Numeric character reference
Encyclopedia
A numeric character reference (NCR) is a common markup construct used in SGML and other SGML-related markup languages such as HTML
and XML
. It consists of a short sequence of character
s that, in turn, represent a single character from the Universal Character Set
(UCS) of Unicode
. NCRs are typically used in order to represent characters that are not directly encodable in a particular document. When the document is interpreted by a markup-aware reader, each NCR is treated as if it were the character it represents.
.
Ideally, when the characters of a document utilizing a markup language are encoded for storage or transmission over a network as a sequence of bit
s, the encoding that is used will be one that supports representing each and every character in the document, if not in the whole of Unicode, directly as a particular bit sequence.
Sometimes, though, for reasons of convenience or due to technical limitations, documents are encoded with an encoding that cannot represent some characters directly. For example, the widely used encodings based on ISO 8859 can only represent, at most, 256 unique characters as one 8-bit byte
each.
Documents are rarely, in practice, ever allowed to use more than one encoding internally, so the onus is usually on the markup language to provide a means for document authors to express unencodable characters in terms of encodable ones. This is generally done through some kind of "escaping" mechanism.
The SGML-based markup languages allow document authors to use special sequences of characters from the ASCII range (the first 128 code points of Unicode) to represent, or reference, any Unicode character, regardless of whether the character being represented is directly available in the document's encoding. These special sequences are character references.
Character references that are based on the referenced character's UCS or Unicode "code point" are called numeric character references. In HTML 4 and in all versions of XHTML
and XML, the code point can be expressed either as a decimal (base 10) number or as a hexadecimal (base 16) number. The syntax is as follows:
Character U+0026 (ampersand), followed by character U+0023 (number sign), followed by one of the following choices:
all followed by character U+003B (semicolon). Older versions of HTML disallowed the hexadecimal syntax.
The characters that comprise a numeric character reference can be represented in every character encoding used in computing and telecommunications today, so there is no risk of the reference itself being unencodable.
There is another kind of character reference called a character entity reference
, which allows a character to be referred to by a name instead of a number. (Naming a character creates a character entity
.) HTML defines some character entities, but not many; all other characters can only be included by direct encoding or using NCRs.
While the syntax of SGML does not prohibit references to unassigned code points, such as
Restrictions may also apply for other reasons. For example, in HTML 4,
encoding: "€", which actually should be represented as
Markup languages also place restrictions on where character references can occur.
HTML
HyperText Markup Language is the predominant markup language for web pages. HTML elements are the basic building-blocks of webpages....
and XML
XML
Extensible Markup Language is a set of rules for encoding documents in machine-readable form. It is defined in the XML 1.0 Specification produced by the W3C, and several other related specifications, all gratis open standards....
. It consists of a short sequence of character
Character (computing)
In computer and machine-based telecommunications terminology, a character is a unit of information that roughly corresponds to a grapheme, grapheme-like unit, or symbol, such as in an alphabet or syllabary in the written form of a natural language....
s that, in turn, represent a single character from the Universal Character Set
Universal Character Set
The Universal Character Set , defined by the International Standard ISO/IEC 10646, Information technology — Universal multiple-octet coded character set , is a standard set of characters upon which many character encodings are based...
(UCS) of Unicode
Unicode
Unicode is a computing industry standard for the consistent encoding, representation and handling of text expressed in most of the world's writing systems...
. NCRs are typically used in order to represent characters that are not directly encodable in a particular document. When the document is interpreted by a markup-aware reader, each NCR is treated as if it were the character it represents.
Example
In SGML, HTML, and XML, the following are all valid numeric character references for the Greek capital letter Sigma| ( = ) | |||
| Unicode character | Numerical base | Numerical reference in markup | Effect |
|---|---|---|---|
| U+03A3 | Decimal | Σ | Σ |
| U+03A3 | Decimal | Σ | Σ |
| U+03A3 | Hexadecimal | Σ | Σ |
| U+03A3 | Hexadecimal | Σ | Σ |
| U+03A3 | Hexadecimal | Σ | Σ |
Discussion
Markup languages are typically defined in terms of UCS or Unicode characters. That is, a document consists, at its most fundamental level of abstraction, of a sequence of characters, which are abstract units that exist independently of any encodingCharacter encoding
A character encoding system consists of a code that pairs each character from a given repertoire with something else, such as a sequence of natural numbers, octets or electrical pulses, in order to facilitate the transmission of data through telecommunication networks or storage of text in...
.
