Getting a textbox to accept negative number

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ASP.NET

I have a form that has three textboxes on it. The user can enter a negative number and the textboxes does the calculations with it but when the user clicks on the submit button the validation fires. How can I get the validation to work and pass the negative numbers?

<td>
                <asp:TextBox ID="TextBoxTA" runat="server" Width="180px">0</asp:TextBox>
                <asp:Label ID="lblreq20" runat="server" CssClass="style52" Text="*"></asp:Label>
                <asp:RegularExpressionValidator ID="RegularExpressionValidator33" 
                    runat="server" ControlToValidate="TextBoxTA" CssClass="style45" 
                    ErrorMessage="You Must Enter A Number For This Field" ForeColor="Red" 
                    ValidationExpression="^\d+$"></asp:RegularExpressionValidator>
            </td>