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Extensible Markup Language (XML) is a
markup language #REDIRECT Markup language #REDIRECT Markup language In computer text processing, a markup language is a system for annotation, annotating a document in a way that is Syntax (logic), syntactically distinguishable from the text, meaning when the do ...
that defines a set of rules for encoding
document A document is a writing, written, drawing, drawn, presented, or memorialized representation of thought, often the manifestation of nonfiction, non-fictional, as well as fictional, content. The word originates from the Latin ''Documentum'', whic ...
s in a format that is both
human-readable 220px, ISBN represented as EAN-13 bar code showing both human-readable and machine-readable data A human-readable medium or human-readable format is any encoding of data Data are units of information Information can be thought of as ...
and machine-readable. The
World Wide Web Consortium The World Wide Web Consortium (W3C) is the main international standards organization A standards organization, standards body, standards developing organization (SDO), or standards setting organization (SSO) is an organization whose primary ...
's XML 1.0 Specification of 1998 and several other related specifications—all of them free
open standard An open standard is a standard Standard may refer to: Flags * Colours, standards and guidons * Standard (flag), a type of flag used for personal identification Norm, convention or requirement * Standard (metrology), an object that bears a d ...
s—define XML. The design goals of XML emphasize simplicity, generality, and usability across the
Internet The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a ''internetworking, network of networks'' that consist ...

Internet
. It is a textual data format with strong support via
Unicode Unicode, formally the Unicode Standard, is an information technology Technical standard, standard for the consistent character encoding, encoding, representation, and handling of Character (computing), text expressed in most of the world's wri ...

Unicode
for different
human languages
human languages
. Although the design of XML focuses on documents, the language is widely used for the representation of arbitrary
data structure In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of ...

data structure
s such as those used in
web service The term Web service (WS) is either: * a service offered by an electronic device to another electronic device, communicating with each other via the World Wide Web The World Wide Web (WWW), commonly known as the Web, is an information ...
s. Several schema systems exist to aid in the definition of XML-based languages, while programmers have developed many
application programming interface In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and so ...
s (APIs) to aid the processing of XML data.


Applications

The essence of why extensible markup languages are necessary is explained at ''
Markup language #REDIRECT Markup language #REDIRECT Markup language In computer text processing, a markup language is a system for annotation, annotating a document in a way that is Syntax (logic), syntactically distinguishable from the text, meaning when the do ...
'' (for example, see ''
Markup language § XML
Markup language § XML
'') and at ''
Standard Generalized Markup Language The Standard Generalized Markup Language (SGML; ISO The International Organization for Standardization (ISO ) is an international standard An international standard is a technical standard A technical standard is an established norm (social) ...
''. Hundreds of document formats using XML syntax have been developed, including
RSS RSS (Resource Description Framework, RDF Site Summary or Really Simple Syndication) is a web feed that allows users and applications to access updates to websites in a Standardization, standardized, computer-readable format. Subscribing to RSS ...

RSS
,
Atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of ato ...
,
SOAP Soap is a salt (chemistry), salt of a fatty acid used in a variety of cleansing and lubricating products. In a domestic setting, soaps are surfactants usually used for washing, bathing, and other types of housekeeping. In industrial settings, ...

SOAP
,
SVG#REDIRECT Scalable Vector Graphics Scalable Vector Graphics (SVG) is an Extensible Markup Language (XML)-based vector image format for two-dimensional graphics with support for interactivity and animation. The SVG specification is an open s ...
, and
XHTML Extensible HyperText Markup Language (XHTML) is part of the family of XML Extensible Markup Language (XML) is a markup language #REDIRECT Markup language In computer text processing, a markup language is a system for annotation, annotating ...

XHTML
. XML-based formats have become the default for many office-productivity tools, including
Microsoft Office Microsoft Office, or simply Office, is a family of client software In computing, a client is a piece of computer hardware or software that accesses a service made available by a Server (computing), server as part of the client–server mo ...
(
Office Open XML Office Open XML (also informally known as OOXML) is a ZIP (file format), zipped, XML-based file format developed by Microsoft for representing spreadsheets, charts, presentations and word processing documents. The format was initially standardi ...
),
OpenOffice.org
OpenOffice.org
and
LibreOffice LibreOffice () is a free and open-source productivity software, office productivity software suite, a project of The Document Foundation (TDF). It was fork (software development), forked in 2010 from OpenOffice.org, an open-sourced version of ...

LibreOffice
(
OpenDocument The Open Document Format for Office Applications (ODF), also known as OpenDocument, is an open standard An open standard is a standard that is publicly available and has various rights to use associated with it and may also have vario ...
), and
Apple An apple is an edible fruit In botany Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this fie ...
's
iWork iWork is an office suite Productivity software (also called personal productivity software or office productivity software) is application software used for producing information (such as document A document is a writing, written, drawing, ...
. XML has also provided the base language for
communication protocol A communication protocol is a system of rules that allows two or more entities of a communications system 400px, Communication system A communications system or communication system is a collection of individual telecommunications networ ...
s such as
XMPP Extensible Messaging and Presence Protocol (XMPP, originally named Jabber) is an open Open or OPEN may refer to: Music * Open (band) Open is a band. Background Drummer Pete Neville has been involved in the Sydney/Australian music scene for ...
. Applications for the
Microsoft Microsoft Corporation is an American multinational corporation, multinational technology company, technology corporation which produces Software, computer software, consumer electronics, personal computers, and related services. Its best-know ...

