These are crowd sourced programs, contributed to the project by an ever-changing self-selected group.
We include both un-optimised programs and heavily optimised programs (look at the gz source code size). We include both single-thread programs and multi-thread programs (look at the cpu secs and cpu load). We include [pdf] vector parallelism and even hand-written vector instructions.
(See rebar.)
Regex might be provided by wrapping a third-party library written in some-other programming language. How would you know?
Always look at the source code.