Client-side validation for two separate conditions - asp.net

Question

Is there a way to validate asp.net text box for input using built-in validators for two distinct conditions? The text box is used in conjunction with a date picker and presently I have RegularExpressionValidator like this to validate:
<asp:TextBox ID="tbStartDate" runat="server" Width="100" CssClass="TextCompact">yyyy</asp:TextBox>
<asp:HyperLink ID="hlStartDate" runat="server" ImageUrl="~/Images/iconCalendar.gif"></asp:HyperLink><br />
<asp:RegularExpressionValidator ID="revStartDate" ControlToValidate="tbStartDate"
CssClass="TextCompact" runat="server" ErrorMessage="* Invalid Date" ValidationExpression="^(\d{4})$" />
which fires everytime an invalid date is entered - you can use the date picker or you can type in a date string.
So the issue is that I only want to validate if there is data in the text box. If the text box is empty I want it to pass through without validation.
Is this possible or would a custom client side validation be required?
Thanks,
Risho.
Update:
I've decided to write my own validator class derived from the BaseValidator class and wired it up but I get a compiler error.
namespace Abcdefg1234
{
public class Custom_Validator : BaseValidator
{
public Custom_Validator()
{
}
protected override bool EvaluateIsValid()
{
String value = this.GetControlValidationValue(this.ControlToValidate);
return ValidateControls(value.Trim());
}
public static bool ValidateControls(string value)
{
if(Regex.IsMatch(value, #"^(\d{4})$") == true)
return true;
else if(value == "")
return true;
else
return false;
}
}
}
Then I included the page directive tag:
<%# Register TagPrefix="Cv" Namespace="Abcdefg1234" TagName="CustomValidaitonX" %>
and the actual tag:
Cv:Custom_Validator ID="cvtbStartDate" ControlToValidate="tbStartDate" runat="server" ErrorMessage="* Invalid Date" />
I get a unknown server tag. Any ideas? Thanks.

Answer 1

The only built-in validator that fires on an empty text box would be the RequiredFieldValidator. If a textbox is empty, the other validators associated with it won't fire on form submission.
In other words, you should be fine just as you are, with the RegularExpressionValidator not firing if you submit with the textbox still empty. Are you experiencing different behavior?
ETA: You need to specify the Assembly name in your Register page directive for the Custom_Validator to be recognized. I don't believe the tag name is required for a custom control.