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. This article lists the character entity references that are valid in HTML and XML documents.
Contents
- Standard public entity sets for characters
- Predefined entities in XML
- Entities representing special characters in XHTML
- References
A character entity reference refers to the content of a named entity. An entity declaration is created by using the <!ENTITY name "value"> syntax in a Document Type Definition (DTD).
Standard public entity sets for characters
ISO Entity Sets: SGML supplied a comprehensive set of entity declarations for characters widely used in Western technical and reference publishing, for Latin, Greek and Cyrillic scripts. The American Mathematical Society also contributed entities for mathematical characters.
HTML Entity Sets: Early versions of HTML built in small subsets of these, relating to characters found in three Western 8-bit fonts.
MathML Entity Sets: The W3C developed a set of entity declarations for MathML characters.
XML Entity Sets: The W3C MathML Working Group took over maintenance of the ISO public entity sets, combined with the MathML and documents them in XML Entity Definitions for Characters. This set can support the requirements of XHTML, MathML and as an input to future versions of HTML.
HTML 5: HTML5 adopts the XML entities as named character references, however it restates them without reference to their sources and does not group them into sets. The HTML 5 specification additionally provides mappings from the names to Unicode character sequences using JSON.
Numerous other entity sets have been developed for special requirements, and for major and minority scripts. However, the advent of Unicode has largely superseded them.
Predefined entities in XML
The XML specification does not use the term "character entity" or "character entity reference". The XML specification defines five "predefined entities" representing special characters, and requires that all XML processors honor them. The entities can be explicitly declared in a DTD, as well, but if this is done, the replacement text must be the same as the built-in definitions. XML also allows other named entities of any size to be defined on a per-document basis.
The table below lists the five XML predefined entities. The "Name" column mentions the entity's name. The "Character" column shows the character. To render the character, the format &name; is used; for example, & renders as &. The "Unicode code point" column cites the character via standard UCS/Unicode "U+" notation, which shows the character's code point in hexadecimal. The decimal equivalent of the code point is then shown in parentheses. The "Standard" column indicates the first version of XML that includes the entity. The "Description" column cites the character via its canonical UCS/Unicode name, in English.
Entities representing special characters in XHTML
The XHTML DTDs explicitly declare 253 entities (including the 5 predefined entities of XML 1.0) whose expansion is a single character, which can therefore be informally referred to as "character entities". These (with the exception of the ' entity) have the same names and represent the same characters as the 252 character entities in HTML. Also, by virtue of being XML, XHTML documents may reference the predefined ' entity, which is not one of the 252 character entities in HTML. Additional entities of any size may be defined on a per-document basis. However, the usability of entity references in XHTML is affected by how the document is being processed:
' or custom entity references may not be supported and may produce unpredictable results.Because of the special ' case mentioned above, only ", &, <, and > will work in all processing situations.