Macos – Text Editor that hilights all instances of selection for Mac

Emacs can do it. Once you've loaded a file, press:

Alt + s then h and finally r

or alternatively if your version of Emacs doesn't recognize the previous shortcut:

Alt + x then type highlight-regexp and press Enter

Emacs will ask you the regexp (a simple word or phrase will work too) to highlight, then it will ask you the color name too, usually starting with hi- e.g. hi-yellow (you can use Tab to complete). That's all.

It may sound complicated but that's Emacs; there are plenty of ways to speed up this process if you want to mess up with it. And I suggest you to do so.

I will always answer Vim (or MacVim in this case): it does everything you want, some more and runs on every platform which gets the next hype!

You can split horizontally and vertically until you bleed. If you shy away from the steep learning curve of pure VIM, you can make Cream to tune Vim into an easier and "more to the standards" version ... but of course you can still vertically split and tab yourself to death.