Autopair is an extension to the Emacs text editor that
automatically pairs braces and quotes:

• Opening braces/quotes are autopaired;
• Closing braces/quotes are autoskipped;
• Backspacing an opening brace/quote autodeletes its pair.
• Newline between newly-opened brace pairs open an extra indented line.

Autopair works well across all Emacs major-modes, deduces from the
language's syntax table which characters to pair, skip or delete. It
should work even with extensions that redefine such keys. It also
works with YASnippet, another package I maintain.

Important: in Emacs 24.4 you can try electric-pair-mode as an alternative to autopair. See below

Installation and basic use:

To try it out, download the
latest version
add to your .emacs

(add-to-list 'load-path "/path/to/autopair") ;; comment if autopair.el is in standard load path 
(require 'autopair)
(autopair-global-mode) ;; enable autopair in all buffers

Alternatives to autopair

I developed autopair to work just like
Textmate or better, be minimally intrusive
to my existing hacks and need very little customization. You might
prefer it to the following:

• https://github.com/Fuco1/smartparens
• http://www.emacswiki.org/emacs/ParEdit
• the options listed in http://www.emacswiki.org/emacs/AutoPairs

Differences to smartparens

Note in particular that smartparens claims in its
README that it provides "the basic funcionality [of autopair]". I
don't know why this claim is made, since at time of writing:

1. it does not automatically set itself up for any mode according
   to the mode's syntax table.
2. it does not attempt to automatically balance parentheses

Autopair provides these things out-of-the-box. Smartparens provides
other features, that I personally use Yasnippet for.

electric-pair-mode in Emacs 24.4

electric-pair-mode is a minor mode in Emacs 24.3, but it's not very
useful or widely used. In Emacs 24.4, electric-pair-mode
minor-mode is much improved and actually supersedes autopair in
functionality and general all-around Emacs integration, and I recommend
you use it instead.

Neat tricks

• autopair-autowrap tells autopair to automatically wrap the
  selection region with the delimiters you're trying to insert.

• autopair-blink makes the cursor quickly blink over matching
  braces and quotes just inserted or skipped over. If you find this
  behaviour annoying, set this to nil.

• autopair-skip-whitespace, when set to non-nil, tells autopair to
  skip over whitespace when attempting to skip over a closing
  brace. If you set this to 'chomp, the whitespace is not only
  jumped over but also deleted.

Autopair's idea is to always do-what-you-mean, but since some people
are never satisfied, have a look at the following:

Optional tricks

You shouldn't need this, but to enable autopair in just some types of
buffers, comment out the autopair-global-mode and turn on
autopair-mode in some major-mode hook, like:

;; use autopair just in c buffers
 
(add-hook 'c-mode-common-hook 
          #'(lambda () (autopair-mode)))

Alternatively, do use autopair-global-mode and create exceptions
using the major mode hooks (watch out for the change in behaviour
emacs 24).

;; use autopair everywhere but c buffers
 
(add-hook 'c-mode-common-hook
           #'(lambda () 
               (setq autopair-dont-activate t) ;; for emacsen < 24
               (autopair-mode -1))             ;; for emacsen >= 24
)

More tricks

autopair-dont-pair lets you define special cases of characters you
don't want paired. Its default value skips pairing single-quote
characters when inside a comment literal, even if the language syntax
tables does pair these characters.

As a further example, to also prevent the { (opening brace)
character from being autopaired in C++ comments use this in your
.emacs.

(add-hook 'c++-mode-hook
          #'(lambda ()
               (push ?{
                     (getf autopair-dont-pair :comment))))

autopair-handle-action-fns lets you write some emacs-lisp that
overrides/extends the actions taken by autopair after it decides
something must be paired, skipped or deleted. To work with triple
quoting in python mode, you can use this for example:

(add-hook 'python-mode-hook
          #'(lambda ()
              (setq autopair-handle-action-fns
                    (list #'autopair-default-handle-action
                          #'autopair-python-triple-quote-action))))

where autopair-python-triple-quote-action is an example of a
user-written function (which is bundled in autopair.el).

See
this issue
for an example of clever use of this variable (thanks
Alex Duller.

autopair-extra-pairs lets you define extra pairing and skipping
behaviour for pairs not programmed into the syntax table. Watch out,
this is work-in-progress, a little unstable and does not help
balancing at all. To have <= and =>= pair in =c++-mode buffers, but
only in code, use:

(add-hook 'c++-mode-hook
          #'(lambda ()
              (push '(?< . ?>)
                    (getf autopair-extra-pairs :code))))

if you program in emacs-lisp you might also like the following to
pair backtick (``=) and quote (='`).

(add-hook 'emacs-lisp-mode-hook
          #'(lambda ()
              (push '(?` . ?')
                    (getf autopair-extra-pairs :comment))
              (push '(?` . ?')
                    (getf autopair-extra-pairs :string))))

Workarounds

Once you set autopair-global-mode everything mostly just works but
a few extensions use tricks that interfere with autopair's own tricks,
disabling autopair or some of the extension's functionality. Using the
customization techniques described above, there are plenty of very
good workarounds for slime-mode, latex-mode, term-mode and even
viper-mode.

See this workaround list

How it works

The extension works by rebinding the braces and quotes keys, but can
still be minimally intrusive, since the original binding is always
called as if autopair did not exist.

The decision of which keys to actually rebind is taken at minor-mode
activation time, based on the current major mode's syntax tables. To
achieve this kind of behaviour, an Emacs variable
emulation-mode-map-alists was used.

If you set autopair-pair-criteria and autopair-skip-criteria to
the symbol help-balance (which, by the way, is the default), braces
are not autopaired/autoskipped in all situations; the decision to
autopair or autoskip a brace is taken according to the following
table:

+---------+------------+-----------+-------------------+, 1234567, autopair?, autoskip?, notes, +---------+------------+-----------+-------------------+, (()), yyyyyyy, ---yy--, balanced, +---------+------------+-----------+-------------------+, (())), ------y, ---yyy-, too many closings, +---------+------------+-----------+-------------------+, ((()), yyyyyyy, -------, too many openings, +---------+------------+-----------+-------------------+

The table is read like this: in a buffer with 7 characters laid out
like the first column, an "y" marks points where an opening brace is
autopaired and in which places would a closing brace be
autoskipped. Quote pairing tries to support similar "intelligence".