My tests agree that this works. Thank you so very much! I will have to learn these extended regex functions better.... Also thanks for the tip on the explain package.

These lookahead functions appear to go beyond the computational power of traditional regular expressions. (At least, I can't think of a way to implement them fully using normal regex's.) I am starting to wonder whether I was trying to literally do the impossible, though I suspect there is a "pure" regex that could do the job.