Hi, I've come across a circumstance, using boost::regex, where I think the interface is exposing less flexibility than the underlying architecture provides. Specifically, I'd like to do a regex_replace(), but instead of a format string specifying the output of the transformation, I'd like to provide a functor. The reason I need this is: the replacement I'd like to do is not a straighforward text substitution, but rather involves rummaging through a database to retrieve stored information about the match results. Aside from that, however, the overall algorithm for finding matches and outputting the transformed text is identical. The good news, as far as I can tell, is that the guts of boost::regex are already structured so as to make this easy. The basic_regex_formatter<> seems to express a concept that can be factored out and exposed in the public interface. The end result I'd hope for are additions something like: template <class OutputIterator, class Formatter> OutputIterator match_results::format(OutputIterator out, Formatter fmt, match_flag_type flags = format_default) const; template <class Formatter> string_type match_results::format(Formatter fmt, match_flag_type flags = format_default) const; template <class OutputIterator, class BidirectionalIterator, class traits, class charT, class Formatter> OutputIterator regex_replace(OutputIterator out, BidirectionalIterator first, BidirectionalIterator last, const basic_regex<charT, traits>& e, Formatter fmt, match_flag_type flags = match_default); template <class traits, class charT, class Formatter> basic_string<charT> regex_replace(const basic_string<charT>& s, const basic_regex<charT, traits>& e, Formatter fmt, match_flag_type flags = match_default); This way, I could specify my custom formatter and use it in place of the default basic_regex_formatter. My present workaround is to essentially reinvent regex_replace() in my own code, by cobbling together a while-loop predicated on regex_search(), but it's not very elegant. Does this make sense / need clarification? Is it a desirable design goal? I'm happy to do the work and provide a patch for review, particularly if John approves. Cheers, dr