Regular expressing error message is not displaying .net - html

Question

My reqular expression is not working but error is not popping
Here is my brief code - It looks simple, I could not able figure out the error in my code.
<TR id="ROW2" runat="server" visible="false">
<TD class="main" align=left>
<asp:Label id="Lab3" runat="server" Text="Cheque No: "></asp:Label>
</TD>
<TD vAlign=top align=left>
<asp:TextBox id="TxtChequeno" runat="server" MaxLength="20" cssclass="textbox"></asp:TextBox>
<asp:RegularExpressionValidator style="VERTICAL-ALIGN: top" id="REVMobile" runat="server"
ControlToValidate="TxtChequeno"
ErrorMessage="Please enter valid Cheque number"
ValidationExpression="^(?=.*\d).{10}$"
ValidationGroup="tripSheet">*</asp:RegularExpressionValidator>
</TD>
</TR>
<TR id="ROW3" runat="server" visible="false">
<TD class="main" align=left>
<asp:Label id="Lab4" runat="server" Text="Cheque Date: "></asp:Label>
</TD>
<TD vAlign=top align=left>
<asp:TextBox id="txtFromDate" runat="server" CssClass="textbox" MaxLength="11" Width="128px"></asp:TextBox>
<asp:Image style="CURSOR: pointer" id="ImaArr1" runat="server" ImageUrl="./images/CAL.GIF"></asp:Image>
<cc1:CalendarExtender id="CalendarExtender2" runat="server" CssClass="yui" TargetControlID="TxtCardDte" PopupButtonID="ImaArr1" Format="dd-MMM-yyyy"></cc1:CalendarExtender>
<asp:RequiredFieldValidator id="RFtxtFromDate" runat="server" ControlToValidate="txtFromDate" ErrorMessage="Select Arrival date" SetFocusOnError="True" ValidationGroup="tripSheet">*</asp:RequiredFieldValidator>
<asp:CustomValidator ID="CustxtFromDate" runat="server" ClientValidationFunction="CallDateFunForToDate1" ControlToValidate="txtFromDate" ValidationGroup="tripSheet" errormessage="Invalid Cheque Date." style="vertical-align:top;">*</asp:CustomValidator>
</td>
</TD>
</TR>
<TR id="RowBtn" runat="server" visible="false">
<td>
<asp:Button ID="BtnAdd" runat="server" Text="Add" CssClass="button" ValidationGroup="tripSheet" />
</td>
</TR>