The Standard Generalized Markup Language (ISO 8879:1986 SGML) is an ISO The International Organization for Standardization , widely known as ISO (pronounced /ˈaɪsoʊ/ EYE-soe), is an international-standard-setting body composed of representatives from various national standards organizations. Founded on 23 February 1947, the organization promulgates worldwide proprietary industrial and commercial standards. It has-standard technology for defining generalized markup languages A markup language is a modern system for annotating a text in a way that is syntactically distinguishable from that text. The idea and terminology evolved from the "marking up" of manuscripts, i.e. the revision instructions by editors, traditionally written with a blue pencil on authors' manuscripts. Examples are typesetting instructions for documents. ISO 8879 Annex A.1 defines generalized markup:

Generalized markup is based on two novel postulates:

  • Markup should describe a document's structure and other attributes, rather than specify the processing to be performed on it, as descriptive markup needs be done only once, and will suffice for future processing.
  • Markup should be rigorous so that the techniques available for processing rigorously-defined objects like programs and data bases, can be used for processing documents as well.

Contents

Standard versions

SGML is an ISO The International Organization for Standardization , widely known as ISO (pronounced /ˈaɪsoʊ/ EYE-soe), is an international-standard-setting body composed of representatives from various national standards organizations. Founded on 23 February 1947, the organization promulgates worldwide proprietary industrial and commercial standards. It has standard: "ISO 8879:1986 Information processing — Text and office systems — Standard Generalized Markup Language (SGML)", of which there are three versions:

SGML is part of a trio of enabling ISO standards for electronic documents developed by ISO/IEC JTC1/SC34 ISO/IEC JTC 1/SC 34 is a subcommittee of the ISO/IEC JTC1 joint technical committee, which is a collaborative effort of both the International Organization for Standardization and the International Electrotechnical Commission[1][2] (ISO/IEC Joint Technical Committee 1, Subcommittee 34 - Document description and processing languages) :

SGML is supported by various technical reports, in particular

Origins

SGML descended from IBM International Business Machines , abbreviated IBM, is a multinational computer, technology and IT consulting corporation head-quartered in Armonk, North Castle, New York, United States. IBM is the world's largest technology company and the second most valuable by global brand (after Coca-Cola). IBM is one of the few information technology's Generalized Markup Language Generalized Markup Language is a set of macros that implement intent-based markup tags for the IBM text formatter, SCRIPT/VS. SCRIPT/VS is the main component of IBM's Document Composition Facility (DCF). A starter set of tags in GML is provided with the DCF product (GML) that Charles Goldfarb Charles F. Goldfarb is known as the father of SGML and is a co-inventor of the concept of markup languages. In 1969 Charles Goldfarb, leading a small team at IBM, developed the first markup language, called Generalized Markup Language, or GML. In an interview with Web Techniques Magazine editor Michael Floyd, Dr. Goldfarb explains that he coined, Edward Mosher, and Raymond Lorie developed in the 1960s. Goldfarb, editor of the international standard, coined the “GML” term using their surname initials.[3]. As a document markup language, SGML was originally designed to enable the sharing of machine-readable In telecommunication, a machine-readable medium is a medium capable of storing data in a machine-readable format that can be accessed by an automated sensing device and capable of being turned into (practically in every case) some form of binary large-project documents in government, law, and industry. Many of these documents must remain readable for several decades — a long time in the information technology Information technology is "the study, design, development, implementation, support or management of computer-based information systems, particularly software applications and computer hardware", according to the Information Technology Association of America (ITAA). IT deals with the use of electronic computers and computer software to field. SGML also was extensively applied by the military, and the aerospace, technical reference, and industrial publishing businesses. The advent of the 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 profile has made SGML suitable for widespread application for small-scale, general-purpose use.

