Sorry if this has already been fixed, but seeing how little used effects are and that I've heard no mention of it, I assume not, and it is a bug in 0.3. As the title explains, effects cannot bring regeneration past zero (aka: no regen). In short, you cannot use effects to create negative hp regeneration, which would be used to create HP draining, sapping health, poison, injuries, etc etc.
This is one effect tag is even used in an example on the wiki (worth mentioning the wiki's examples are outdated and things like "movement-speed" need to be changed to "move-speed" and so on...
Also worth noting that you can reduce HP regen if it's already over zero, but you can't make it a negative number. (ie: an effect that gives -50 HP regen that affects a unit with 3 HP regen will end up with no HP regen instead of the -47 it should be).