How to allow only numeric values in a textbox using ASP.NET Validator control

<asp:TextBox id='txtNumber' Runat='server' />
<asp:RegularExpressionValidator ID='vldNumber' ControlToValidate='txtNumber' Display='Dynamic' ErrorMessage='Not a number' ValidationExpression='(^([0-9]*|\d*\d{1}?\d*)$)' Runat='server'>
</asp:RegularExpressionValidator>