Microsoft
.NET Framework The .NET Framework (pronounced as "''dot net"'') is a software framework In computer programming Computer programming is the process of designing and building an executable computer program to accomplish a specific computing result or t ...
use XML files for configuration, and
property list In the macOS, IOS (Apple), iOS, NeXTSTEP, and GNUstep programming Software framework, frameworks, property list files are files that store serialization, serialized object (computer science), objects. Property list files use the filename extensio ...
s are an implementation of configuration storage built on XML. Many industry data standards, such as
Health Level 7 Health Level Seven or HL7 refers to a set of international standards for transfer of clinical and administrative data between software applications used by various healthcare providers. These standards focus on the application layer, which is "la ...
,
OpenTravel Alliance The OpenTravel Alliance is a not-for-profit trade association founded in 1999. History OpenTravel was founded in 1999 by a consortium of travel companies - including air, hotel, car rental, and supporting software companies - with a primary focu ...
, FpML, MISMO, and
National Information Exchange Model The National Information Exchange Model (NIEM) ( ) is an XML-based information exchange framework from the United States. NIEM represents a collaborative partnership of agencies and organizations across all levels of government (federal, state, tri ...
are based on XML and the rich features of the XML schema specification. Many of these standards are quite complex and it is not uncommon for a specification to comprise several thousand pages. In publishing,
Darwin Information Typing Architecture The Darwin Information Typing Architecture (DITA) specification defines a set of document types for authoring and organizing topic-oriented information, as well as a set of mechanisms for combining, extending, and constraining document types. It i ...
is an XML industry data standard. XML is used extensively to underpin various publishing formats. XML is widely used in a
Service-oriented architecture In software engineering Software engineering is the systematic application of engineering approaches to the software development, development of software. A software engineer is a person who applies the principles of software engineering to desi ...
(SOA). Disparate systems communicate with each other by exchanging XML messages. The message exchange format is standardised as an
XML schema #REDIRECT XML schema#REDIRECT XML schema 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 the basic syntactical constra ...
(XSD). This is also referred to as the canonical schema. XML has come into common use for the interchange of data over the Internet.
IETF The Internet Engineering Task Force (IETF) is an open standards organization A standards organization, standards body, standards developing organization (SDO), or standards setting organization (SSO) is an organization whose primary functi ...
:RFC:3023, now superseded by :RFC:7303, gave rules for the construction of Internet Media Types for use when sending XML. It also defines the media types application/xml and text/xml, which say only that the data is in XML, and nothing about its
semantics Semantics (from grc, σημαντικός ''sēmantikós'', "significant") is the study of reference Reference is a relationship between objects in which one object designates, or acts as a means by which to connect to or link to, another ...
. RFC 7303 also recommends that XML-based languages be given media types ending in +xml; for example image/svg+xml for
SVG#REDIRECT Scalable Vector Graphics Scalable Vector Graphics (SVG) is an Extensible Markup Language (XML)-based vector image format for two-dimensional graphics with support for interactivity and animation. The SVG specification is an open s ...
. Further guidelines for the use of XML in a networked context appear in RFC 3470, also known as IETF BCP 70, a document covering many aspects of designing and deploying an XML-based language.


Key terminology

The material in this section is based on the XML Specification. This is not an exhaustive list of all the constructs that appear in XML; it provides an introduction to the key constructs most often encountered in day-to-day use. * An XML document is a string of ''characters''.
Almost In set theory Set theory is the branch of mathematical logic that studies Set (mathematics), sets, which can be informally described as collections of objects. Although objects of any kind can be collected into a set, set theory, as a branch ...
every legal
Unicode Unicode, formally the Unicode Standard, is an information technology Technical standard, standard for the consistent character encoding, encoding, representation, and handling of Character (computing), text expressed in most of the world's wri ...

Unicode
character may appear in an XML document. * The ''processor'' analyzes the markup and passes structured information to an ''application''. The specification places requirements on what an XML processor must do and not do, but the application is outside its scope. The processor (as the specification calls it) is often referred to colloquially as an ''XML parser''. * The characters making up an XML document are divided into ''markup'' and ''content'', which may be distinguished by the application of simple syntactic rules. Generally, strings that constitute markup either begin with the character < and end with a >, or they begin with the character & and end with a ;. Strings of characters that are not markup are content. However, in a
CDATA The term CDATA, meaning character data, is used for distinct, but related, purposes in the markup language In computer text processing, a markup language is a system for annotation, annotating a document in a way that is Syntax (logic), syntacti ...
section, the delimiters <! DATA[_and_>_are_classified_as_markup,_while_the_text_between_them_is_classified_as_content._In_addition,_whitespace_before_and_after_the_outermost_element_is_classified_as_markup. *_A_''tag''_is_a_markup_construct_that_begins_with_<_and_ends_with_>._There_are_three_types_of_tag: :*_''start-tag'',_such_as_<section>; :*_''end-tag'',_such_as_</section>; :*_''empty-element_tag'',_such_as_<line-break_/>. *_An_''element''_is_a_logical_document_component_that_either_begins_with_a_start-tag_and_ends_with_a_matching_end-tag_or_consists_only_of_an_empty-element_tag._The_characters_between_the_start-tag_and_end-tag,_if_any,_are_the_element's_''content'',_and_may_contain_markup,_including_other_elements,_which_are_called_''child_elements''._An_example_is_<greeting>Hello,_world!</greeting>._Another_is_<line-break />. *_An_''attribute''_is_a_markup_construct_consisting_of_a_attribute–value_pair.html" ;"title="/code>_and_.html" ;"title="DATA[ and ">DATA[ and > are classified as markup, while the text between them is classified as content. In addition, whitespace before and after the outermost element is classified as markup. * A ''tag'' is a markup construct that begins with < and ends with >. There are three types of tag: :* ''start-tag'', such as <section>; :* ''end-tag'', such as </section>; :* ''empty-element tag'', such as <line-break />. * An ''element'' is a logical document component that either begins with a start-tag and ends with a matching end-tag or consists only of an empty-element tag. The characters between the start-tag and end-tag, if any, are the element's ''content'', and may contain markup, including other elements, which are called ''child elements''. An example is <greeting>Hello, world!</greeting>. Another is <line-break />. * An ''attribute'' is a markup construct consisting of a attribute–value pair">name–value pair that exists within a start-tag or empty-element tag. An example is <img src="madonna.jpg" alt="Madonna" />, where the names of the attributes are "src" and "alt", and their values are "madonna.jpg" and "Madonna" respectively. Another example is <step number="3">Connect A to B.</step>, where the name of the attribute is "number" and its value is "3". An XML attribute can only have a single value and each attribute can appear at most once on each element. In the common situation where a list of multiple values is desired, this must be done by encoding the list into a well-formed XML attribute with some format beyond what XML defines itself. Usually this is either a comma or semi-colon delimited list or, if the individual values are known not to contain spaces, a space-delimited list can be used. <div class="inner greeting-box">Welcome!</div>, where the attribute "class" has both the value "inner greeting-box" and also indicates the two CSS class names "inner" and "greeting-box". * XML documents may begin with an ''XML declaration'' that describes some information about themselves. An example is <?xml version="1.0" encoding="UTF-8"?>.


