Regular expression is correct but still get error message in asp.net

Question

I have a textbox and it accepts blank and number only. I test the valiadtionExpression on REGULAR EXPRESSION. It work as expect. However on my page, when I input a number, it show the error message.

There is my code snippet

<asp:TextBox id="txtQty" Runat="server" class="w20" MaxLength="3"></asp:TextBox>
<asp:RegularExpressionValidator ID="valQty" runat="server" 
     ErrorMessage="must be 0 or more than 0"  ControlToValidate="txtQty" 
   ValidationExpression="/^\s*\d*\s*$/"  Display="Dynamic" />