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Source: Standard Generalized Markup Language

The Standard Generalized Markup Language (SGML; ISO 8879:1986) is a standard for defining generalized markup languages for documents. ISO 8879 Annex A.1 states that generalized markup is "based on two postulates":

Declarative: Markup should describe a document's structure and other attributes rather than specify the processing that needs to be performed, because it is less likely to conflict with future developments.
Rigorous: In order to allow markup to take advantage of the techniques available for processing, markup should rigorously define objects like programs and databases.
DocBook SGML and LinuxDoc are examples which used SGML tools.

Standard versions

standard

History

Document validity

standard

= Terminology

Syntax

markup

= Optional features

markup

= Concrete and abstract syntaxes

markup

standard

, , and are equivalent. (A concrete syntax might change this rule via the NAMECASE NAMING declarations.)

= Markup minimization

=
SGML has features for reducing the number of characters required to mark up a document, which must be enabled in the SGML Declaration. SGML processors need not support every available feature, thus allowing applications to tolerate many types of inadvertent markup omissions; however, SGML systems usually are intolerant of invalid structures. XML is intolerant of syntax omissions, and does not require a DTD for checking well-formedness.

OMITTAG

Both start tags and end tags may be omitted from a document instance, provided:

the OMITTAG feature is enabled in the SGML Declaration,
the DTD indicates that the tags are permitted to be omitted,
(for start tags) the element has no associated required (#REQUIRED) attributes, and
the tag can be unambiguously inferred by context.
For example, if OMITTAG YES is specified in the SGML Declaration (enabling the OMITTAG feature), and the DTD includes the following declarations:

then this excerpt:

which omits two tags and two tags, would represent valid markup.
Omitting tags is optional – the same excerpt could be tagged like this:

and would still represent valid markup.

Note: The OMITTAG feature is unrelated to the tagging of elements whose declared content is EMPTY as defined in the DTD:

Elements defined like this have no end tag, and specifying one in the document instance would result in invalid markup. This is syntactically different from XML empty elements in this regard.

SHORTREF

Tags can be replaced with delimiter strings, for a terser markup, via the SHORTREF feature. This markup style is now associated with wiki markup, e.g. wherein two equals-signs (

), at the start of a line, are the "heading start-tag", and two equals signs (

) after that are the "heading end-tag".

SHORTTAG

SGML markup languages whose concrete syntax enables the SHORTTAG VALUE feature, do not require attribute values containing only alphanumeric characters to be enclosed within quotation marks—either double " " (LIT) or single ' ' (LITA)—so that the previous markup example could be written:

One feature of SGML markup languages is the "presumptuous empty tagging", such that the empty end tag in this "inherits" its value from the nearest previous full start tag, which, in this example, is (in other words, it closes the most recently opened item). The expression is thus equivalent to this.

NET

Another feature is the NET (Null End Tag) construction: this.

Other features

Additionally, the SHORTTAG NETENABL IMMEDNET feature allows shortening tags surrounding an empty text value, but forbids shortening full tags:

can be written as

wherein the first slash ( / ) stands for the NET-enabling "start-tag close" (NESTC), and the second slash stands for the NET. NOTE: XML defines NESTC with a /, and NET with an > (angled bracket)—hence the corresponding construct in XML appears as .
The third feature is 'text on the same line', allowing a markup item to be ended with a line-end; especially useful for headings and such, requiring using either SHORTREF or DATATAG minimization. For example, if the DTD includes the following declarations:

(and "&#RE;&#RS;" is a short-reference delimiter in the concrete syntax), then:

is equivalent to:

Formal characterization

SGML has many features that defied convenient description with the popular formal automata theory and the contemporary parser technology of the 1980s and the 1990s. The standard warns in Annex H:

The SGML model group notation was deliberately designed to resemble the regular expression notation of automata theory, because automata theory provides a theoretical foundation for some aspects of the notion of conformance to a content model. No assumption should be made about the general applicability of automata to content models.
A report on an early implementation of a parser for basic SGML, the Amsterdam SGML Parser, notes the DTD-grammar in SGML must conform to a notion of unambiguity which closely resembles the LL(1) conditions and specifies various differences.
There appears to be no definitive classification of full SGML against a known class of formal grammar. Plausible classes may include tree-adjoining grammars and adaptive grammars.
XML is described as being generally parsable like a two-level grammar for non-validated XML and a Conway-style pipeline of coroutines (lexer, parser, validator) for valid XML. The SGML productions in the ISO standard are reported to be LL(3) or LL(4). XML-class subsets are reported to be expressible using a W-grammar. According to one paper, and probably considered at an information set or parse tree level rather than a character or delimiter level:

The class of documents that conform to a given SGML document grammar forms an LL(1) language. ... The SGML document grammars by themselves are, however, not LL(1) grammars.
The SGML standard does not define SGML with formal data structures, such as parse trees; however, an SGML document is constructed of a rooted directed acyclic graph (RDAG) of physical storage units known as "entities", which is parsed into a RDAG of structural units known as "elements". The physical graph is loosely characterized as an entity tree, but entities might appear multiple times. Moreover, the structure graph is also loosely characterized as an element tree, but the ID/IDREF markup allows arbitrary arcs.
The results of parsing can also be understood as a data tree in different notations; where the document is the root node, and entities in other notations (text, graphics) are child nodes. SGML provides apparatus for linking to and annotating external non-SGML entities.
The SGML standard describes it in terms of maps and recognition modes (s9.6.1). Each entity, and each element, can have an associated notation or declared content type, which determines the kinds of references and tags which will be recognized in that entity and element. Also, each element can have an associated delimiter map (and short reference map), which determines which characters are treated as delimiters in context. The SGML standard characterizes parsing as a state machine switching between recognition modes. During parsing, there is a stack of maps that configure the scanner, while the tokenizer relates to the recognition modes.
Parsing involves traversing the dynamically-retrieved entity graph, finding/implying tags and the element structure, and validating those tags against the grammar. An unusual aspect of SGML is that the grammar (DTD) is used both passively — to recognize lexical structures, and actively — to generate missing structures and tags that the DTD has declared optional. End- and start- tags can be omitted, because they can be inferred. Loosely, a series of tags can be omitted only if there is a single, possible path in the grammar to imply them. It was this active use of grammars that made concrete SGML parsing difficult to formally characterize.
SGML uses the term validation for both recognition and generation. XML does not use the grammar (DTD) to change delimiter maps or to inform the parse modes, and does not allow tag omission; consequently, XML validation of elements is not active in the sense that SGML validation is active. SGML without a DTD (e.g. simple XML), is a grammar or a language; SGML with a DTD is a metalanguage. SGML with an SGML declaration is, perhaps, a meta-metalanguage, since it is a metalanguage whose declaration mechanism is a metalanguage.
SGML has an abstract syntax implemented by many possible concrete syntaxes; however, this is not the same usage as in an abstract syntax tree and as in a concrete syntax tree. In the SGML usage, a concrete syntax is a set of specific delimiters, while the abstract syntax is the set of names for the delimiters. The XML Infoset corresponds more to the programming language notion of abstract syntax introduced by John McCarthy.

Derivatives

= XML

=

The W3C XML (Extensible Markup Language) is a profile (subset) of SGML designed to ease the implementation of the parser compared to a full SGML parser, primarily for use on the World Wide Web. In addition to disabling many SGML options present in the reference syntax (such as omitting tags and nested subdocuments) XML adds a number of additional restrictions on the kinds of SGML syntax. For example, despite enabling SGML shortened tag forms, XML does not allow unclosed start or end tags. It also relied on many of the additions made by the WebSGML Annex. XML currently is more widely used than full SGML. XML has lightweight internationalization based on Unicode. Applications of XML include XHTML, XQuery, XSLT, XForms, XPointer, JSP, SVG, RSS, Atom, XML-RPC, RDF/XML, and SOAP.