Characters and escaping

XML documents consist entirely of characters from the
Unicode Unicode, formally the Unicode Standard, is an information technology Technical standard, standard for the consistent character encoding, encoding, representation, and handling of Character (computing), text expressed in most of the world's wri ...

Unicode
repertoire. Except for a small number of specifically excluded
control characters In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and softw ...
, any character defined by Unicode may appear within the content of an XML document. XML includes facilities for identifying the ''encoding'' of the Unicode characters that make up the document, and for expressing characters that, for one reason or another, cannot be used directly.


Valid characters

Unicode code points in the following ranges are valid in XML 1.0 documents: * U+0009 (Horizontal Tab), U+000A (Line Feed), U+000D (Carriage Return): these are the only C0 controls accepted in XML 1.0; * U+0020–U+D7FF, U+E000–U+FFFD: this excludes some non-characters in the BMP (all surrogates, U+FFFE and U+FFFF are forbidden); * U+10000–U+10FFFF: this includes all code points in supplementary planes, including non-characters. XML 1.1 extends the set of allowed characters to include all the above, plus the remaining characters in the range U+0001–U+001F. At the same time, however, it restricts the use of C0 and C1 control characters other than U+0009 (Horizontal Tab), U+000A (Line Feed), U+000D (Carriage Return), and U+0085 (Next Line) by requiring them to be written in escaped form (for example U+0001 must be written as &#x01; or its equivalent). In the case of C1 characters, this restriction is a backwards incompatibility; it was introduced to allow common encoding errors to be detected. The code point U+0000 (Null) is the only character that is not permitted in any XML 1.0 or 1.1 document.


Encoding detection

The Unicode character set can be encoded into bytes for storage or transmission in a variety of different ways, called "encodings". Unicode itself defines encodings that cover the entire repertoire; well-known ones include
UTF-8 UTF-8 is a variable-width character encoding Character encoding is the process of assigning numbers to Graphics, graphical character (computing), characters, especially the written characters of Language, human language, allowing them to be ...
and
UTF-16 UTF-16 (16-bit 16-bit microcomputer A microcomputer is a small, relatively inexpensive computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations automatically. Modern computers ...
. There are many other text encodings that predate Unicode, such as
ASCII ASCII ( ), abbreviated from American Standard Code for Information Interchange, is a character encoding Character encoding is the process of assigning numbers to graphical Graphics (from Greek Greek may refer to: Greece Anything of, ...
and
ISO/IEC 8859 ISO/IEC 8859 is a joint ISO The International Organization for Standardization (ISO ) is an international standard An international standard is a technical standard A technical standard is an established norm (social), norm or requirement for ...
; their character repertoires in almost every case are subsets of the Unicode character set. XML allows the use of any of the Unicode-defined encodings and any other encodings whose characters also appear in Unicode. XML also provides a mechanism whereby an XML processor can reliably, without any prior knowledge, determine which encoding is being used. Encodings other than UTF-8 and UTF-16 are not necessarily recognized by every XML parser.


Escaping

XML provides '' escape'' facilities for including characters that are problematic to include directly. For example: * The characters "<" and "&" are key syntax markers and may never appear in content outside a
CDATA The term CDATA, meaning character data, is used for distinct, but related, purposes in the markup language In computer text processing, a markup language is a system for annotation, annotating a document in a way that is Syntax (logic), syntacti ...
section. It is allowed, but not recommended, to use "<" in XML entity values. * Some character encodings support only a subset of Unicode. For example, it is legal to encode an XML document in ASCII, but ASCII lacks code points for Unicode characters such as "é". * It might not be possible to type the character on the author's machine. * Some characters have
glyphs The term glyph is used in typography Typography is the art and technique of arranging type to make written language A written language is the representation of a spoken or gestural language A language is a structured system o ...
that cannot be visually distinguished from other characters, such as the
non-breaking space In word processing In linguistics Linguistics is the science, scientific study of language. It encompasses the analysis of every aspect of language, as well as the methods for studying and modeling them. The traditional areas of lin ...
(&#xa0;) " " and the
space Space is the boundless three-dimensional Three-dimensional space (also: 3-space or, rarely, tri-dimensional space) is a geometric setting in which three values (called parameter A parameter (from the Ancient Greek language, Ancient Gre ...

