Thanks for the suggestions, guys. It's true that there probably is enough memory to read both of them at once. I can open both files into poxy notepad, so it must fit into memory.

And I'll see how far I can get with your suggestion oliver, at least it has syntax I can understand. The big problem with outputting non-matches into another file is that there is a variable number of lines between matching the <username> the first time and the next(last) time - all of these lines need to be omitted. I suppose you match once, read all subsequent lines into a dummy hash until the next match, where you start reading lines into your output hash again. I can see the concept, but not quite how to execute it...

Thanks again for the food for thought