Font Lock mode is a minor mode, always local to a particular buffer, which assigns faces to (or fontifies) the text in the buffer. Each buffer’s major mode tells Font Lock mode which text to fontify; for instance, programming language modes fontify syntactically relevant constructs like comments, strings, and function names.
Font Lock mode is enabled by default. To toggle it in the current buffer, type M-x font-lock-mode. A positive numeric argument unconditionally enables Font Lock mode, and a negative or zero argument disables it.
Type M-x global-font-lock-mode to toggle Font Lock mode in all
buffers. To impose this setting for future Emacs sessions, customize
global-font-lock-mode (see Easy Customization), or add the following line to your init file:
If you have disabled Global Font Lock mode, you can still enable Font
Lock for specific major modes by adding the function
font-lock-mode to the mode hooks (see Hooks). For example,
to enable Font Lock mode for editing C files, you can do this:
(add-hook 'c-mode-hook 'font-lock-mode)
Font Lock mode uses several specifically named faces to do its job,
and others. The easiest way to find them all is to use M-x
customize-group RET font-lock-faces RET. You can then
use that customization buffer to customize the appearance of these
faces. See Face Customization.
You can customize the variable
to alter the amount of fontification applied by Font Lock mode, for
major modes that support this feature. The value should be a number
(with 1 representing a minimal amount of fontification; some modes
support levels as high as 3); or
t, meaning “as high as
possible” (the default). You can also specify different numbers for
particular major modes; for example, to use level 1 for C/C++ modes,
and the default level otherwise, use the value
'((c-mode . 1) (c++-mode . 1)))
Comment and string fontification (or “syntactic” fontification) relies on analysis of the syntactic structure of the buffer text. For the sake of speed, some modes, including Lisp mode, rely on a special convention: an open-parenthesis or open-brace in the leftmost column always defines the beginning of a defun, and is thus always outside any string or comment. Therefore, you should avoid placing an open-parenthesis or open-brace in the leftmost column, if it is inside a string or comment. See Left Margin Paren, for details.
font-lock-beginning-of-syntax-function, which is
always buffer-local, specifies how Font Lock mode can find a position
guaranteed to be outside any comment or string. In modes which use
the leftmost column parenthesis convention, the default value of the
beginning-of-defun—that tells Font Lock mode to
use the convention. If you set this variable to
nil, Font Lock
no longer relies on the convention. This avoids incorrect results,
but the price is that, in some cases, fontification for a changed text
must rescan buffer text from the beginning of the buffer. This can
considerably slow down redisplay while scrolling, particularly if you
are close to the end of a large buffer.
Font Lock highlighting patterns already exist for most modes, but
you may want to fontify additional patterns. You can use the function
font-lock-add-keywords, to add your own highlighting patterns
for a particular mode. For example, to highlight ‘FIXME:’ words
in C comments, use this:
(add-hook 'c-mode-hook (lambda () (font-lock-add-keywords nil '(("\\<\\(FIXME\\):" 1 font-lock-warning-face t)))))
To remove keywords from the font-lock highlighting patterns, use the
font-lock-remove-keywords. See Search-based
Fontification in The Emacs Lisp Reference Manual.
Fontifying large buffers can take a long time. To avoid large delays when a file is visited, Emacs initially fontifies only the visible portion of a buffer. As you scroll through the buffer, each portion that becomes visible is fontified as soon as it is displayed; this type of Font Lock is called Just-In-Time (or JIT) Lock. You can control how JIT Lock behaves, including telling it to perform fontification while idle, by customizing variables in the customization group ‘jit-lock’. See Specific Customization.