33.5 Character Properties

A character property is a named attribute of a character that specifies how the character behaves and how it should be handled during text processing and display. Thus, character properties are an important part of specifying the character's semantics.

On the whole, Emacs follows the Unicode Standard in its implementation of character properties. In particular, Emacs supports the Unicode Character Property Model, and the Emacs character property database is derived from the Unicode Character Database (UCD). See the Character Properties chapter of the Unicode Standard, for a detailed description of Unicode character properties and their meaning. This section assumes you are already familiar with that chapter of the Unicode Standard, and want to apply that knowledge to Emacs Lisp programs.

In Emacs, each property has a name, which is a symbol, and a set of possible values, whose types depend on the property; if a character does not have a certain property, the value is nil. As a general rule, the names of character properties in Emacs are produced from the corresponding Unicode properties by downcasing them and replacing each ‘_’ character with a dash ‘-’. For example, Canonical_Combining_Class becomes canonical-combining-class. However, sometimes we shorten the names to make their use easier.

Here is the full list of value types for all the character properties that Emacs knows about:

name

This property corresponds to the Unicode Name property. The value is a string consisting of upper-case Latin letters A to Z, digits, spaces, and hyphen ‘-’ characters.

general-category

This property corresponds to the Unicode General_Category property. The value is a symbol whose name is a 2-letter abbreviation of the character's classification.

canonical-combining-class

Corresponds to the Unicode Canonical_Combining_Class property. The value is an integer number.

bidi-class

Corresponds to the Unicode Bidi_Class property. The value is a symbol whose name is the Unicode directional type of the character.

decomposition

Corresponds to the Unicode Decomposition_Type and Decomposition_Value properties. The value is a list, whose first element may be a symbol representing a compatibility formatting tag, such as small¹; the other elements are characters that give the compatibility decomposition sequence of this character.

decimal-digit-value

Corresponds to the Unicode Numeric_Value property for characters whose Numeric_Type is ‘Digit’. The value is an integer number.

digit

Corresponds to the Unicode Numeric_Value property for characters whose Numeric_Type is ‘Decimal’. The value is an integer number. Examples of such characters include compatibility subscript and superscript digits, for which the value is the corresponding number.

numeric-value

Corresponds to the Unicode Numeric_Value property for characters whose Numeric_Type is ‘Numeric’. The value of this property is an integer or a floating-point number. Examples of characters that have this property include fractions, subscripts, superscripts, Roman numerals, currency numerators, and encircled numbers. For example, the value of this property for the character U+2155 (vulgar fraction one fifth) is 0.2.

mirrored

Corresponds to the Unicode Bidi_Mirrored property. The value of this property is a symbol, either Y or N.

old-name

Corresponds to the Unicode Unicode_1_Name property. The value is a string.

iso-10646-comment

Corresponds to the Unicode ISO_Comment property. The value is a string.

uppercase

Corresponds to the Unicode Simple_Uppercase_Mapping property. The value of this property is a single character.

lowercase

Corresponds to the Unicode Simple_Lowercase_Mapping property. The value of this property is a single character.

titlecase

Corresponds to the Unicode Simple_Titlecase_Mapping property. Title case is a special form of a character used when the first character of a word needs to be capitalized. The value of this property is a single character.