space
(&#x20;) " ", and the
Cyrillic capital letter A
Cyrillic capital letter A
(&#x410;) "А" and the
Latin capital letter A
Latin capital letter A
(&#x41;) "A". There are five predefined entities: * &lt; represents "<"; * &gt; represents ">"; * &amp; represents "&"; * &apos; represents ""; * &quot; represents ''. All permitted Unicode characters may be represented with a ''
numeric character reference A numeric character reference (NCR) is a common markup construct used in SGML The Standard Generalized Markup Language (SGML; ISO The International Organization for Standardization (ISO ) is an international standard An international stand ...
''. Consider the Chinese character "中", whose numeric code in Unicode is hexadecimal 4E2D, or decimal 20,013. A user whose keyboard offers no method for entering this character could still insert it in an XML document encoded either as &#20013; or &#x4e2d;. Similarly, the string "I <3 Jörg" could be encoded for inclusion in an XML document as I &lt;3 J&#xF6;rg. &#0; is not permitted, however, because the
null character The null character (also null terminator) is a control character In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithm of ...
is one of the control characters excluded from XML, even when using a numeric character reference. An alternative encoding mechanism such as
Base64 In programming, Base64 is a group of binary-to-text encoding A binary-to-text encoding is code, encoding of data (computing), data in plain text. More precisely, it is an encoding of binary data in a sequence of character (computing), printable ...
is needed to represent such characters.


Comments

Comments may appear anywhere in a document outside other markup. Comments cannot appear before the XML declaration. Comments begin with <!-- and end with -->. For compatibility with
SGML The Standard Generalized Markup Language (SGML; ISO The International Organization for Standardization (ISO ) is an international standard An international standard is a technical standard A technical standard is an established norm (social) ...

SGML
, the string "--" (double-hyphen) is not allowed inside comments; this means comments cannot be nested. The ampersand has no special significance within comments, so entity and character references are not recognized as such, and there is no way to represent characters outside the character set of the document encoding. An example of a valid comment: <!--no need to escape <code> & such in comments-->


International use

XML 1.0 (Fifth Edition) and XML 1.1 support the direct use of almost any
Unicode Unicode, formally the Unicode Standard, is an information technology Technical standard, standard for the consistent character encoding, encoding, representation, and handling of Character (computing), text expressed in most of the world's wri ...

Unicode
character in element names, attributes, comments, character data, and processing instructions (other than the ones that have special symbolic meaning in XML itself, such as the less-than sign, "<"). The following is a well-formed XML document including
Chinese Chinese can refer to: * Something related to China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the List of countries and dependencies by population, world's most populous country, with a populat ...

Chinese
,
Armenian Armenian may refer to: * Something of, from, or related to Armenia, a country in the South Caucasus region of Eurasia * Armenians, the national people of Armenia, or people of Armenian descent ** Armenian language, the Indo-European language spoken ...

Armenian
and
Cyrillic , bg, кирилица , mk, кирилица , russian: кириллица , sr, ћирилица, uk, кирилиця , fam1 = Egyptian hieroglyphs Egyptian hieroglyphs () were the formal writing system A writing system is ...
characters: <俄语 լեզու="ռուսերեն">данные


Syntactical correctness and error-handling

The XML specification defines an XML document as a
well-formed Well-formed or wellformed indicate syntactic correctness and may refer to: * Well-formedness, quality of linguistic elements that conform to grammar rules * Well-formed formula, a string that is generated by a formal grammar in logic * Well-formed ...
text, meaning that it satisfies a list of syntax rules provided in the specification. Some key points in the fairly lengthy list include: * The document contains only properly encoded legal Unicode characters. * None of the special syntax characters such as < and & appear except when performing their markup-delineation roles. * The start-tag, end-tag, and empty-element tag that delimit elements are correctly nested, with none missing and none overlapping. * Tag names are case-sensitive; the start-tag and end-tag must match exactly. * Tag names cannot contain any of the characters !"#$%&'()*+,/;<=>?@ `~, nor a space character, and cannot begin with "-", ".", or a numeric digit. * A single root element contains all the other elements. The definition of an XML document excludes texts that contain violations of well-formedness rules; they are simply not XML. An XML processor that encounters such a violation is required to report such errors and to cease normal processing. This policy, occasionally referred to as "
draconian Draconian is an adjective meaning "of great severity", that derives from Draco, an Athenian law scribe under whom small offenses had heavy punishments ( Draconian laws). Draconian may also refer to: * Draconian (band), a death/doom metal band fr ...
error handling," stands in notable contrast to the behavior of programs that process
HTML The HyperText Markup Language, or HTML is the standard markup language #REDIRECT Markup language In computer text processing, a markup language is a system for annotation, annotating a document in a way that is Syntax (logic), syntacticall ...

HTML
, which are designed to produce a reasonable result even in the presence of severe markup errors. XML's policy in this area has been criticized as a violation of
Postel's law In computing, the robustness principle is a design guideline for software that states: "be conservative in what you do, be liberal in what you accept from others". It is often reworded as: "be conservative in what you send, be liberal in what you ac ...
("Be conservative in what you send; be liberal in what you accept"). The XML specification defines a valid XML document as a well-formed XML document which also conforms to the rules of a Document Type Definition (DTD).


Schemas and validation

In addition to being well-formed, an XML document may be ''valid''. This means that it contains a reference to a Document Type Definition (DTD), and that its elements and attributes are declared in that DTD and follow the grammatical rules for them that the DTD specifies. XML processors are classified as ''validating'' or ''non-validating'' depending on whether or not they check XML documents for validity. A processor that discovers a validity error must be able to report it, but may continue normal processing. A DTD is an example of a ''
schema The word schema comes from the Greek word ('), which means ''shape'', or more generally, ''plan''. The plural is ('). In English, both ''schemas'' and ''schemata'' are used as plural forms. Schema may refer to: Science and technology * SCHEMA ...
'' or ''grammar''. Since the initial publication of XML 1.0, there has been substantial work in the area of schema languages for XML. Such schema languages typically constrain the set of elements that may be used in a document, which attributes may be applied to them, the order in which they may appear, and the allowable parent/child relationships.