= HTML

=

While HTML (Hyper Text Markup Language) was developed partially independently and in parallel with SGML, its creator, Tim Berners-Lee, intended it to be an application of SGML. The design of HTML was therefore inspired by SGML tagging, but, since no clear expansion and parsing guidelines were established, most actual HTML documents are not valid SGML documents. Later, HTML was reformulated (version 2.0) to be more of an SGML application; however, the HTML markup language has many legacy- and exception-handling features that differ from SGML's requirements. HTML 4 is an SGML application that fully conforms to ISO 8879 – SGML.
The charter for the 2006 revival of the World Wide Web Consortium HTML Working Group says, "the Group will not assume that an SGML parser is used for 'classic HTML'". Although HTML syntax closely resembles SGML syntax with the default reference concrete syntax, HTML5 abandons any attempt to define HTML as an SGML application, explicitly defining its own parsing rules, which more closely match existing implementations and documents. It does, however, define an alternative XHTML serialization, which conforms to XML and therefore to SGML as well.

= OED

=

The second edition of the Oxford English Dictionary (OED) is entirely marked up with an SGML-based markup language using the LEXX text editor.
The third edition is marked up as XML.

= Others

=
Other document markup languages are partly related to SGML and XML, but—because they cannot be parsed or validated or other-wise processed using standard SGML and XML tools—they are not considered either SGML or XML languages; the Z Format markup language for typesetting and documentation is an example.
Several modern programming languages support tags as primitive token types, or now support Unicode and regular expression pattern-matching. An example is the Scala programming language.

Applications

Document markup languages defined using SGML are called "applications" by the standard; many pre-XML SGML applications were proprietary property of the organizations which developed them, and thus unavailable in the World Wide Web. The following list is of pre-XML SGML applications.

Text Encoding Initiative (TEI) is an academic consortium that designs, maintains, and develops technical standards for digital-format textual representation applications.
DocBook is a markup language originally created as an SGML application, designed for authoring technical documentation; DocBook currently is an XML application.
CALS (Continuous Acquisition and Life-cycle Support) is a US Department of Defense (DoD) initiative for electronically capturing military documents and for linking related data and information.
HyTime defines a set of hypertext-oriented element types that allow SGML document authors to build hypertext and multimedia presentations.
EDGAR (Electronic Data-Gathering, Analysis, and Retrieval) system effects automated collection, validation, indexing, acceptance, and forwarding of submissions, by companies and others, who are legally required to file data and information forms with the US Securities and Exchange Commission (SEC).
LinuxDoc. Documentation for Linux packages has used the LinuxDoc SGML DTD and Docbook XML DTD.
AAP DTD is a document type definition for scientific documents, defined by the Association of American Publishers.
ISO 12083, a successor to AAP DTP, is an international SGML standard for document interchange between authors and publishers.
SGMLguid was an early SGML document type definition created, developed and used at CERN.

Open-source implementations

Significant open-source implementations of SGML have included:

ASP-SGML
ARC-SGML, by Standard Generalized Markup Language Users', 1991, C language
SGMLS, by James Clark, 1993, C language
Project YAO, by Yuan-ze Institute of Technology, Taiwan, with Charles Goldfarb, 1994.
SP and Jade by James Clark, C++ language
SP and Jade, the associated DSSSL processors, are maintained by the OpenJade project, and are common parts of Linux distributions. A general archive of SGML software and materials resides at SUNET. The original HTML parser class, in Sun System's implementation of Java, is a limited-features SGML parser, using SGML terminology and concepts.

References

External links

Overview of SGML Resources at W3C's website.
Introduction and Examples of Software Documentation in SGML
SC34 Committee Records, Charles Babbage Institute – Collection on the development of SGML and other standards influential in the development of current XML tools; documents include early drafts of SGML administrative materials, documentation, working group papers, and standards for computer languages.
SGML Syntax Summary by Charles Goldfarb
SGML document introducing you to SGML; Some reasons why SGML is important
The SGML Declaration, in SGML and HTML Explained, Martin Bryan (1997) (the original URL is broken at http://www.is-thought.co.uk/book/sgml-4.htm#Fig4-2)
SGML Declarations Wayne Wohler, IBM Corporation, 1994.
ISO 9069:1988 – Information processing – SGML support facilities – SGML Document Interchange Format (SDIF)
ISO/IEC 9070:1991 – Information technology – SGML support facilities – Registration procedures for public text owner identifiers