Apologies if these are really simple questions to answer (first time developing using the new membership provider)

I'm trying to mess around with the extended membership profile properties and have a few questions relating to this.

How do I return a list of Users that have a blank value for a particular extended profile property?

For instance I've added a leave date property to the profile and want to return a list of all users that do not have a value in that field (I've read the stuff about there not being a date data type so have gone for the text option with regular expression validation)

I've tried things such as:

lstStaff.DataSource = UserController.GetUsersByProfileProperty(Me.PortalId, "EndDate", "", 0, 1000, 1000)
lstStaff.DataValueField = "UserId"
lstStaff.DataTextField = "DisplayName"
lstStaff.DataBind()

and

but to no avail.

Also how would I go about returning a list of users whereby a particular extended profile property value only partially matched the 'search' value?

Thanks in advance