bug#5393: Patch for lookaround assertion in regexp

[Mattias Engdegård]

>That looks pretty cool to me, but there was next to no discussion about this 
>at the time (2009), which is a bit odd... 

Didn't see it until now myself, and it looks useful, but perhaps it's unwise to 
add features that couldn't easily make it into a DFA-based engine. Positive 
lookahead maybe, the rest probably not. (The old engine would need to be kept 
around for backrefs, but those are fairly rare in actual use.)