Undoing Changes
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Emacs allows all changes made in the text of a buffer to be undone, up to a certain amount of change.  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 worth (undo).

C-_
     The same.

The command C-x u or C-_ is how you undo.  The first time you give this command, it undoes the last change.  Point moves back to where it was before the command that made the change.

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 will 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.  At this time, all the modifications you made have been cancelled.  If you do not remember whether you changed the buffer deliberately, type C-_ once, and when you see the last change you made undone, you will remember why you made it.  If it 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, it means that the buffer contents are the same as they were 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.

You cannot undo mere cursor motion; only changes in the buffer contents save undo information.  However, some cursor motion commands set the mark, so if you use these commands from time to time, you can move back to the neighborhoods you have moved through by popping the mark ring (see Mark Ring).

When the undo information of a buffer becomes too large, Emacs discards the oldest undo information from time to time (during garbage collection).  You can specify how much undo information to keep by setting two variables: undo-limit and undo-strong-limit. Their values are expressed in units of bytes of space.

The variable undo-limit sets a soft limit: Emacs keeps undo data for enough commands to reach this size, and perhaps exceed it, but does not keep data for any earlier commands beyond that.  Its default value is 20000.  The variable undo-strong-limit sets a harsher limit: the command which pushes the size past this amount is itself forgotten.  Its default value is 30000.

Regardless of the values of those variables, the most recent change is never discarded, so there is no danger that garbage collection occurring right after an unintentional large change might prevent you from undoing it.

The reason the undo command has two keys, C-x u and C-_, set up to run it is that it is worthy of a single-character key, but 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.