Validate one number with regular expression

I'm trying to use the RegularExpressionValidator to validate that only one number is inputed into a TextBox.
Shouldn't this work?
<asp:Label ID="lblValImg" runat="server" CssClass="form_person_label"> <asp:Image runat="server" ID="ValidateImage" ImageUrl="~/img/val-image.png" AlternateText="Validate Image" /> </asp:Label> <asp:TextBox ID="Validate" runat="server"></asp:TextBox> <asp:RegularExpressionValidator ID="RegValidate" ControlToValidate="Validate" runat="server" CssClass="Sorry, you have entered an invalid character" ValidationExpression="[0-9]{1,}"></asp:RegularExpressionValidator>