No, it's just a DSL for standard regular expressions. But it seems that passing the `case-fold` flag with `font-lock-defaults`, as I have done on line 107, is the standard way to deal with the case-insensitive keywords here. `rx` is pretty nice, but even the plain regex version can be made more readable using the `regexp-opt` function, e.g. `(concat "^" (regexp-opt '("from" "run" ...)))`. Both `rx` and `regexp-opt` merge regexps to minimise backtracking, so `(regexp-opt '("foobar" "floop")) => "\\(?:f\\(?:loop\\|oobar\\)\\)"`, which is pretty neat.