I'm a diet coke addict and surprised it didn't come right out with water. Try Oxyclean or white vinegar. Peroxide works on blood, not sure it'll help here or not. Don't dry it or it will be set forever. Good luck, this will surely happen to me one day, though I try to drink favored selzer water in the sewing room. One of the many blessings in sewing is that when I'm sewing, I'm not eating.