Document type definition

The oldest schema language for XML is the document type definition (DTD), inherited from SGML. DTDs have the following benefits: * DTD support is ubiquitous due to its inclusion in the XML 1.0 standard. * DTDs are terse compared to element-based schema languages and consequently present more information in a single screen. * DTDs allow the declaration of standard public entity sets for publishing characters. * DTDs define a ''document type'' rather than the types used by a namespace, thus grouping all constraints for a document in a single collection. DTDs have the following limitations: * They have no explicit support for newer
features Feature may refer to: Computing * Feature (CAD), could be a hole, pocket, or notch * Feature (computer vision), could be an edge, corner or blob * Feature (software design) is an intentional distinguishing characteristic of a software item ( ...
of XML, most importantly
namespaces In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and softw ...
. * They lack expressiveness. XML DTDs are simpler than SGML DTDs and there are certain structures that cannot be expressed with regular grammars. DTDs only support rudimentary datatypes. * They lack readability. DTD designers typically make heavy use of parameter entities (which behave essentially as textual macros), which make it easier to define complex grammars, but at the expense of clarity. * They use a syntax based on
regular expression A regular expression (shortened as regex or regexp; also referred to as rational expression) is a sequence of Character (computing), characters that specifies a ''search pattern matching, pattern''. Usually such patterns are used by string-se ...
syntax, inherited from SGML, to describe the schema. Typical XML APIs such as SAX do not attempt to offer applications a structured representation of the syntax, so it is less accessible to programmers than an element-based syntax may be. Two peculiar features that distinguish DTDs from other schema types are the syntactic support for embedding a DTD within XML documents and for defining ''entities'', which are arbitrary fragments of text or markup that the XML processor inserts in the DTD itself and in the XML document wherever they are referenced, like character escapes. DTD technology is still used in many applications because of its ubiquity.


Schema

A newer schema language, described by the W3C as the successor of DTDs, is
XML Schema #REDIRECT XML schema#REDIRECT XML schema 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 the basic syntactical constra ...
, often referred to by the
initialism An acronym is a word In linguistics Linguistics is the scientific study of language A language is a structured system of communication used by humans, including speech (spoken language), gestures (Signed language, sign langu ...
for XML Schema instances, XSD (XML Schema Definition). XSDs are far more powerful than DTDs in describing XML languages. They use a rich datatyping system and allow for more detailed constraints on an XML document's logical structure. XSDs also use an XML-based format, which makes it possible to use ordinary XML tools to help process them. xs:schema element that defines a schema:


RELAX NG

RELAX NG In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and softwar ...
(Regular Language for XML Next Generation) was initially specified by
OASIS In geography, an oasis (, plural oases, ) is a fertile land in a desert or semi-desert environment.ISO/IEC 19757 – DSDL). RELAX NG schemas may be written in either an XML based syntax or a more compact non-XML syntax; the two syntaxes are
isomorphic In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). I ...

isomorphic
and James Clark's conversion tool
Trang
can convert between them without loss of information. RELAX NG has a simpler definition and validation framework than XML Schema, making it easier to use and implement. It also has the ability to use
datatype In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of c ...
framework
plug-ins Plug-in, plug in or plugin may refer to: * Plug-in (computing), a software component that adds a specific feature to an existing computer program. ** Audio plug-in, adds audio signal processing features * Plug-in hybrid electric vehicle * Plug-in e ...
; a RELAX NG schema author, for example, can require values in an XML document to conform to definitions in XML Schema Datatypes.


Schematron

Schematron Schematron is a rule-based XML validation, validation language for making assertions about the presence or absence of patterns in XML trees. It is a structural schema language expressed in XML using a small number of elements and XPath. In a typi ...
is a language for making assertions about the presence or absence of patterns in an XML document. It typically uses
XPath XPath (XML Path Language) is a query language for selecting Node (computer science), nodes from an XML document. In addition, XPath may be used to compute values (e.g., String (computer science), strings, numbers, or Boolean datatype, Boolean val ...
expressions. Schematron is now a standard (Part 3: ''Rule-based validation'' of ISO/IEC 19757 – DSDL).


DSDL and other schema languages

DSDL (Document Schema Definition Languages) is a multi-part ISO/IEC standard (ISO/IEC 19757) that brings together a comprehensive set of small schema languages, each targeted at specific problems. DSDL includes
RELAX NG In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and softwar ...
full and compact syntax,
Schematron Schematron is a rule-based XML validation, validation language for making assertions about the presence or absence of patterns in XML trees. It is a structural schema language expressed in XML using a small number of elements and XPath. In a typi ...
assertion language, and languages for defining datatypes, character repertoire constraints, renaming and entity expansion, and namespace-based
routing Routing is the process of selecting a path for traffic in a Network theory, network or between or across multiple networks. Broadly, routing is performed in many types of networks, including circuit switching, circuit-switched networks, such as ...

routing
of document fragments to different validators. DSDL schema languages do not have the vendor support of XML Schemas yet, and are to some extent a grassroots reaction of industrial publishers to the lack of utility of XML Schemas for
publishing Publishing is the activity of making information, literature, music, software and other content available to the public for sale or for free. Traditionally, the term refers to the creation and distribution of printed works, such as book A ...
. Some schema languages not only describe the structure of a particular XML format but also offer limited facilities to influence processing of individual XML files that conform to this format. DTDs and XSDs both have this ability; they can for instance provide the infoset augmentation facility and attribute defaults. RELAX NG and Schematron intentionally do not provide these.


