Ah, I see. Ok. So we're dealing with a sort of tri-state thing here: `undefined | false | regexp()`, where `undefined` is the initial state (and should never be seen by the backend), `false` means `unsupported` or `not_applicable`, and tells the backend there is no regexp filter, and anything else _is_ a compiled regexp to apply. I get it now. I guess I'm ok with `false`, I was thrown 'cos we're not dealing with a boolean.