I have a user who says they have a bunch of expressions written for the Oniguruma regex library that use \k<name> syntax. According to http://www.boost.org/doc/libs/1_44_0/libs/regex/doc/html/boost_regex/syntax/... Boost.regex should also support that syntax, but it does not seem to work. For example: boost::regex_replace( std::string("xyyz"), boost::regex(".*(?<first>x)(?<second>y).*", boost::regex_constants::perl), std::string("$+{second}\\k<first>"), boost::match_default | boost::format_all | boost::match_extra); results in "yk<first>" This result shows that $+{second} evaluated to "y" but \k<first> evaluated to "k<first>". I thought maybe the \k<name> syntax was only supported as a backreference within the first regex instead of the replace string, but this doesn't work either: boost::regex_replace( std::string("xyyz"), boost::regex(".*(?<first>x)(?<second>y)\\k<second>.*", boost::regex_constants::perl), std::string("$+{second}"), boost::match_default | boost::format_all | boost::match_extra); The result is "xyyz" meaning nothing was replaced (the regex did not match). However, if modify the regex to use \\g{name} syntax, it works: boost::regex_replace( std::string("xyyz"), boost::regex(".*(?<first>x)(?<second>y)\\g{second}.*", boost::regex_constants::perl), std::string("$+{second}"), boost::match_default | boost::format_all | boost::match_extra); The result is "y". Am I doing something wrong or have I found a bug? I'm using the latest stable 1_44_0 release compiled from source in VC9 and I also tried the pre-compiled library (http://sourceforge.net/projects/boost/files/boost-binaries/1.44.0/libboost_r...). The OS is Windows 7. Thanks for the help!