The following is an example of how to validate a field using the asp:customvalidator control. I am going to be validating a textbox to ensure that its entry is alphanumeric without any spaces.

This is the field to be validated...

<asp:TextBox id='PersonId' runat='server' />

Here is the asp:CustomValidator on the aspx page.

<asp:CustomValidator id='PersonIdCustomValidator' runat='server'
	Display='Dynamic'
	ControlToValidate='PersonId'
	EnableClientScript='true'
	ClientValidationFunction='ValidatePersonId'
	OnServerValidate='ValidatePersonId'
	Text='* PersonId must be alphanumeric without any spaces' />

Javascript implementation of the client-side validation.

function ValidatePersonId(source, args)
{	
	var re = new RegExp("^\\w*$");
	
	args.IsValid = (window.event.srcElement.value.match(re));
}

C# codebehind implementation of the server-side validation.

protected void ValidatePersonId(object source, ServerValidateEventArgs args)
{
	bool valid = false;
	Regex re = new Regex("^\\w*$");
	if (args.Value.Length == 0)
	{
		valid = true;
	}
	else if (re.IsMatch(args.Value))
	{
		valid = true;
	}

	if (valid)
	{
		args.IsValid = true;
	}
	else
	{
		args.IsValid = false;
	}
}