regular expression to not accept multiple 9's

 the text box should not accept eith one or multiple nines (only nines number). I know the regular expression works fine when I test it on regular expression tool but for some strange reason it does not work on asp.net validation control.
Here is the code:
<asp:RegularExpressionValidator ID="txtboxRegExp" runat="server" ControlToValidate="txtbox" ToolTip="Can not contain only 9's" ValidationExpression="^9+$" ValidationGroup="VGroup" SetFocusOnError="True">!</asp:RegularExpressionValidator> ...