URGENT plz! How to clear an asp TextBox when RegularExpressionValidator returns an error value

Hi

I am using a asp Text box having runat = server.
<asp:TextBox ID="tphone" runat="server" MaxLength="10" />
For validations I used RegularExpressionValidator
<asp:RegularExpressionValidator ID="RegularExpressionValidator1" runat="server" ControlToValidate="tphone"
                                                                ErrorMessage="Enter Any Valid Mobile No" ValidationExpression="^\d{10}$"></asp:RegularExpressionValidator>


This is working fine , however I want to clear the textbox if there is an error. Currently ...