Basically this regex library (Boost.Xpressive) supports what I like about perl's regex engine: The (??{ code }) feature (except with different syntax). This means what I had to hack around in grok-perl I can easily express in C++ code. Awesome.

The docs only show examples of using static regexes with this great feature. I'm going to try using it with dynamic regexes. If it works, I'll be converting grok to C++.