Emacs is completely programmable. Everything is written in LISP, and everything can be overruled. You can make the ‘a’ key insert the character ‘a’, but 1 in a 100 times insert ‘b’. Emacs is the originator of incremental regex search, it had auto-complete long before even Visual Studio, and it has more than one way to use snippets. It doesn’t simply have levels of indent and regions, but an extensive org-mode that can do the weirdest things wit them. You can select a piece of text, pipe it through a shell command, and have it replace the original. It has a symbolic expression solver! But that comes at a price: emacs is complex. But if you want to have an idea of how flexible an editor can be, take a look. It’s still going strong, despite its age.