The part that makes it difficult is that it doesn't return the same matches, it returns almost the same matches but not exactly.
But if PCRE semantics isn't set in stone then i hope leftmost longest could be the default some day. There's a lot of nice things you get for free with the two pass approach
Ah, there is a post with more detail about RE# and discussion here recently that I must have missed: https://news.ycombinator.com/item?id=47206647