A fragment of the OED The Oxford English Dictionary , published by the Oxford University Press, is a dictionary of the English language. Two fully-bound print editions of the OED have been published under its current name, in 1928 and 1989. As of December 2008[update], the editors had completed one quarter of a third edition (1985), showing SGML markup

Validity

SGML (ENR+WWW) defines two kinds of validity. According to the revised Terms and Definitions of IS 8879 (from the public draft):

A conforming SGML document must be either a type-valid SGML document, a tag-valid SGML document, or both. Note: A user may wish to enforce additional constraints on a document, such as whether a document instance is integrally-stored or free of entity references.

A type-valid SGML document is defined by the standard as

An SGML document in which, for each document instance, there is an associated document type declaration to whose DTD that instance conforms.

A tag-valid SGML document is defined by the standard as

An SGML document, all of whose document instances are fully-tagged. There need not be a document type declaration associated with any of the instances. Note: If there is a document type declaration, the instance can be parsed with or without reference to it.

NOTE Tag-validity was introduced in SGML (ENR+WWW) to support 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 which allows documents with no DOCTYPE declaration but which can be parsed without needing a grammar—the standard calls this fully-tagged--, and standalone documents which have a DOCTYPE declaration but this makes no XML Infoset contributions to the document. Integrally stored supports the 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 requirement that elements end in the same entity in which they started. Reference-free supports the HTML HTML, which stands for HyperText Markup Language, is the predominant markup language for web pages. It is written in the form of HTML elements consisting of "tags" surrounded by angle brackets within the web page content requirement that entity references are for special characters and do not contain markup. Commentary, especially commentary that was made before 1997 or that is unaware of SGML (ENR+WWW), that discusses SGML validity will be limited to type-validity only.

The SGML emphasis on validity supports the requirement for generalized markup that markup should be rigorous. (ISO 8879 A.1)

Syntax

An SGML document may have three parts:

An SGML document may be composed from many entities In the Standard Generalized Markup Language , 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, discrete physical parts. In SGML, the entities and elements types used in the document may be specified with a DTD, the different character sets, features, delimiter sets, and keywords are specified in the SGML Declaration to create the concrete syntax of the document.

The 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 specification (s4.3.1) puts it the following way, though note that full SGML allows implicit markup and some other kinds of tags.

Each XML document has both a logical and a physical structure. Physically, the document is composed of units called entities. An entity may refer to other entities to cause their inclusion in the document. A document begins in a "root" or document entity. Logically, the document is composed of declarations, elements, comments, character references, and processing instructions, all of which are indicated in the document by explicit markup.

For introductory information on basic, modern SGML syntax, see the 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 entry. The following material concentrates on features that are additional to those found in XML and are not a comprehensive summary of SGML syntax.

Optional features

SGML was developed to generalize and support a wide range of markup languages as found in the mid 1980s. These ranged from Wiki Wikis may exist to serve a specific purpose, and in such cases, users use their editorial rights to remove material that is considered "off topic." Such is the case of the collaborative encyclopedia Wikipedia. In contrast, open purpose wikis accept content without firm rules as to how the content should be organized-like terse syntaxes to RTF The Rich Text Format is a proprietary document file format with published specification developed by Microsoft Corporation in 1987 for Microsoft products and for cross-platform document interchange-like bracketed languages to HTML HTML, which stands for HyperText Markup Language, is the predominant markup language for web pages. It is written in the form of HTML elements consisting of "tags" surrounded by angle brackets within the web page content-like matching-tag languages. SGML did this by a relatively simple default reference concrete syntax augmented with a large number of optional features that could be enabled in the SGML Declaration. Thus not every SGML parser could necessarily process every SGML document, however because the processor's System Declaration could be compared to the document's SGML Declaration it was always possible to know whether a document was supported by a particular processor.

Many of the features related to markup minimization. However other features related to parallel asynchronous markup (CONCUR), to linking processing attributes (LINK), and to embedded SGML documents directly within SGML documents (SUBDOC).

The notion of customizable features was not appropriate for use over the WWW, so a goal of 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 was to reduce optional features to a minimum. However in making its well-formedness rules, XML does not have capabilities to support the declaration and parsing of Wiki-like languages, leaving them unstandardized and difficult to integrate with non-text information systems.

