Categories provide an alternate way of classifying characters syntactically. You can define several categories as needed, then independently assign each character to one or more categories. Unlike syntax classes, categories are not mutually exclusive; it is normal for one character to belong to several categories.
Each buffer has a category table which records which categories are defined and also which characters belong to each category. Each category table defines its own categories, but normally these are initialized by copying from the standard categories table, so that the standard categories are available in all modes.
Each category has a name, which is an ASCII printing character in the range ‘
~’. You specify the name of a category when you define it with
The category table is actually a char-table (see Char-Tables). The element of the category table at index
c is a category set—a bool-vector—that indicates which categories character
c belongs to. In this category set, if the element at index
t, that means category
cat is a member of the set, and that character
c belongs to category
For the next three functions, the optional argument
table defaults to the current buffer’s category table.
function define-category char docstring \&optional table
This function defines a new category, with name
char and documentation
docstring, for the category table
Here’s an example of defining a new category for characters that have strong right-to-left directionality (see Bidirectional Display) and using it in a special category table. To obtain the information about the directionality of characters, the example code uses the ‘
bidi-class’ Unicode property (see bidi-class).
;; Make an empty category-table.
(let ((category-table (make-category-table))
;; Create a char-table which gives the 'bidi-class' Unicode
;; property for each character.
(define-category ?R "Characters of bidi-class R, AL, or RLO"
;; Modify the category entry of each character whose
;; 'bidi-class' Unicode property is R, AL, or RLO --
;; these have a right-to-left directionality.
(lambda (key val)
(if (memq val '(R AL RLO))
(modify-category-entry key ?R category-table)))
function category-docstring category \&optional table
This function returns the documentation string of category
category in category table
function get-unused-category \&optional table
This function returns a category name (a character) which is not currently defined in
table. If all possible categories are in use in
table, it returns
This function returns the current buffer’s category table.
function category-table-p object
This function returns
object is a category table, otherwise
This function returns the standard category table.
function copy-category-table \&optional table
This function constructs a copy of
table and returns it. If
table is not supplied (or is
nil), it returns a copy of the standard category table. Otherwise, an error is signaled if
table is not a category table.
function set-category-table table
This function makes
table the category table for the current buffer. It returns
This creates and returns an empty category table. In an empty category table, no categories have been allocated, and no characters belong to any categories.
function make-category-set categories
This function returns a new category set—a bool-vector—whose initial contents are the categories listed in the string
categories. The elements of
categories should be category names; the new category set has
t for each of those categories, and
nil for all other categories.
function char-category-set char
This function returns the category set for character
char in the current buffer’s category table. This is the bool-vector which records which categories the character
char belongs to. The function
char-category-set does not allocate storage, because it returns the same bool-vector that exists in the category table.
function category-set-mnemonics category-set
This function converts the category set
category-set into a string containing the characters that designate the categories that are members of the set.
(category-set-mnemonics (char-category-set ?a))
function modify-category-entry char category \&optional table reset
This function modifies the category set of
char in category table
table (which defaults to the current buffer’s category table).
char can be a character, or a cons cell of the form
(min . max); in the latter case, the function modifies the category sets of all characters in the range between
Normally, it modifies a category set by adding
category to it. But if
reset is non-
nil, then it deletes
command describe-categories \&optional buffer-or-name
This function describes the category specifications in the current category table. It inserts the descriptions in a buffer, and then displays that buffer. If
buffer-or-name is non-
nil, it describes the category table of that buffer instead.