Alas, I haven't the capital to subscribe to ACM, so I cannot read the paper.
However, I think you are likely stuck on the idea of a regular expression derivative. You can read more about them -- how they are constructed and used -- here: http://www.ccs.neu.edu/home/turon/re-deriv.pdf
You do need to be familiar with basic computational theory and the notation commonly used. If not you'll need to google around a little for it.
Also, you should be familiar with graph theory and, particularly as relates to re derivatives, functional list processing idioms.
This is a lot to take in at once. Let us know where exactly you get stuck.