Font Lock Basics

The Font Lock functionality is based on several basic functions. Each of these calls the function specified by the corresponding variable. This indirection allows major and minor modes to modify the way fontification works in the buffers of that mode, and even use the Font Lock mechanisms for features that have nothing to do with fontification. (This is why the description below says “should” when it describes what the functions do: the mode can customize the values of the corresponding variables to do something entirely different.) The variables mentioned below are described in Other Font Lock Variables.

There are several variables that control how Font Lock mode highlights text. But major modes should not set any of these variables directly. Instead, they should set font-lock-defaults as a buffer-local variable. The value assigned to this variable is used, if and when Font Lock mode is enabled, to set all the other variables.

— Variable: font-lock-defaults

This variable is set by modes to specify how to fontify text in that mode. It automatically becomes buffer-local when set. If its value is nil, Font Lock mode does no highlighting, and you can use the ‘Faces’ menu (under ‘Edit’ and then ‘Text Properties’ in the menu bar) to assign faces explicitly to text in the buffer.
If non-nil, the value should look like this:
          (keywords [keywords-only [case-fold
           [syntax-alist other-vars...]]])
The first element, keywords, indirectly specifies the value of font-lock-keywords which directs search-based fontification. It can be a symbol, a variable or a function whose value is the list to use for font-lock-keywords. It can also be a list of several such symbols, one for each possible level of fontification. The first symbol specifies the ‘mode default’ level of fontification, the next symbol level 1 fontification, the next level 2, and so on. The ‘mode default’ level is normally the same as level 1. It is used when font-lock-maximum-decoration has a nil value. See Levels of Font Lock.
The second element, keywords-only, specifies the value of the variable font-lock-keywords-only. If this is omitted or nil, syntactic fontification (of strings and comments) is also performed. If this is non-nil, syntactic fontification is not performed. See Syntactic Font Lock.
The third element, case-fold, specifies the value of font-lock-keywords-case-fold-search. If it is non-nil, Font Lock mode ignores case during search-based fontification.
If the fourth element, syntax-alist, is non-nil, it should be a list of cons cells of the form (char-or-string.string). These are used to set up a syntax table for syntactic fontification; the resulting syntax table is stored in font-lock-syntax-table. If syntax-alist is omitted or nil, syntactic fontification uses the syntax table returned by the syntax-table function. See Syntax Table Functions.
All the remaining elements (if any) are collectively called other-vars. Each of these elements should have the form (variable . value)—which means, make variable buffer-local and then set it to value. You can use these other-vars to set other variables that affect fontification, aside from those you can control with the first five elements. See Other Font Lock Variables.

If your mode fontifies text explicitly by adding font-lock-face properties, it can specify (nil t) for font-lock-defaults to turn off all automatic fontification. However, this is not required; it is possible to fontify some things using font-lock-face properties and set up automatic fontification for other parts of the text.

23.6.1 Font Lock Basics