@c This is part of the Emacs manual.
@c Copyright (C) 1985-1987, 1993-1995, 1997, 2001-2016 Free Software
@c Foundation, Inc.
@c See file emacs.texi for copying conditions.
@node Fixit
@chapter Commands for Fixing Typos
@cindex typos, fixing
@cindex mistakes, correcting

  In this chapter we describe commands that are useful when you catch
a mistake while editing.  The most fundamental of these commands is
the undo command @kbd{C-/} (also bound to @kbd{C-x u} and @kbd{C-_}).
This undoes a single command, or a
part of a command (as in the case of @code{query-replace}), or several
consecutive character insertions.  Consecutive repetitions of
@kbd{C-/} undo earlier and earlier changes, back to the limit of the
undo information available.

  Aside from the commands described here, you can erase text using
deletion commands such as @key{DEL} (@code{delete-backward-char}).
These were described earlier in this manual.  @xref{Erasing}.

@menu
* Undo::        The Undo commands.
* Transpose::   Exchanging two characters, words, lines, lists...
* Fixing Case:: Correcting case of last word entered.
* Spelling::    Apply spelling checker to a word, or a whole file.
@end menu

@node Undo
@section Undo
@cindex undo
@cindex changes, undoing

  The @dfn{undo} command reverses recent changes in the buffer's text.
Each buffer records changes individually, and the undo command always
applies to the current buffer.  You can undo all the changes in a
buffer for as far back as the buffer's records go.  Usually, each editing
command makes a separate entry in the undo records, but some commands
such as @code{query-replace} divide their changes into multiple
entries for flexibility in undoing.  Consecutive character insertion
commands are usually grouped together into a single undo record, to
make undoing less tedious.

@table @kbd
@item C-/
@itemx C-x u
@itemx C-_
Undo one entry in the current buffer's undo records (@code{undo}).
@end table

@kindex C-x u
@kindex C-_
@kindex C-/
@findex undo
  To begin to undo, type @kbd{C-/} (or its aliases, @kbd{C-_} or
@kbd{C-x u})@footnote{Aside from @kbd{C-/}, the @code{undo} command is
also bound to @kbd{C-x u} because that is more straightforward for
beginners to remember: @samp{u} stands for ``undo''.  It is also bound
to @kbd{C-_} because typing @kbd{C-/} on some text terminals actually
enters @kbd{C-_}.}.  This undoes the most recent change in the buffer,
and moves point back to where it was before that change.
  Consecutive repetitions of @kbd{C-/} (or its aliases) undo earlier
and earlier changes in the current buffer.  If all the recorded
changes have already been undone, the undo command signals an error.

@cindex redo
@findex undo-only
  Any command other than an undo command breaks the sequence of undo
commands.  Starting from that moment, the entire sequence of undo
commands that you have just performed are themselves placed into the
undo record, as a single set of changes.  Therefore, to re-apply
changes you have undone, type @kbd{C-f} or any other command that
harmlessly breaks the sequence of undoing; then type @kbd{C-/} to undo
the undo command.

  Alternatively, if you want to resume undoing, without redoing
previous undo commands, use @kbd{M-x undo-only}.  This is like
@code{undo}, but will not redo changes you have just undone.

@c What about @kbd{M-x revert-buffer}? --xfq
  If you notice that a buffer has been modified accidentally, the
easiest way to recover is to type @kbd{C-/} repeatedly until the stars
disappear from the front of the mode line (@pxref{Mode Line}).
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.  If you do not remember whether you
changed the buffer deliberately, type @kbd{C-/} once.  When you see
the last change you made undone, you will see whether it was an
intentional change.  If it was an accident, leave it undone.  If it
was deliberate, redo the change as described above.

@cindex selective undo
@kindex C-u C-/
  When there is an active region, any use of @code{undo} performs
@dfn{selective undo}: it undoes the most recent change within the
region, instead of the entire buffer.  However, when Transient Mark
mode is off (@pxref{Disabled Transient Mark}), @kbd{C-/} always
operates on the entire buffer, ignoring the region.  In this case, you
can perform selective undo by supplying a prefix argument to the
@code{undo} command: @kbd{C-u C-/}.  To undo further changes in the
same region, repeat the @code{undo} command (no prefix argument is
needed).

