33.6 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.

Some codepoints are left unassigned by the UCD—they don't correspond to any character. The Unicode Standard defines default values of properties for such codepoints; they are mentioned below for each property.

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

name

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

general-category

Corresponds to the General_Category Unicode property. The value is a symbol whose name is a 2-letter abbreviation of the character's classification. For unassigned codepoints, the value is Cn.

canonical-combining-class

Corresponds to the Canonical_Combining_Class Unicode property. The value is an integer. For unassigned codepoints, the value is zero.

bidi-class

Corresponds to the Unicode Bidi_Class property. The value is a symbol whose name is the Unicode directional type of the character. Emacs uses this property when it reorders bidirectional text for display (see Bidirectional Display). For unassigned codepoints, the value depends on the code blocks to which the codepoint belongs: most unassigned codepoints get the value of L (strong L), but some get values of AL (Arabic letter) or R (strong R).

decomposition

Corresponds to the Unicode properties Decomposition_Type and Decomposition_Value. 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. For characters that don't have decomposition sequences, and for unassigned codepoints, the value is a list with a single member, the character itself.

decimal-digit-value

Corresponds to the Unicode Numeric_Value property for characters whose Numeric_Type is ‘Decimal’. The value is an integer, or nil if the character has no decimal digit value. For unassigned codepoints, the value is nil, which means NaN, or “not a number”.

digit-value

Corresponds to the Unicode Numeric_Value property for characters whose Numeric_Type is ‘Digit’. The value is an integer. Examples of such characters include compatibility subscript and superscript digits, for which the value is the corresponding number. For characters that don't have any numeric value, and for unassigned codepoints, the value is nil, which means NaN.

numeric-value

Corresponds to the Unicode Numeric_Value property for characters whose Numeric_Type is ‘Numeric’. The value of this property is a 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. For characters that don't have any numeric value, and for unassigned codepoints, the value is nil, which means NaN.

mirrored

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

mirroring

Corresponds to the Unicode Bidi_Mirroring_Glyph property. The value of this property is a character whose glyph represents the mirror image of the character's glyph, or nil if there's no defined mirroring glyph. All the characters whose mirrored property is N have nil as their mirroring property; however, some characters whose mirrored property is Y also have nil for mirroring, because no appropriate characters exist with mirrored glyphs. Emacs uses this property to display mirror images of characters when appropriate (see Bidirectional Display). For unassigned codepoints, the value is nil.

paired-bracket

Corresponds to the Unicode Bidi_Paired_Bracket property. The value of this property is the codepoint of a character's paired bracket, or nil if the character is not a bracket character. This establishes a mapping between characters that are treated as bracket pairs by the Unicode Bidirectional Algorithm; Emacs uses this property when it decides how to reorder for display parentheses, braces, and other similar characters (see Bidirectional Display).

bracket-type

Corresponds to the Unicode Bidi_Paired_Bracket_Type property. For characters whose paired-bracket property is non-nil, the value of this property is a symbol, either o (for opening bracket characters) or c (for closing bracket characters). For characters whose paired-bracket property is nil, the value is the symbol n (None). Like paired-bracket, this property is used for bidirectional display.

old-name

Corresponds to the Unicode Unicode_1_Name property. The value is a string. For unassigned codepoints, and characters that have no value for this property, the value is nil.

iso-10646-comment

Corresponds to the Unicode ISO_Comment property. The value is either a string or nil. For unassigned codepoints, the value is nil.

uppercase

Corresponds to the Unicode Simple_Uppercase_Mapping property. The value of this property is a single character. For unassigned codepoints, the value is nil, which means the character itself.

lowercase

Corresponds to the Unicode Simple_Lowercase_Mapping property. The value of this property is a single character. For unassigned codepoints, the value is nil, which means the character itself.

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. For unassigned codepoints, the value is nil, which means the character itself.

special-uppercase

Corresponds to Unicode language- and context-independent special upper-casing rules. The value of this property is a string (which may be empty). For example mapping for U+00DF latin small letter sharp s is "SS". For characters with no special mapping, the value is nil which means uppercase property needs to be consulted instead.

special-lowercase

Corresponds to Unicode language- and context-independent special lower-casing rules. The value of this property is a string (which may be empty). For example mapping for U+0130 latin capital letter i with dot above the value is "i\u0307" (i.e. 2-character string consisting of latin small letter i followed by U+0307 combining dot above). For characters with no special mapping, the value is nil which means lowercase property needs to be consulted instead.

special-titlecase

Corresponds to Unicode unconditional special title-casing rules. The value of this property is a string (which may be empty). For example mapping for U+FB01 latin small ligature fi the value is "Fi". For characters with no special mapping, the value is nil which means titlecase property needs to be consulted instead.