string split at xth position

Hi,

I have a vector of strings like:
c("a1b1","a2b2","a1b2") which I want to spilt into two parts like:
c("a1","a2","a2") and c("b1","b2,"b2"). So there is
always a first part with a+number and a second part with b+number.
Unfortunately there is no separator I could use to directly split
the vectors.. Any idea how to handle such cases?

/Johannes