/^\w+((\.\w+){0,3})@\w+(\.\w{2,3}){1,3}$/

Jquery

<script type="text/javascript">
        $(function () {
            $("#txtEmail").blur(function () {
                var value = this.value;
                if (value.length > 0 && !value.match(/^\w+((\.\w+){0,3}$/)) {
                    alert("Email format errors.");
                    this.focus();
                    return false;
                }
            });
        })
    </script>