I have been having regular coke on a regular basis (maybe once a fortnight). I hate Diet coke so I can't help you on that one! I hate all diet drinks they just taste yuck to me. I do take Panadine rapid solubles tho which have phenelethane/aspertame (fake sugar) in them. There is nothing on there to say pg women shouldn't take them.
HTH
IMO it's not really that bad, and it's not like you are drinking litres of the stuff or having it with scotch!