The title pretty much covers it. For reference, see http://www.d20srd.org/srd/monsters/troll.htm and http://www.d20srd.org/srd/specialAbilities.htm#regeneration
There is, of course, no definitive answer in the rulebooks on this. However if you know of some relevant ruling by the game developers that has been published, that would be useful. I’m also interested to hear your personal interpretations.