Undoing Changes
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Emacs allows you to undo all changes you make to the text of a buffer, up to a certain amount of change (8000 characters).  Each buffer records changes individually, and the undo command always applies to the current buffer.  Usually each editing command makes a separate entry in the undo records, but some commands such as query-replace make many entries, and very simple commands such as self-inserting characters are often grouped to make undoing less tedious.

C-x u
     Undo one batch of changes (usually, one command's worth) (undo).

C-_
     The same.

The command C-x u or C-_ allows you to undo changes.  The first time you give this command, it undoes the last change.  Point moves to the text affected by the undo, so you can see what was undone.

Consecutive repetitions of the C-_ or C-x u commands undo earlier and earlier changes, back to the limit of what has been recorded.  If all recorded changes have already been undone, the undo command prints an error message and does nothing.

Any command other than an undo command breaks the sequence of undo commands.  Starting at this moment, the previous undo commands are considered ordinary changes that can themselves be undone.  Thus, you can redo changes you have undone by typing C-f or any other command that have no important effect, and then using more undo commands.

If you notice that a buffer has been modified accidentally, the easiest way to recover is to type C-_ repeatedly until the stars disappear from the front of the mode line.  When that happens, all the modifications you made have been canceled.  If you do not remember whether you changed the buffer deliberately, type C-_ once. When you see Emacs undo the last change you made, you probably remember why you made it.  If the change was an accident, leave it undone.  If it was deliberate, redo the change as described in the preceding paragraph.

Whenever an undo command makes the stars disappear from the mode line, the buffer contents is the same as it was when the file was last read in or saved.

Not all buffers record undo information.  Buffers whose names start with spaces don't; these buffers are used internally by Emacs and its extensions to hold text that users don't normally look at or edit.  Minibuffers, help buffers, and documentation buffers also don't record undo information.

Emacs can remember at most 8000 or so characters of deleted or modified text in any one buffer for reinsertion by the undo command. There is also a limit on the number of individual insert, delete, or change actions that Emacs can remember.

There are two keys to run the undo command, C-x u and C-_, because on some keyboards, it is not obvious how to type C-_. C-x u is an alternative you can type in the same fashion on any terminal.