I have This weapon below, and it has "Fire Damage". My build is all Physical damage. I want to roll the Fire Damage for Physical damage, however that's not an options.. Is it regular damage, or how does this work?
+X-Y Damage is indeed Physical damage, but you shouldn't care anyway.
For several patches, Blizzard has gradually removed the interaction between weapon bonus element and skill element. Currently weapon element does not matter at all.
Every skill converts all weapon damage to element of the skill. The only point in rerolling is increasing the +damage roll range itself, and you can pick any element that rolls.