex = /^[a-zA-Z0-9._-]+@([a-zA-Z0-9.-]+\.)+[a-zA-Z0-9.-]{2,4}$/;
if (regex.test(obj.value))
 {
   return true;
 }
else
 {
  alert(msg);
  obj.focus();
  return false;
 }
}

function ReqField1Validator()
{
if (document.getElementById('txtName').value == ' ')  /*txtName is the name of field (server side control )you want to validate*/ 
{
  alert('Full Name cannot be Empty')
   return false;
}

/* here you can add more field  you  want to validate*/

if(!test(document.getElementById('txtLogin'),"Please Enter Valid Email Address"))
    return false;
return true ;
}

/* Write this code in this page Load Event  -- where btnSubmit is the name of Button on which you want to call the function. */

this.btnSubmit.Attributes.Add("onclick","return ReqField1Validator()");