Related specifications

A cluster of specifications closely related to XML have been developed, starting soon after the initial publication of XML 1.0. It is frequently the case that the term "XML" is used to refer to XML together with one or more of these other technologies that have come to be seen as part of the XML core. * XML namespaces enable the same document to contain XML elements and attributes taken from different vocabularies, without any naming collisions occurring. Although XML Namespaces are not part of the XML specification itself, virtually all XML software also supports XML Namespaces. * XML Base defines the xml:base attribute, which may be used to set the base for resolution of relative URI references within the scope of a single XML element. * XML Information Set or XML Infoset is an abstract data model for XML documents in terms of ''information items''. The infoset is commonly used in the specifications of XML languages, for convenience in describing constraints on the XML constructs those languages allow. * XSL (Extensible Stylesheet Language) is a family of languages used to transform and render XML documents, split into three parts: * XSLT (XSL Transformations), an XML language for transforming XML documents into other XML documents or other formats such as HTML, plain text, or XSL-FO. XSLT is very tightly coupled with XPath, which it uses to address components of the input XML document, mainly elements and attributes. * XSL-FO (XSL Formatting Objects), an XML language for rendering XML documents, often used to generate PDFs. *
XPath XPath (XML Path Language) is a query language for selecting Node (computer science), nodes from an XML document. In addition, XPath may be used to compute values (e.g., String (computer science), strings, numbers, or Boolean datatype, Boolean val ...
(XML Path Language), a non-XML language for addressing the components (elements, attributes, and so on) of an XML document. XPath is widely used in other core-XML specifications and in programming libraries for accessing XML-encoded data. * XQuery (XML Query) is an XML query language strongly rooted in XPath and XML Schema. It provides methods to access, manipulate and return XML, and is mainly conceived as a query language for XML databases. * XML Signature defines syntax and processing rules for creating digital signatures on XML content. * XML Encryption defines syntax and processing rules for encryption, encrypting XML content. * XML model (Part 11: ''Schema Association'' of ISO/IEC 19757 – DSDL) defines a means of associating any xml document with any of the schema types mentioned #Schemas and validation, above. Some other specifications conceived as part of the "XML Core" have failed to find wide adoption, including XInclude, XLink, and XPointer.


Programming interfaces

The design goals of XML include, "It shall be easy to write programs which process XML documents." Despite this, the XML specification contains almost no information about how programmers might go about doing such processing. The XML Infoset specification provides a vocabulary to refer to the constructs within an XML document, but does not provide any guidance on how to access this information. A variety of APIs for accessing XML have been developed and used, and some have been standardized. Existing APIs for XML processing tend to fall into these categories: * Stream-oriented APIs accessible from a programming language, for example SAX and StAX. * Tree-traversal APIs accessible from a programming language, for example DOM (XML API), DOM. * XML data binding, which provides an automated translation between an XML document and programming-language objects. * Declarative transformation languages such as XSLT and XQuery. * Syntax extensions to general-purpose programming languages, for example LINQ and Scala (programming language), Scala. Stream-oriented facilities require less memory and, for certain tasks based on a linear traversal of an XML document, are faster and simpler than other alternatives. Tree-traversal and data-binding APIs typically require the use of much more memory, but are often found more convenient for use by programmers; some include declarative retrieval of document components via the use of XPath expressions. XSLT is designed for declarative description of XML document transformations, and has been widely implemented both in server-side packages and Web browsers. XQuery overlaps XSLT in its functionality, but is designed more for searching of large XML databases.


Simple API for XML

Simple API for XML (SAX) is a Lexical analysis, lexical, Event-driven programming, event-driven API in which a document is read serially and its contents are reported as Callback (computer programming), callbacks to various Method (computer science), methods on a Event handler, handler object of the user's design. SAX is fast and efficient to implement, but difficult to use for extracting information at random from the XML, since it tends to burden the application author with keeping track of what part of the document is being processed. It is better suited to situations in which certain types of information are always handled the same way, no matter where they occur in the document.


Pull parsing

Pull parsing treats the document as a series of items read in sequence using the Iterator pattern, iterator design pattern. This allows for writing of recursive descent parsers in which the structure of the code performing the parsing mirrors the structure of the XML being parsed, and intermediate parsed results can be used and accessed as local variables within the functions performing the parsing, or passed down (as function parameters) into lower-level functions, or returned (as function return values) to higher-level functions. Examples of pull parsers include Data::Edit::Xml in Perl (programming language), Perl, StAX in the Java (programming language), Java programming language, XMLPullParser in Smalltalk, XMLReader in PHP, ElementTree.iterparse in Python (programming language), Python, System.Xml.XmlReader in the
.NET Framework The .NET Framework (pronounced as "''dot net"'') is a software framework In computer programming Computer programming is the process of designing and building an executable computer program to accomplish a specific computing result or t ...
, and the DOM traversal API (NodeIterator and TreeWalker). A pull parser creates an iterator that sequentially visits the various elements, attributes, and data in an XML document. Code that uses this iterator can test the current item (to tell, for example, whether it is a start-tag or end-tag, or text), and inspect its attributes (local name, XML namespace, namespace, values of XML attributes, value of text, etc.), and can also move the iterator to the next item. The code can thus extract information from the document as it traverses it. The recursive-descent approach tends to lend itself to keeping data as typed local variables in the code doing the parsing, while SAX, for instance, typically requires a parser to manually maintain intermediate data within a stack of elements that are parent elements of the element being parsed. Pull-parsing code can be more straightforward to understand and maintain than SAX parsing code.


