Scheme 48 is an implementation of the Scheme programming language as
described in the Revised5 Report on the Algorithmic Language Scheme. It
is based on a compiler and interpreter for a virtual Scheme machine.
Scheme 48 tries to be faithful to the Revised5 Scheme Report, providing
neither more nor less in the initial user environment. It is based on a
byte-code interpreter and is designed to be used as a testbed for
experiments in implementation techniques and as an expository tool.

It has a foreign function interface for calling functions from C and
comes with a regex library, and a POSIX interface. It is supported by
SLIB the portable Scheme library.