Okay, I figured out that flex is the GNU version of lex, which is free
software.
FYI ...
Although rms has always intended flex to be the lexer in the GNU system,
and yes, flex is free software, it is not an official GNU package. (A
situation similar to TeX, X11, etc.)
To make things even more confusing, a manual for flex written by
Gavin Nico *was* made a GNU package, but hasn't been updated in umpteen
years (to the point where it is no longer useful), and I've had no luck
in straightening out the situation.
