asp.net code not working validation
===

i can't figure out what exactly is wrong in this code. I an new to asp.net.

    <body>
        <form id="form1" runat="server">
        <div>
        <asp:TextBox ID="txt" runat="server" />
            <asp:RegularExpressionValidator ID="RegularExpressionValidator1" runat="server" 
                ErrorMessage="RegularExpressionValidator" ValidationExpression="^\d{4}$"></asp:RegularExpressionValidator>
        
        </div>
        </form>
    </body>