Next: , Previous: Character Codes, Up: Non-ASCII Characters

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.

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 an empty string.


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 number. 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 unassigned codepoints, the value is the character itself.
decimal-digit-value
Corresponds to the Unicode Numeric_Value property for characters whose Numeric_Type is ‘Digit’. The value is an integer number. 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 ‘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. 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 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. 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.
old-name
Corresponds to the Unicode Unicode_1_Name property. The value is a string. For unassigned codepoints, the value is an empty string.
iso-10646-comment
Corresponds to the Unicode ISO_Comment property. The value is a string. For unassigned codepoints, the value is an empty string.
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.
— Function: get-char-code-property char propname

This function returns the value of char's propname property.

          (get-char-code-property ?  'general-category)
               ⇒ Zs
          (get-char-code-property ?1  'general-category)
               ⇒ Nd
          ;; subscript 4
          (get-char-code-property ?\u2084 'digit-value)
               ⇒ 4
          ;; one fifth
          (get-char-code-property ?\u2155 'numeric-value)
               ⇒ 0.2
          ;; Roman IV
          (get-char-code-property ?\u2163 'numeric-value)
               ⇒ 4
— Function: char-code-property-description prop value

This function returns the description string of property prop's value, or nil if value has no description.

          (char-code-property-description 'general-category 'Zs)
               ⇒ "Separator, Space"
          (char-code-property-description 'general-category 'Nd)
               ⇒ "Number, Decimal Digit"
          (char-code-property-description 'numeric-value '1/5)
               ⇒ nil
— Function: put-char-code-property char propname value

This function stores value as the value of the property propname for the character char.

— Variable: unicode-category-table

The value of this variable is a char-table (see Char-Tables) that specifies, for each character, its Unicode General_Category property as a symbol.

— Variable: char-script-table

The value of this variable is a char-table that specifies, for each character, a symbol whose name is the script to which the character belongs, according to the Unicode Standard classification of the Unicode code space into script-specific blocks. This char-table has a single extra slot whose value is the list of all script symbols.

— Variable: char-width-table

The value of this variable is a char-table that specifies the width of each character in columns that it will occupy on the screen.

— Variable: printable-chars

The value of this variable is a char-table that specifies, for each character, whether it is printable or not. That is, if evaluating (aref printable-chars char) results in t, the character is printable, and if it results in nil, it is not.


Footnotes

[1] The Unicode specification writes these tag names inside ‘<..>’ brackets, but the tag names in Emacs do not include the brackets; e.g., Unicode specifies ‘<small>’ where Emacs uses ‘small’.