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A numeric character reference (NCR) is a common markup construct used in SGML and SGML-derived markup languages such as HTML and XML.It consists of a short sequence of characters that, in turn, represents a single character.
In SGML, HTML and XML documents, the logical constructs known as character data and attribute values consist of sequences of characters, in which each character can manifest directly (representing itself), or can be represented by a series of characters called a character reference, of which there are two types: a numeric character reference and a character entity reference.
Grouped by their numerical property as used in a text, Unicode has four values for Numeric Type. First there is the "not a number" type. Then there are decimal-radix numbers, commonly used in Western style decimals (plain 0–9), there are numbers that are not part of a decimal system such as Roman numbers, and decimal numbers in typographic context, such as encircled numbers.
A character encoding form (CEF) is the mapping of code points to code units to facilitate storage in a system that represents numbers as bit sequences of fixed length (i.e. practically any computer system). For example, a system that stores numeric information in 16-bit units can only directly represent code points 0 to 65,535 in each unit, but ...
A numeric character reference uses the format &#nnnn; or &#xhhhh; where nnnn is the code point in decimal form, and hhhh is the code point in hexadecimal form. The x must be lowercase in XML documents. The nnnn or hhhh may be any number of digits and may include leading zeros. The hhhh may mix uppercase and lowercase, though uppercase is the ...
To support these rules and algorithms, Unicode adds many properties to each character in the set such as properties determining a character's default bidirectional class and properties to determine how the character combines with other characters. If the character represents a numeric value such as the European number '8', or the vulgar ...
ASCII was incorporated into the Unicode (1991) character set as the first 128 symbols, so the 7-bit ASCII characters have the same numeric codes in both sets. This allows UTF-8 to be backward compatible with 7-bit ASCII, as a UTF-8 file containing only ASCII characters is identical to an ASCII file containing the same sequence of characters.
The intended effect is that a simple parser can use these decimal numeric values, without being distracted by say a numeric superscript or a fraction. Eighty-three CJK Ideographs that represent a number, including those used for accounting, are typed Numeric. On the other hand, characters that could have a numeric value as a second meaning are ...