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Document Type Definition (DTD) is one of several SGML and XML schema languages, and is also the term used to describe a document or portion thereof that is authored in the DTD language. The Standard Generalized Markup Language ( ISO 88791986 SGML) is an ISO Standard Metalanguage in which one can define Markup languages An XML schema is a description of a type of XML document typically expressed in terms of constraints on the structure and content of documents of that type above and beyond A DTD is primarily used for the expression of a schema via a set of declarations that conform to a particular markup syntax and that describe a class, or type, of document, in terms of constraints on the structure of that document. A markup language is an Artificial language using a set of annotations to text that give instructions regarding the structure of text or how it is to be displayed A DTD may also declare constructs that are not always required to establish document structure, but that may affect the interpretation of some documents. XML documents are described using a subset of DTD which imposes a number of restrictions on the document's structure, as required per the XML standard (XML is in itself an application of SGML optimized for automated parsing). Don't change "Extensible"

DTD is native to the SGML and XML specifications, and since its introduction other specification languages such as XML Schema and RELAX NG have been released with additional functionality. XML Schema, published as a W3C recommendation in May 2001 is one of several XML schema languages. In Computing, RELAX NG ( REgular LAnguage for XML Next Generation) is a schema language for XML, based on Murata Makoto 's RELAX

As an expression of a schema, a DTD specifies, in effect, the syntax of an "application" of SGML or XML, such as the derivative language HTML or XHTML. HTML, an initialism of HyperText Markup Language, is the predominant Markup language for Web pages It provides a means to describe the structure The Extensible Hypertext Markup Language, or XHTML, is a This syntax is usually a less general form of the syntax of SGML or XML.

In a DTD, the structure of a class of documents is described via element and attribute-list declarations. Element declarations name the allowable set of elements within the document, and specify whether and how declared elements and runs of character data may be contained within each element. Attribute-list declarations name the allowable set of attributes for each declared element, including the type of each attribute value, if not an explicit set of valid value(s). A data type in Programming languages is an attribute of a datum which tells the computer (and the programmer something about the kind of datum it is

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Associating DTDs with documents

A DTD is associated with an XML document via a Document Type Declaration, which is a tag that appears near the start of the XML document. A Document Type Declaration, or DOCTYPE, is an instruction that associates a particular SGML or XML document (for example a Webpage) with a The declaration establishes that the document is an instance of the type defined by the referenced DTD.

The declarations in a DTD are divided into an internal subset and an external subset. The declarations in the internal subset are embedded in the Document Type Declaration in the document itself. The declarations in the external subset are located in a separate text file. The external subset may be referenced via a public identifier and/or a system identifier. A public identifier is a document processing construct in SGML and XML. A system identifier is a document processing construct introduced in the HyTime markup language as a supplement to SGML. Programs for reading documents may not be required to read the external subset.

Examples

Here is an example of a Document Type Declaration containing both public and system identifiers:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1. 0 Transitional//EN""http://www. w3. org/TR/xhtml1/DTD/xhtml1-transitional. dtd">


<!DOCTYPE foo [ <!ENTITY greeting "hello"> ]> <!DOCTYPE bar [ <!ENTITY greeting "hello"> ]>

All HTML 4. 01 documents are expected to conform to one of three SGML DTDs. The public identifiers of these DTDs are constant and are as follows:

The system identifiers of these DTDs, if present in the Document Type Declaration, will be URI references. System identifiers can vary, but are expected to point to a specific set of declarations in a resolvable location. SGML allows for public identifiers to be mapped to system identifiers in catalogs that are optionally made available to the URI resolvers used by document parsing software.

XML DTDs and schema validation

The XML DTD syntax is one of several XML schema languages. An XML schema is a description of a type of XML document typically expressed in terms of constraints on the structure and content of documents of that type above and beyond

A common misconception is that non-validating XML parsers are not required to read DTDs, when in fact, the DTD must still be scanned for correct syntax as well as for declarations of entities and default attributes. A non-validating parser may, however, elect not to read external entities, including the external subset of the DTD. If the XML document depends on declarations found only in external entities, it should assert standalone="no" in its XML declaration.

Differences between SGML and XML DTD syntax

The syntax of SGML and XML DTDs are very similar, but not identical. The Standard Generalized Markup Language ( ISO 88791986 SGML) is an ISO Standard Metalanguage in which one can define Markup languages

XML DTD Example

An example of a very simple XML DTD to describe a list of persons is given below:

<!ELEMENT people_list (person*)><!ELEMENT person (name, birthdate?, gender?, socialsecuritynumber?)><!ELEMENT name (#PCDATA)><!ELEMENT birthdate (#PCDATA)><!ELEMENT gender (#PCDATA)><!ELEMENT socialsecuritynumber (#PCDATA)>

Taking this line by line, it says:

  1. people_list is a valid element name, and an instance of such an element contains any number of person elements. The * denotes there can be 0 or more person elements within the people_list element.
  2. person is a valid element name, and an instance of such an element contains one element named name, followed by one named birthdate (optional), then gender (also optional) and socialsecuritynumber (also optional). The ? indicates that an element is optional. The reference to the name element name has no ?, so a person element must contain a name element.
  3. name is a valid element name, and an instance of such an element contains "parsed character data" (#PCDATA).
  4. birthdate is a valid element name, and an instance of such an element contains character data.
  5. gender is a valid element name, and an instance of such an element contains character data.
  6. socialsecuritynumber is a valid element name, and an instance of such an element contains character data.

An example of an XML file which makes use of and conforms to this DTD follows. It assumes the DTD is identifiable by the relative URI reference "example. dtd", and the "people_list" after "!DOCTYPE" tells us that the root tags, or the first element defined in the DTD, is called "people_list":

<?xml version="1. 0" encoding="UTF-8"?><!DOCTYPE people_list SYSTEM "example. dtd"><people_list>  <person>    <name>Fred Bloggs</name>    <birthdate>27/11/2008</birthdate>    <gender>Male</gender>  </person></people_list>

It is possible to render this in an XML-enabled browser (such as IE5 or Mozilla) by pasting and saving the DTD component above to a text file named example. A web browser is a software application which enables a user to display and interact with text images videos music games and other information typically located on a Windows Internet Explorer (formerly Microsoft Internet Explorer abbreviated MSIE) commonly abbreviated to IE, is a series of graphical Mozilla was the official public original name of Mozilla Application Suite by the Mozilla Foundation, currently known as SeaMonkey suite. dtd and the XML file to a differently-named text file, and opening the XML file with the browser. The files should both be saved in the same directory. However, many browsers do not check that an XML document conforms to the rules in the DTD; they are only required to check that the DTD is syntactically correct. For security reasons, they may also choose not to read the external DTD.

DTD criticisms and alternatives

While DTD support in XML tools is widespread due to its inclusion in the XML 1. 0 standard, it is seen as limiting for the following reasons:

Three newer XML schema languages that are much more powerful are increasingly favored over DTDs:

See also

External links

XML Schema, published as a W3C recommendation in May 2001 is one of several XML schema languages. POJO is an acronym for Plain Old Java Object, and is favoured by advocates of the idea that the simpler the design the better
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