ASP.NET Textbox client side validation with out postback

function Validation()
{
    var txtDob = document.getElementById("<%=txtDob.ClientID%>").value;
    var returnVal = "true";
    if (txtDob != "") {
        if (isDate(txtDob) == false)
            $("#<%=txtDob.ClientID%>").addClass("txtValidate");
        else
            $("#<%=txtDob.ClientID%>").removeClass("txtValidate");
    }
    if (returnVal == "true")
        return true;
    else
        return false;
}

<asp:TextBox ID="txtDob" runat="server" AutoPostBack="true" onchange="rerurn Validation();"  OnTextChanged="txtExample_TextChanged"></asp:TextBox>

protected void txtExample_TextChanged(object sender, EventArgs e)
 {
      // If the client side function returns true then only this will execute..
 }