Globstar operator in Bash

I learned today today a code like:

echo test/**/*_test.rb

prints all files matching this pattern. However, I noted that certain files were missing in subdirectories of subdirectories, like test/a/b/c_test.rb.

Turns out, this globstar operator needs to be activated on most systems.

Bonus learning: On Mac OS X, it does not work at all since the bash version they ship is too old:

bash --version
GNU bash, version 3.2.57(1)-release (x86_64-apple-darwin22)
Copyright (C) 2007 Free Software Foundation, Inc.

Globstar was released with Bash v4 in 2009.

(yes I still need to switch to zsh).