Document Object Model

Document Object Model (DOM) is an API that allows for navigation of the entire document as if it were a tree of Node (computer science), node Object (computer science), objects representing the document's contents. A DOM document can be created by a parser, or can be generated manually by users (with limitations). Data types in DOM nodes are abstract; implementations provide their own programming language-specific language binding, bindings. DOM implementations tend to be memory intensive, as they generally require the entire document to be loaded into memory and constructed as a tree of objects before access is allowed.


Data binding

XML data binding is the binding of XML documents to a hierarchy of custom and strongly typed objects, in contrast to the generic objects created by a DOM parser. This approach simplifies code development, and in many cases allows problems to be identified at compile time rather than run-time. It is suitable for applications where the document structure is known and fixed at the time the application is written. Example data binding systems include the Java Architecture for XML Binding (JAXB), XML Serialization in
.NET Framework The .NET Framework (pronounced as "''dot net"'') is a software framework In computer programming Computer programming is the process of designing and building an executable computer program to accomplish a specific computing result or t ...
. and XML serialization in gSOAP.


XML as data type

XML has appeared as a first-class data type in other languages. The E4X, ECMAScript for XML (E4X) extension to the ECMAScript/JavaScript language explicitly defines two specific objects (XML and XMLList) for JavaScript, which support XML document nodes and XML node lists as distinct objects and use a dot-notation specifying parent-child relationships. E4X is supported by the Mozilla 2.5+ browsers (though now deprecated) and Adobe Actionscript, but has not been adopted more universally. Similar notations are used in Microsoft's LINQ implementation for Microsoft .NET 3.5 and above, and in Scala (programming language), Scala (which uses the Java VM). The open-source xmlsh application, which provides a Linux-like shell with special features for XML manipulation, similarly treats XML as a data type, using the <[ ]> notation. The Resource Description Framework defines a data type rdf:XMLLiteral to hold wrapped, canonical XML. Facebook has produced extensions to the PHP and JavaScript languages that add XML to the core syntax in a similar fashion to E4X, namely XHP and React (JavaScript library)#JSX, JSX respectively.


History

XML is an application Profile (engineering), profile of SGML (ISO 8879). The versatility of SGML for dynamic information display was understood by early digital media publishers in the late 1980s prior to the rise of the Internet. By the mid-1990s some practitioners of SGML had gained experience with the then-new World Wide Web, and believed that SGML offered solutions to some of the problems the Web was likely to face as it grew. Dan Connolly (computer scientist), Dan Connolly added SGML to the list of W3C's activities when he joined the staff in 1995; work began in mid-1996 when Sun Microsystems engineer Jon Bosak developed a charter and recruited collaborators. Bosak was well connected in the small community of people who had experience both in SGML and the Web. XML was compiled by a working group of eleven members, supported by a (roughly) 150-member Interest Group. Technical debate took place on the Interest Group mailing list and issues were resolved by consensus or, when that failed, majority vote of the Working Group. A record of design decisions and their rationales was compiled by Michael Sperberg-McQueen on December 4, 1997. James Clark (XML expert), James Clark served as Technical Lead of the Working Group, notably contributing the empty-element <empty /> syntax and the name "XML". Other names that had been put forward for consideration included "MAGMA" (Minimal Architecture for Generalized Markup Applications), "SLIM" (Structured Language for Internet Markup) and "MGML" (Minimal Generalized Markup Language). The co-editors of the specification were originally Tim Bray and Michael Sperberg-McQueen. Halfway through the project Bray accepted a consulting engagement with Netscape Communications Corporation, Netscape, provoking vociferous protests from Microsoft. Bray was temporarily asked to resign the editorship. This led to intense dispute in the Working Group, eventually solved by the appointment of Microsoft's Jean Paoli as a third co-editor. The XML Working Group never met face-to-face; the design was accomplished using a combination of email and weekly teleconferences. The major design decisions were reached in a short burst of intense work between August and November 1996, when the first Working Draft of an XML specification was published. Further design work continued through 1997, and XML 1.0 became a W3C Recommendation on February 10, 1998.


Sources

XML is a profile of an ISO standard SGML, and most of XML comes from SGML unchanged. From SGML comes the separation of logical and physical structures (elements and entities), the availability of grammar-based validation (DTDs), the separation of data and metadata (elements and attributes), mixed content, the separation of processing from representation (processing instructions), and the default angle-bracket syntax. The SGML declaration was removed; thus XML has a fixed delimiter set and adopts
Unicode Unicode, formally the Unicode Standard, is an information technology Technical standard, standard for the consistent character encoding, encoding, representation, and handling of Character (computing), text expressed in most of the world's wri ...

Unicode
as the document Character encoding, character set. Other sources of technology for XML were the Text Encoding Initiative, TEI (Text Encoding Initiative), which defined a profile of SGML for use as a "transfer syntax"; and
HTML The HyperText Markup Language, or HTML is the standard markup language #REDIRECT Markup language In computer text processing, a markup language is a system for annotation, annotating a document in a way that is Syntax (logic), syntacticall ...

