Yes you can
. When pg the limit is 200mg a day
http://www.nhs.uk/news/2008/11November/Pages/Newcaffeineadviceinpregnancy.aspx
Red bull has 80mg a tin
http://www.energyfiend.com/the-caffeine-database
So yes technically you could have 2 a day, but no coffee, cola, chocolate etc on top of it.
At the start of my pg I was drinking 3-4 tins of coke every day, which was within limits but not ideal. I told MWs and they said it was prob my body craving energy/sugar as I was so sick and not eating much. They said not to worry if it passed in a few weeks BUT their main concern was the amount of sugar and the weight id put on if I continued drinking it.
I dont think there any problem with the odd tin occasionally. They contain less caffeine than a mug of coffee, which most pg women who enjoy it have the odd time.