In the US the current ingredients list for Coke Zero is:

CARBONATED WATER, CARAMEL COLOR, PHOSPHORIC ACID, ASPARTAME, POTASSIUM BENZOATE (TO PROTECT TASTE), NATURAL FLAVORS, POTASSIUM CITRATE, ACESULFAME POTASSIUM, CAFFEINE, STEVIA EXTRACT.

whereas for Diet Coke it's:

CARBONATED WATER, CARAMEL COLOR, ASPARTAME, PHOSPHORIC ACID, POTASSIUM BENZOATE (TO PROTECT TASTE), NATURAL FLAVORS, CITRIC ACID, CAFFEINE.