HTML
, in which elements were synchronous with their resource, document character sets were separate from resource encoding, the xml:lang attribute was invented, and (like HTTP) metadata accompanied the resource rather than being needed at the declaration of a link. The ERCS(Extended Reference Concrete Syntax) project of the SPREAD (Standardization Project Regarding East Asian Documents) project of the ISO-related China/Japan/Korea Document Processing expert group was the basis of XML 1.0's naming rules; SPREAD also introduced hexadecimal numeric character references and the concept of references to make available all Unicode characters. To support ERCS, XML and HTML better, the SGML standard IS 8879 was revised in 1996 and 1998 with WebSGML Adaptations. The XML header followed that of ISO HyTime. Ideas that developed during discussion that are novel in XML included the algorithm for encoding detection and the encoding header, the processing instruction target, the xml:space attribute, and the new close delimiter for empty-element tags. The notion of well-formedness as opposed to validity (which enables parsing without a schema) was first formalized in XML, although it had been implemented successfully in the Electronic Book Technology "Dynatext" software; the software from the University of Waterloo New Oxford English Dictionary Project; the RISP LISP SGML text processor at Uniscope, Tokyo; the US Army Missile Command IADS hypertext system; Mentor Graphics Context; Interleaf and Xerox Publishing System.


Versions

There are two current versions of XML:


XML 1.0

The first (''XML 1.0'') was initially defined in 1998. It has undergone minor revisions since then, without being given a new version number, and is currently in its fifth edition, as published on November 26, 2008. It is widely implemented and still recommended for general use.


XML 1.1

The second (''XML 1.1'') was initially published on February 4, 2004, the same day as XML 1.0 Third Edition, and is currently in its second edition, as published on August 16, 2006. It contains features (some contentious) that are intended to make XML easier to use in certain cases. The main changes are to enable the use of line-ending characters used on EBCDIC platforms, and the use of scripts and characters absent from Unicode 3.2. XML 1.1 is not very widely implemented and is recommended for use only by those who need its particular features.


Valid Unicode characters in XML 1.0 and XML 1.1

Prior to its fifth edition release, XML 1.0 differed from XML 1.1 in having stricter requirements for characters available for use in element and attribute names and unique identifiers: in the first four editions of XML 1.0 the characters were exclusively enumerated using a specific version of the
Unicode Unicode, formally the Unicode Standard, is an information technology Technical standard, standard for the consistent character encoding, encoding, representation, and handling of Character (computing), text expressed in most of the world's wri ...

Unicode
standard (Unicode 2.0 to Unicode 3.2.) The fifth edition substitutes the mechanism of XML 1.1, which is more future-proof but reduces Redundancy (information theory), redundancy. The approach taken in the fifth edition of XML 1.0 and in all editions of XML 1.1 is that only certain characters are forbidden in names, and everything else is allowed to accommodate suitable name characters in future Unicode versions. In the fifth edition, XML names may contain characters in the Balinese script, Balinese, Cham script, Cham, or Phoenician alphabet, Phoenician scripts among many others added to Unicode since Unicode 3.2. Almost any Unicode code point can be used in the character data and attribute values of an XML 1.0 or 1.1 document, even if the character corresponding to the code point is not defined in the current version of Unicode. In character data and attribute values, XML 1.1 allows the use of more control characters than XML 1.0, but, for "robustness", most of the control characters introduced in XML 1.1 must be expressed as numeric character references (and #x7F through #x9F, which had been allowed in XML 1.0, are in XML 1.1 even required to be expressed as numeric character references). Among the supported control characters in XML 1.1 are two line break codes that must be treated as whitespace. Whitespace characters are the only control codes that can be written directly.


XML 2.0

There has been discussion of an XML 2.0, although no organization has announced plans for work on such a project. XML-SW (SW for Skunkworks project, skunkworks), written by one of the original developers of XML, contains some proposals for what an XML 2.0 might look like: elimination of DTDs from syntax, integration of XML namespace, namespaces, XML Base and XML Information Set into the base standard.


Binary XML

The World Wide Web Consortium also has an XML Binary Characterization Working Group doing preliminary research into use cases and properties for a binary encoding of XML Information Set. The working group is not chartered to produce any official standards. Since XML is by definition text-based, ITU-T and ISO are using the name Fast Infoset for their own binary infoset to avoid confusion (see ITU-T Rec. X.891 and ISO/IEC 24824-1).


Criticism

XML and its extensions have regularly been criticized for verbosity, complexity and redundancy. Mapping the basic tree model of XML to type systems of programming languages or databases can be difficult, especially when XML is used for exchanging highly structured data between applications, which was not its primary design goal. However, XML data binding systems allow applications to access XML data directly from objects representing a
data structure In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of ...

data structure
of the data in the programming language used, which ensures type safety, rather than using the Document Object Model, DOM or SAX to retrieve data from a direct representation of the XML itself. This is accomplished by automatically creating a mapping between elements of the XML schema XSD of the document and members of a class to be represented in memory. Other criticisms attempt to refute the claim that XML is a self-describing language (though the XML specification itself makes no such claim). JSON, YAML, and S-expression, S-Expressions are frequently proposed as simpler alternatives (see Comparison of data serialization formats) that focus on representing highly structured data rather than documents, which may contain both highly structured and relatively unstructured content. However, W3C standardized XML schema specifications offer a broader range of structured XSD data types compared to simpler serialization formats and offer modularity and reuse through XML namespaces.


See also

* List of XML markup languages * List of XML schemas * Extensible programming * Comparison of layout engines (XML) * Comparison of data serialization formats * Binary XML * EBML * WBXML * XML Protocol


Notes


References


Further reading

* Annex A of ISO 8879:1986 (SGML) * * * * * *


External links


W3C XML homepage

XML 1.0 Specification


by Rick Jelliffe
''XML, Java and the Future of the Web''
(1997) by Jon Bosak
http://validator.w3.org/
The Official [W3C] Markup Validation Service
The XML FAQ
originally for the W3C's XML SIG by Peter Flynn {{Authority control XML, Application layer protocols Articles with example code Bibliography file formats Computer file formats Data modeling languages Data serialization formats Markup languages Open formats Presentation layer protocols Technical communication World Wide Web Consortium standards