The (real) risk of misunderstanding exists with all options that include "null", "empty", "void", "nothing", "nada". The name refers to a set of matches but can be understood as qualifying the match itself, as a set of chars. If you teach programming and mark code written by students, you know that the confusion will definitely happen.

This is why I much prefer Alan's re-nomatch suggestion.

On Sun, 19 May 2019 at 09:33, Mattias Engdegård <mattiase@acm.org> wrote:

19 maj 2019 kl. 07.00 skrev HaiJun Zhang <netjune@outlook.com>:
>
> > regexp-null? It's also a little cryptic, but it's short!
>
> What about regexp-nothing or re-nothing?

Those are fair suggestions and close to the mathematical meaning, but could possibly be misunderstood as patterns (only) matching the empty string.