How to use Undo functionality in Nano?

Starting with nano version 2.3.5 in July 2014, undo/redo became standard:

• Alt-U or Option-U or EscU - undo
• Alt-E or Option-E or EscE - redo

It's in the help:

And if you stretch the screen wider than about 1400 pixels, there's a hint at bottom right:

If these keys don't work, check nano --version.

• nano versions 2.1.10 - 2.3.4 should use the command line option -u (thanks @sil). Tip: add alias nano="nano -u" to your .bashrc (thanks @Benia). The implementation of undo during this period appears to have been buggy or incomplete.

• nano versions 2.1.3 - 2.1.9 had some kind of undo feature. The -u option was not necessary.

M stands for meta.

Nano's undo code is experimental. As you'll see from the nano manual (type "man nano" in a Terminal to read that), you'll need to start nano with the -u option (so "nano -u somefile.txt"), and then you can use Alt-U to undo.

You can also add the line

set undo

in the .nanorc file. This enables the experimental undo feature on startup in versions below 2.4, such as 2.2.6 which seems to be the standard installation on many Linux distros.