The standalone GNU regex library has long been decommissioned.
For a current regex implementation, we recommend using the regex code from Gnulib; see also the Regular expressions chapter in the Gnulib manual, which is essentially the old standalone regex manual. (Also, if all you need is POSIX regexp functionality, any modern C library should provide that.)
The one-time rx replacement for regex has likewise been decommissioned in favor of the above.
The Free Software Foundation is the principal organizational sponsor of the GNU Operating System. Support GNU and the FSF by buying manuals and gear, joining the FSF as an associate member, or making a donation, either directly to the FSF or via Flattr.