The discovery functions were not working in our environment. After some digging it turns out this was because both the regexps and expressions tables were empty.
I believe this may have happened during migration work from 1.8 => 2.0 => 2.2.

It would be nice that the interface (and/or the server logs) would mention that regexps are used but not defined.
This would've made it more clear that the configuration in its current form is not possible.