  Some specialized buffers do not make undo records.  Buffers whose
names start with spaces never do; these buffers are used internally by
Emacs to hold text that users don't normally look at or edit.

@vindex undo-limit
@vindex undo-strong-limit
@vindex undo-outer-limit
@cindex undo limit
  When the undo information for a buffer becomes too large, Emacs discards
the oldest records from time to time (during @dfn{garbage
collection}).  You can specify how much undo information to keep by
setting the variables @code{undo-limit}, @code{undo-strong-limit}, and
@code{undo-outer-limit}.  Their values are expressed in bytes.

  The variable @code{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 80000.  The variable @code{undo-strong-limit} sets a
stricter limit: any previous command (though not the most recent one)
that pushes the size past this amount is forgotten.  The default value
of @code{undo-strong-limit} is 120000.

  Regardless of the values of those variables, the most recent change
is never discarded unless it gets bigger than @code{undo-outer-limit}
(normally 12,000,000).  At that point, Emacs discards the undo data and
warns you about it.  This is the only situation in which you cannot
undo the last command.  If this happens, you can increase the value of
@code{undo-outer-limit} to make it even less likely to happen in the
future.  But if you didn't expect the command to create such large
undo data, then it is probably a bug and you should report it.
@xref{Bugs,, Reporting Bugs}.

@node Transpose
@section Transposing Text

@table @kbd
@item C-t
Transpose two characters (@code{transpose-chars}).
@item M-t
Transpose two words (@code{transpose-words}).
@item C-M-t
Transpose two balanced expressions (@code{transpose-sexps}).
@item C-x C-t
Transpose two lines (@code{transpose-lines}).
@end table

@kindex C-t
@findex transpose-chars
  The common error of transposing two characters can be fixed, when they
are adjacent, with the @kbd{C-t} command (@code{transpose-chars}).  Normally,
@kbd{C-t} transposes the two characters on either side of point.  When
given at the end of a line, rather than transposing the last character of
the line with the newline, which would be useless, @kbd{C-t} transposes the
last two characters on the line.  So, if you catch your transposition error
right away, you can fix it with just a @kbd{C-t}.  If you don't catch it so
fast, you must move the cursor back between the two transposed
characters before you type @kbd{C-t}.  If you transposed a space with
the last character of the word before it, the word motion commands are
a good way of getting there.  Otherwise, a reverse search (@kbd{C-r})
is often the best way.  @xref{Search}.

@kindex C-x C-t
@findex transpose-lines
@kindex M-t
@findex transpose-words
@c Don't index C-M-t and transpose-sexps here, they are indexed in
@c programs.texi, in the "List Commands" node.
@c @kindex C-M-t
@c @findex transpose-sexps
  @kbd{M-t} transposes the word before point with the word after point
(@code{transpose-words}).  It moves point forward over a word,
dragging the word preceding or containing point forward as well.  The
punctuation characters between the words do not move.  For example,
@w{@samp{FOO, BAR}} transposes into @w{@samp{BAR, FOO}} rather than
@samp{@w{BAR FOO,}}.

  @kbd{C-M-t} (@code{transpose-sexps}) is a similar command for
transposing two expressions (@pxref{Expressions}), and @kbd{C-x C-t}
(@code{transpose-lines}) exchanges lines.  They work like @kbd{M-t}
except as regards what units of text they transpose.

  A numeric argument to a transpose command serves as a repeat count: it
tells the transpose command to move the character (word, expression, line)
before or containing point across several other characters (words,
expressions, lines).  For example, @kbd{C-u 3 C-t} moves the character before
point forward across three other characters.  It would change
@samp{f@point{}oobar} into @samp{oobf@point{}ar}.  This is equivalent to
repeating @kbd{C-t} three times.  @kbd{C-u - 4 M-t} moves the word
before point backward across four words.  @kbd{C-u - C-M-t} would cancel
the effect of plain @kbd{C-M-t}.

  A numeric argument of zero is assigned a special meaning (because
otherwise a command with a repeat count of zero would do nothing): to
transpose the character (word, expression, line) ending after point
with the one ending after the mark.

