Date Format Validation in asp.net

<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %>
 <asp:TextBox id="txtDate" runat="server" CssClass="txt_borderNo" Width="204px"
                                             ></asp:TextBox>
                                             <br />
    <asp:RegularExpressionValidator ID="RegularExpressionValidator1" ControlToValidate="txtDate" runat="server"
     ErrorMessage="Enter valid date" ForeColor="Red"
     ValidationExpression="^(0[1-9]|[1-9]|[12][0-9]|3[01])-([Jj][Aa][Nn]|[Ff][Ee][Bb]|[Mm][Aa][Rr]|[Aa][Pp][Rr]|[Mm][Aa][Yy]|[Jj][Uu][Nn]|[Jj][Uu][Lj]|[Aa][Uu][Gg]|[Ss][Ee][Pp]|[Oo][Cc][Tt]|[Nn][Oo][Vv]|[Dd][Ee][Cc])-(19|20)\d\d$">
    </asp:RegularExpressionValidator>
<cc1:CalendarExtender id="CalendarExtender1" runat="server" Format="dd-MMM-yyyy"  TargetControlID="txtDate"></cc1:CalendarExtender>