Ideally, when the characters of a document utilizing a markup language are encoded for storage or transmission over a network as a sequence of bit
Bit
A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...
s, the encoding that is used will be one that supports representing each and every character in the document, if not in the whole of Unicode, directly as a particular bit sequence.
Sometimes, though, for reasons of convenience or due to technical limitations, documents are encoded with an encoding that cannot represent some characters directly. For example, the widely used encodings based on ISO 8859 can only represent, at most, 256 unique characters as one 8-bit byte
Byte
The byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...
each.
Documents are rarely, in practice, ever allowed to use more than one encoding internally, so the onus is usually on the markup language to provide a means for document authors to express unencodable characters in terms of encodable ones. This is generally done through some kind of "escaping" mechanism.
The SGML-based markup languages allow document authors to use special sequences of characters from the ASCII range (the first 128 code points of Unicode) to represent, or reference, any Unicode character, regardless of whether the character being represented is directly available in the document's encoding. These special sequences are character references.
Character references that are based on the referenced character's UCS or Unicode "code point" are called numeric character references. In HTML 4 and in all versions of XHTML
XHTML
XHTML is a family of XML markup languages that mirror or extend versions of the widely-used Hypertext Markup Language , the language in which web pages are written....
and XML, the code point can be expressed either as a decimal (base 10) number or as a hexadecimal (base 16) number. The syntax is as follows:
Character U+0026 (ampersand), followed by character U+0023 (number sign), followed by one of the following choices:
- one or more decimal digits zero (U+0030) through nine (U+0039); or
- character U+0078 ("x") followed by one or more hexadecimal digits, which are zero (U+0030) through nine (U+0039), Latin capital letter A (U+0041) through F (U+0046), and Latin small letter a (U+0061) through f (U+0066);
all followed by character U+003B (semicolon). Older versions of HTML disallowed the hexadecimal syntax.
The characters that comprise a numeric character reference can be represented in every character encoding used in computing and telecommunications today, so there is no risk of the reference itself being unencodable.
There is another kind of character reference called a character entity reference
Character entity reference
In the markup languages SGML, HTML, XHTML and XML, a character entity reference is a reference to a particular kind of named entity that has been predefined or explicitly declared in a Document Type Definition . The "replacement text" of the entity consists of a single character from the Universal...
, which allows a character to be referred to by a name instead of a number. (Naming a character creates a character entity
SGML entity
In the Standard Generalized Markup Language , an entity is a primitive data type, which associates a string with either a unique alias or an SGML reserved word . Entities are foundational to the organizational structure and definition of SGML documents...
.) HTML defines some character entities, but not many; all other characters can only be included by direct encoding or using NCRs.
Restrictions
The Universal Character Set defined by ISO 10646 is the "document character set" of SGML, HTML 4, so by default, any character in such a document, and any character referenced in such a document, must be in the UCS.While the syntax of SGML does not prohibit references to unassigned code points, such as
, SGML-derived markup languages such as HTML and XML can, and often do, restrict numeric character references to only those code points that are assigned to characters or that have not been permanently left unassigned.Restrictions may also apply for other reasons. For example, in HTML 4,
, which is a reference to a non-printing "form feed" control character, is allowed because a form feed character is allowed. But in XML, the form feed character cannot be used, not even by reference. As another example, €, which is a reference to another control character, is not allowed to be used or referenced in either HTML or XML, but when used in HTML, it is usually not flagged as an error by web browsers—some of which attempt to interpret it as a reference to the character represented by code value 128 in the Windows-1252Windows-1252
Windows-1252 or CP-1252 is a character encoding of the Latin alphabet, used by default in the legacy components of Microsoft Windows in English and some other Western languages. It is one version within the group of Windows code pages...
encoding: "€", which actually should be represented as
€. As a further example, prior to the publication of XML 1.0 Second Edition on October 6, 2000, XML 1.0 was based on an older version of ISO 10646 and prohibited using characters above U+FFFD, except in character data, thus making a reference like 𐀀 (U+10000) illegal. In XML 1.1 and newer editions of XML 1.0, such a reference is allowed, because the available character repertoire was explicitly extended.Markup languages also place restrictions on where character references can occur.
See also
- Character entity referenceCharacter entity referenceIn the markup languages SGML, HTML, XHTML and XML, a character entity reference is a reference to a particular kind of named entity that has been predefined or explicitly declared in a Document Type Definition . The "replacement text" of the entity consists of a single character from the Universal...
- List of XML and HTML character entity references