Flex 2.5.11 is rather old now. When I have time I will also add another stage to compile a more recent version. But I believe that flex 2.5.11 can generate the lexer of the latest flex release. Also, it seems that all flex versions can be compiled with a yacc that is not bison. This is what I recall from my experiments of a few weeks ago, that I still have not consolidated in a working script. Surprises might happen!
The other steps of the project are described in the README (where flex is not mentioned yet, but all the rest is). Later steps will be bison, as I said, and then who know. I should aim at Perl, which then would unlock autotools, but Perl itself has a lot of dependencies and I haven't yet begun unravelling them. Also bash and coreutils should be bootstrapped at some point, but for the moment dash and Heirloom can do their job without problems, so it is not a priority (although it might become at some point). Later targets will be binutils, gcc and glibc, but do not hold your breath. Eventually I would like to bootstrap Debian packaging tools and start building Debian packages. That might require that I train my offspring into programming and convince them to continue my mission.
It is also possible (though tedious) to build GCC and binutils without autotools.
Getting from epsilon to Debian seems a bit like following stepping stones across a swamp. It's unclear which route is most likely to work, but whichever way you pick you end up ankle-deep, or waist-deep, in mud.
Also, it's not clear what "epsilon" should be. My feeling is that the ideal minimum prerequisites would be something like a C89 native compiler on an operating system that is much simpler than Unix. So you could read and write files and execute a subcommand, perhaps, but no pipes and no fork. So the first major goal would be to build GCC and binutils as a cross-compiler, and the second major goal would be to cross-compile those things plus the Linux kernel and some kind of shell so that you could then move to Linux.
The whole thing is a big project, but if you can describe a way of getting from stepping stone A to stepping stone B, where B is a bit closer to Debian than A was, then that may help future explorers, and that use of Heirloom Project's lex to build flex sounds like a good example of that.
To further reduce epsilon (or, in somewhat standard terminology, the "binary seed") I am also writing asmc (https://gitlab.com/giomasce/asmc), which is a minimal operating system with a minimal compiler, totalling less than 6 KiB binary seed. This compiler is not for C, but for G, which is a C-inspired language that I invented for this purpose (see the project README and links for more details). The target for asmc is to be able to build and boot Linux and a static copy of tcc (the latter is more or less already there; Linux is, of course, the hard part), at which point it can bootstrap nbs. I'm not there yet, though.
Also, to be fair, although asmc's seed is 6 KiB, you still need an x86 CPU (I'm not aware of free implementations), a loader and a BIOS to have a functional environment. This is not little thing, but I guess it is a reasonable target so far.
