There are three bodies of source code that can be referred to as "Henry Spencer's regular expression library".
Firstly the "old library" or the "book library". This dates back to
around 1986 and was updated for the
book Software Solutions In C, ed. Dale Schumacher, Academic
Press, 1994 [ref: Usenet]. This version is a bit later: the man page is dated 5 September 1996 and the tar archive was created on 4 April 1998.
Originally from an ftp server at zoo.toronto.edu.
The second is the "BSD library". This is a POSIX.2 compliant library that was included in 4.4BSD Unix. Spencer wrote that it was basically an alpha release, and pretty slow [ref: Usenet].
Originally from an ftp server at zoo.toronto.edu, this version is dated 10 August 1999.
I have also prepared a shared library version, using Autoconf etc., to allow installation as a system library in GNU/Linux or similar systems. See the WHATSNEW page for details of changes.
rxspencer-alpha3.8.g5.tar.gz (342860 bytes, December 15, 2013)
The third is the "Tcl library", which was added to Tcl in 1999 (version 8.1) and supports
wide-character Unicode. Although Spencer intended to release this
library as a standalone package [ref: Usenet], I don't think
he ever got around to it.
However, a port by Walter Waldo's is available: hsrex.tar.gz (79908 bytes). Documentation is somewhat lacking for this library, but here's a basic example for the ASCII version: hsrex-char.c. The Tcl library documentation may also be useful.