Renaming Tags

I'm stumped. I need to be able to import and rename tags as follows:

Take the composers last name from the %composer% field formatted as Haydn, Joseph and append it to the %album% field thus: Haydn: String Quartets, Op. 50

Can anyone help please?

Use as format string: $regexp(%composer%,'(.*), .*',$1): %album%

Thanks very much for your help. It's exactly what I was after and I'm rather happy that I wasn't being stupid. I would never have been able to derive that expression!