Automating analysis of forum posts

I am looking for a way to automatically analyse a large quantity of message board posts along with each posts' username. The idea would be to match up the writing styles of each username to identify people who post using multiple accounts.

If anyone could give me some pointers in the right direction on what types of algorithms exist to do something like this I would be very grateful. I am presuming the algorithm might look at things like types of sentence constructs and semantics, but I am not sure exactly where to start researching this.

Many thanks in advance,
Chris