ISBN 123456789X is parsed as an ISBN. ISBN 123456789 is not, because it doesn't have enough digits. That means you need quite a lot of lookahead and backtracking for ISBNs, at least in the tokenizer. Which was my point, the tokenizer will need to be able to backtrack. It's not a big issue, I don't think, judging by the flex docs, which was the reason for my post: responding to Steve Sanbeg's remark about how much lookahead is needed by the tokenizer.
I may be completely misunderstanding all this, but doesn't ISBN just require lookahead, not backtracking? Nothing that comes before the "ISBN" bit is relevant.