boost::regex documentation runs "... embarrassments / limitations will be removed in due course, mainly dependent upon user demand ...". I just wote for zero-length lookbehind "(?<=pattern)" implementation.

That will be in the next release (1.33; soon to be released): the limitation (which I believe is true of Perl also) is that the lookbehind pattern must be of fixed length. In practice most expressions that use this feature only look behind one character anyway, so hopefully it's not too much of a limitation. John.