Not all sugar-free carbonated drinks contain alternative sweeteners, so check the ingredient list for aspartame, saccharin, sucaralose and acesulfame potassium.
But President Theodore Roosevelt, who used saccharin after his doctor prescribed him a sugar-free diet, personally intervened to prevent it being banned.
So, fructooligosaccharides are basically sugars, but it sounds like they may contain a couple of different types of sugar, fructose along with some saccharin.