Do mana_regen and hp_regen items stack?

Just like the title says - do mana_regen and hp_regen items stack? It has been well discussed (and documented) that save_break items DO NOT stack so curious about the _regen items. Also in the context of sets that grant _regen.

If I wear a set that grants +10% hp_regen and then I am also wearing two non-set item that grant 5% each, is my total hp_regen at 20% or at 10%?

Specific rules supersede general ones. In this case, the general rule is that bonuses from different sources stack. An example of a specific rule superseding this general rule is save breaks, as you, and the relevant help file, already point out. If there's an exception to this that isn't explicitly stated in a helpfile, please file it as a bug.

I think the above statement answers your question, but feel free to test it yourself to determine if there's indeed a bug there.