<form runat="server">
<asp:RegularExpressionValidator id="RegularExpressionValidator1"
ControlToValidate="Numbers"
ValidationExpression="\d{3}-\d{3}-\d{4}"
Display="Static"
EnableClientScript="false"
runat="server">
Must be in the format XXX-XXX-XXXX
</asp:RegularExpressionValidator>
<asp:TextBox id="Numbers" runat="server"/>
<asp:button type="submit" name="btnSubmit" text="Enter in phone number " runat="server"/>
</form>