<asp:textbox id="txt_EMail" runat="server">
<span>(e.g., abc@xyz.com)</span>
<asp:regularexpressionvalidator id="RegularExpressionValidator1" runat="server" xmlns:asp="#unknown">
ControlToValidate="txt_EMail" ErrorMessage="*" SetFocusOnError="True"
ValidationExpression="\w+([-+.']\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*">