The need for a regex solution is as you surmise, partially arbirtrary (no, I'm not doing homework, it's been 15+ years since I had any to do). The problem is that this particular problem is only a small portion of something I'm trying to solve as part of a large regex. I have considered other non-regex solutions, and I may end up returning to them, but there will be certain grace to solving the "big problem" with regex if practical.

As for inefficiency, the question becomes "how inefficient?". While what I hope to do is construct a "very massive" regex, If it can be applied to a 2K string and resolved in under a minute on single processer Intel box, that's probably fast enough for my purposes. (I realize that the question "How inefficient? is difficult to answer without a far better understanding of the entire pattern and the input, but I thought I'd ask anyway :)

I've never used lookaheads, so I'll have to "do some research" to understand your example.

Thanks.