shhhh don't tell anyone but I occasionally have a glass of diet lemonade. It has citric acid in it. It doesn't knock me out of ketosis. Some people are knocked out by citric acid, but not all. I tried a small glass one night, and tested the next morning, no difference. The first week when I was so sick of water it was a life saver for me! Now I have the water flavourings its not soo bad.
If you really want some diet coke, have some and keep an eye on your ketostix tomorrow, you'll soon find out if it affects you or not.