Try using following:
<asp:TextBox ID="tbxEmailAddress" runat="server" CssClass="text"></asp:Textbox>
<asp:RequiredFieldValidator ID="rfvBillingEmailAddress" runat="server" ControlToValidate="tbxEmailAddress" ErrorMessage="* Required" Display="dynamic" SetFocusOnError="true" EnableClientScript="true" ValidationGroup="Vgroup">
</asp:RequiredFieldValidator><br />
<asp:RegularExpressionValidator runat="server" ID="revBillingEmailAddress" ControlToValidate="tbxEmailAddress"
EnableClientScript="true" ErrorMessage="Incorrect Email Format" SetFocusOnError="true"
ValidationExpression="\w+([-+.]\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*" ValidationGroup="Vgroup">
</asp:RegularExpressionValidator>
<br />
<asp:Button ID="Button1" runat="server" Text="Button" ValidationGroup="Vgroup" />