Сущности в SGML: Типы, Объявления и Применение

In the Standard Generalized Markup Language (SGML), an entity is a primitive data type, which associates a string with either a unique alias (such as a user specified name) or an SGML reserved word (such as #DEFAULT). Entities are foundational to the organizational structure and definition of SGML documents. The SGML specification defines numerous entity types, which are distinguished by keyword qualifiers and context. An entity string value may variously consist of plain text, SGML tags, and/or references to previously defined entities. Certain entity types may also invoke external documents. Entities are called by reference.

An internal entity has a value that is either a literal string, or a parsed string comprising markup and entities defined in the same document (such as a Document Type Declaration or subdocument). In contrast, an external entity has a declaration that invokes an external document, thereby necessitating the intervention of an entity manager to resolve the external document reference.

An entity declaration may have a literal value, or may have some combination of an optional SYSTEM identifier, which allows SGML parsers to process an entity's string referent as a resource identifier, and an optional PUBLIC identifier, which identifies the entity independent of any particular representation. In XML, a subset of SGML, an entity declaration may not have a PUBLIC identifier without a SYSTEM identifier.

When an external entity references a complete SGML document, it is known in the calling document as an SGML document entity. An SGML document is a text document with SGML markup defined in an SGML prologue (i. e., the DTD and subdocuments). A complete SGML document comprises not only the document instance itself, but also the prologue and, optionally, the SGML declaration (which defines the document's markup syntax and declares the character encoding).

Standard entity sets for SGML and some of its derivatives have been developed as mnemonic devices, to ease document authoring when there is a need to use characters that are not easily typed or that are not widely supported by legacy character encodings. Each such entity consists of just one character from the Universal Character Set. Although any character can be referenced using a numeric character reference, a character entity reference allows characters to be referenced by name instead of code point. For example, HTML 4 has 252 built in character entities that do not need to be explicitly declared, while XML has five. XHTML has the same five as XML, but if its DTDs are explicitly used, then it has 253 (' being the extra entity beyond those in HTML 4).