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.