RX - A fast replacement for the GNU regex library.
RX has been decommissioned, since no one has come forward to revive it, to improve it over the other free software regex library programs that are available, which would be quite a big task. (If you are interested after all, email <email@example.com>
For a current regex implementation, we recommend using the regex code from Gnulib; see also the Regular expressions chapter in the Gnulib manual. (Also, if all you need is POSIX regexp functionality, any modern C library should provide that.)
The one-time regex standalone library has likewise been decommissioned in favor of the above.