@node Fixing Case
@section Case Conversion

@table @kbd
@item M-- M-l
Convert last word to lower case.  Note @kbd{Meta--} is Meta-minus.
@item M-- M-u
Convert last word to all upper case.
@item M-- M-c
Convert last word to lower case with capital initial.
@end table

@kindex M-@t{-} M-l
@kindex M-@t{-} M-u
@kindex M-@t{-} M-c
  A very common error is to type words in the wrong case.  Because of this,
the word case-conversion commands @kbd{M-l}, @kbd{M-u} and @kbd{M-c} have a
special feature when used with a negative argument: they do not move the
cursor.  As soon as you see you have mistyped the last word, you can simply
case-convert it and go on typing.  @xref{Case}.

@node Spelling
@section Checking and Correcting Spelling
@cindex spelling, checking and correcting
@cindex checking spelling
@cindex correcting spelling

  This section describes the commands to check the spelling of a
single word or of a portion of a buffer.  These commands only work if
the spelling checker program Aspell, Ispell or Hunspell is installed.
These programs are not part of Emacs, but one of them is usually
installed in GNU/Linux and other free operating systems.
@ifnottex
@xref{Top, Aspell,, aspell, The Aspell Manual}.
@end ifnottex

@table @kbd
@item M-$
Check and correct spelling of the word at point (@code{ispell-word}).
If the region is active, do it for all words in the region instead.
@item M-x ispell
Check and correct spelling of all words in the buffer.  If the region
is active, do it for all words in the region instead.
@item M-x ispell-buffer
Check and correct spelling in the buffer.
@item M-x ispell-region
Check and correct spelling in the region.
@item M-x ispell-message
Check and correct spelling in a draft mail message, excluding cited
material.
@item M-x ispell-change-dictionary @key{RET} @var{dict} @key{RET}
Restart the Aspell/Ispell/Hunspell process, using @var{dict} as the dictionary.
@item M-x ispell-kill-ispell
Kill the Aspell/Ispell/Hunspell subprocess.
@item M-@key{TAB}
@itemx @key{ESC} @key{TAB}
Complete the word before point based on the spelling dictionary
(@code{ispell-complete-word}).
@item M-x flyspell-mode
Enable Flyspell mode, which highlights all misspelled words.
@item M-x flyspell-prog-mode
Enable Flyspell mode for comments and strings only.
@end table

@kindex M-$
@findex ispell-word
  To check the spelling of the word around or before point, and
optionally correct it as well, type @kbd{M-$} (@code{ispell-word}).
If a region is active, @kbd{M-$} checks the spelling of all words
within the region.  @xref{Mark}.  (When Transient Mark mode is off,
@kbd{M-$} always acts on the word around or before point, ignoring the
region; @pxref{Disabled Transient Mark}.)

@findex ispell
@findex ispell-buffer
@findex ispell-region
@cindex spell-checking the active region
  Similarly, the command @kbd{M-x ispell} performs spell-checking in
the region if one is active, or in the entire buffer otherwise.  The
commands @kbd{M-x ispell-buffer} and @kbd{M-x ispell-region}
explicitly perform spell-checking on the entire buffer or the region
respectively.  To check spelling in an email message you are writing,
use @kbd{M-x ispell-message}; that command checks the whole buffer,
except for material that is indented or appears to be cited from other
messages.  @xref{Sending Mail}.

  When one of these commands encounters what appears to be an
incorrect word, it asks you what to do.  It usually displays a list of
numbered @dfn{near-misses}---words that are close to the incorrect word.
Then you must type a single-character response.  Here are the valid
responses:

@table @kbd
@item @var{digit}
Replace the word, just this time, with one of the displayed
near-misses.  Each near-miss is listed with a digit; type that digit
to select it.

@item @key{SPC}
Skip this word---continue to consider it incorrect, but don't change it
here.

@item r @var{new} @key{RET}
Replace the word, just this time, with @var{new}.  (The replacement
string will be rescanned for more spelling errors.)

@item R @var{new} @key{RET}
Replace the word with @var{new}, and do a @code{query-replace} so you
can replace it elsewhere in the buffer if you wish.  (The replacements
will be rescanned for more spelling errors.)

@item a
Accept the incorrect word---treat it as correct, but only in this
editing session.

@item A
Accept the incorrect word---treat it as correct, but only in this
editing session and for this buffer.

@item i
Insert this word in your private dictionary file so that Aspell or Ispell
or Hunspell will consider it correct from now on, even in future sessions.

