This is not a primer on ASP.NET Validation controls, there are plenty of those out there. This is just my handy little piece of cheat code - it's easier to copy it out of here when I need it...
First, place the ValidationSummary control on the page. We'll have the individual controls display an '*' for error, and the Summary control will show the error text
<asp:ValidationSummary id="valSummary" runat="server"
HeaderText="Errors:"
ShowSummary="true" DisplayMode="List" />
This RegularExpressionValidator will make sure that the validate field has one to three numeric characters:
<asp:RegularExpressionValidator runat="server" ID="regexpAge"
ControlToValidate="txtReqShipDays"
ValidationExpression="^\d{1,3}$"
ErrorMessage="Req Ship Days must be one to three digits."
Display="Dynamic" Text="*" />
Other regex expressions to use:
"^\d" - numeric only
This RequiredFieldValidator will be sure that a field is not left empty:
<asp:RequiredFieldValidator runat="server" ID="reqAge" ControlToValidate="txtReqShipDays"
ErrorMessage="Missing Req Ship Days" Text="*" Display="Dynamic" />