GNU Emacs is an extensible, customizable, self-documenting real-time display editor. At the core of Emacs lies an Emacs Lisp interpreter, the language in which the majority of Emacs' built-in functionality and extensions are implemented. Also consider asking your question at emacs.stackexchange.com.

GNU Emacs is an extensible, customizable text editor. Begun in the mid-1970s as TECO, it was re-written using C and Emacs Lisp to provide portability and an extendable interface. It continues to be actively developed today.

Emacs provides context-sensitive editing modes with syntax coloring, is self documenting, has full Unicode support and extensions to do almost anything. Die-hard Emacs users do most everything from within Emacs: write, compile, run and debug code; read/compose email; browse the web; do project planning etc.

Notice that there are several popular Mac versions of Emacs (and probably still more unpopular ones! :-)

When asking about Emacs, please specify which package you are using. The URL where you downloaded it is often a good enough indication.

Perhaps see also https://emacs.stackexchange.com/questions/271/what-is-the-difference-between-aquamacs-and-other-mac-versions-of-emacs

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