Concrete and abstract syntaxes

The usual (default) SGML concrete syntax resembles this example, which is the default HTML HTML, which stands for HyperText Markup Language, is the predominant markup language for web pages. It is written in the form of HTML elements consisting of "tags" surrounded by angle brackets within the web page content concrete syntax:

<QUOTE TYPE="example">
typically something like <ITALICS>this</ITALICS>
</QUOTE>

SGML provides an abstract syntax In linguistics, syntax is the study of the principles and rules for constructing sentences in natural languages that can be implemented Implementation is the realization of an application, or execution of a plan, idea, model, design, specification, standard, algorithm, or policy in many different types of concrete syntax. Although the markup norm is using angle brackets Brackets are tall punctuation marks used in matched pairs within text, to set apart or interject other text. In the United States, "bracket" usually refers specifically to the "square" or "box" type; in British usage, it normally refers to a parenthesis mark as start- and end- tag delimiters A delimiter is a sequence of one or more characters used to specify the boundary between separate, independent regions in plain text or other data streams. An example of a delimiter is the comma character, which acts as a field delimiter in a sequence of comma-separated values in an SGML document (per the standard-defined reference concrete syntax), it is possible to use other characters — provided a suitable concrete syntax is defined in the document's SGML declaration.[4] For example, an SGML interpreter might be programmed to parse GML markup, wherein the tags are delimited with a left colon As with many other punctuation marks, the usage of colon varies among languages and, for a given language, among historical periods. As a rule, however, a colon informs the reader that what follows proves and explains, or simply provides elements of, what is referred to before and a right full stop A full stop or period (American English) is the punctuation mark commonly placed at the end of sentences, thus, an :e prefix denotes an end tag: :xmp.Hello, world:exmp.. According to the reference syntax, letter-case (upper- or lower-) is not distinguished in tag names, thus the three tags: (i) <quote>, (ii) <QUOTE>, and (iii) <quOtE> are equivalent. NOTE: 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 markup a document, which must be enabled in the SGML Declaration. The SGML software need not support every available feature, thus allowing SGML applications to tolerate many types of inadvertent markup omissions; however, SGML systems usually are intolerant of invalid element structures, whereas XML is intolerant of syntax omissions, and does not require a DTD for validation.

OMITTAG

The DTD information specifies whether or not a markup element's start- or end- tags might be omitted; SGML has rules for implying omitted tags, the OMITTAG feature. If a tag must be paired or not (as in the previous <QUOTE></QUOTE> pair example) or if it can occur singly (as an HTML <HR>), those specifications are defined in the DTD for the markup language being defined (provided the OMITTAG feature is enabled). In this case, the 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 counterpart would be the specific empty tag <hr/>, equivalent to the SGML NET-enabling start-tag, introduced in the TC2 (International Standard ISO 8879:1986, Technical Corrigendum 2, November 1999).

SHORTREF

Tags can be replaced with delimiter strings, for a terser markup, 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:

<QUOTE TYPE=example>
typically something like <ITALICS>this</ITALICS>
</QUOTE>

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

NET

Another feature is the NET (Null End Tag) construction: <ITALICS/this/, which is structurally equivalent to <ITALICS>this</ITALICS>.

Other features

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

<QUOTE></QUOTE>

can be written as:

<QUOTE/>

Wherein the first virgule ( / ) stands for the NET-enabling “start-tag close” (NETSC), and the second virgule ( / ) stands for the NET. NOTE: XML defines NETSC with a / (virgule), and NET with an > (angled bracket) — hence this construct appears as <QUOTE/>. 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:

<!ELEMENT lines (line*)
<!ELEMENT line O - (#PCDATA)>
<!ENTITY line-tagc "</line>">
<!SHORTREF one-line "&#RE;&#RS;" line-tagc>
<!USEMAP one-line line>

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

<lines>
first line
second line
</lines>

is equivalent to:

<lines>
<line>first line</line>
<line>second line</line>
</lines>

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