@item m
Like @kbd{i}, but you can also specify dictionary completion
information.

@item u
Insert the lower-case version of this word in your private dic@-tion@-ary
file.

@item l @var{word} @key{RET}
Look in the dictionary for words that match @var{word}.  These words
become the new list of near-misses; you can select one of them as
the replacement by typing a digit.  You can use @samp{*} in @var{word} as a
wildcard.

@item C-g
@itemx X
Quit interactive spell checking, leaving point at the word that was
being checked.  You can restart checking again afterward with @kbd{C-u
M-$}.

@item x
Quit interactive spell checking and move point back to where it was
when you started spell checking.

@item q
Quit interactive spell checking and kill the spell-checker subprocess.

@item ?
Show the list of options.
@end table

@findex ispell-complete-word
  In Text mode and related modes, @kbd{M-@key{TAB}}
(@code{ispell-complete-word}) performs in-buffer completion based on
spelling correction.  Insert the beginning of a word, and then type
@kbd{M-@key{TAB}}; this shows a list of completions.  (If your
window manager intercepts @kbd{M-@key{TAB}}, type @kbd{@key{ESC}
@key{TAB}} or @kbd{C-M-i}.)  Each completion is listed with a digit or
character; type that digit or character to choose it.

@cindex @code{ispell} program
@findex ispell-kill-ispell
  Once started, the Aspell or Ispell or Hunspell subprocess continues
to run, waiting for something to do, so that subsequent spell checking
commands complete more quickly.  If you want to get rid of the
process, use @kbd{M-x ispell-kill-ispell}.  This is not usually
necessary, since the process uses no processor time except when you do
spelling correction.

@vindex ispell-dictionary
@vindex ispell-local-dictionary
@vindex ispell-personal-dictionary
@findex ispell-change-dictionary
  Ispell, Aspell and Hunspell look up spelling in two dictionaries:
the standard dictionary and your personal dictionary.  The standard
dictionary is specified by the variable @code{ispell-local-dictionary}
or, if that is @code{nil}, by the variable @code{ispell-dictionary}.
If both are @code{nil}, the spelling program's default dictionary is
used.  The command @kbd{M-x ispell-change-dictionary} sets the
standard dictionary for the buffer and then restarts the subprocess,
so that it will use a different standard dictionary.  Your personal
dictionary is specified by the variable
@code{ispell-personal-dictionary}.  If that is @code{nil}, the
spelling program looks for a personal dictionary in a default
location.

@vindex ispell-complete-word-dict
  A separate dictionary is used for word completion.  The variable
@code{ispell-complete-word-dict} specifies the file name of this
dictionary.  The completion dictionary must be different because it
cannot use root and affix information.  For some languages, there
is a spell checking dictionary but no word completion dictionary.

@cindex Flyspell mode
@cindex mode, Flyspell
@findex flyspell-mode
  Flyspell mode is a minor mode that performs automatic spell checking
as you type.  When it finds a word that it does not recognize, it
highlights that word.  Type @kbd{M-x flyspell-mode} to toggle Flyspell
mode in the current buffer.  To enable Flyspell mode in all text mode
buffers, add @code{flyspell-mode} to @code{text-mode-hook}.
@xref{Hooks}.

@findex flyspell-correct-word
@findex flyspell-auto-correct-word
@findex flyspell-correct-word-before-point
  When Flyspell mode highlights a word as misspelled, you can click on
it with @kbd{mouse-2} (@code{flyspell-correct-word}) to display a menu
of possible corrections and actions.  In addition, @kbd{C-.} or
@kbd{@key{ESC}-@key{TAB}} (@code{flyspell-auto-correct-word}) will
propose various successive corrections for the word at point, and
@kbd{C-c $} (@code{flyspell-correct-word-before-point}) will pop up a
menu of possible corrections.  Of course, you can always correct the
misspelled word by editing it manually in any way you like.

@findex flyspell-prog-mode
  Flyspell Prog mode works just like ordinary Flyspell mode, except
that it only checks words in comments and string constants.  This
feature is useful for editing programs.  Type @kbd{M-x
flyspell-prog-mode} to enable or disable this mode in the current
buffer.  To enable this mode in all programming mode buffers, add
@code{flyspell-prog-mode} to @code{prog-mode-hook} (@pxref{Hooks}).
