I tried this. It seems to work. int lower_case(int c) { return tolower(c); } ... ... ... if (boost::filesystem::is_regular(itr->path())) { path = itr->path(); lpath = path.string(); transform(lpath.begin(), lpath.end(), lpath.begin(), lower_case); std::cout << path << std::endl; std::cout << lpath << std::endl; }