XML

Extensible Markup Language (XML) is a

The main purpose of XML is serialization, i.e. storing, transmitting, and reconstructing arbitrary data. For two disparate systems to exchange information, they need to agree upon a file format. XML standardizes this process. It is therefore analogous to a lingua franca for representing information.^[9]

As a markup language, XML labels, categorizes, and structurally organizes information.^[10] XML tags represent the data structure and contain metadata. What is within the tags is data, encoded in the way the XML standard specifies.^[10] An additional XML schema (XSD) defines the necessary metadata for interpreting and validating XML. (This is also referred to as the canonical schema.)^[11] An XML document that adheres to basic XML rules is "well-formed"; one that adheres to its schema is "valid".^[11]

IETF RFC 7303 (which supersedes the older RFC 3023), provides rules for the construction of media types for use in XML message. It defines three media types: application/xml (text/xml is an alias), application/xml-external-parsed-entity (text/xml-external-parsed-entity is an alias) and application/xml-dtd. They are used for transmitting raw XML files without exposing their internal semantics. RFC 7303 further recommends that XML-based languages be given media types ending in +xml, for example, image/svg+xml for SVG.

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.^[8]

XML has come into common use for the interchange of data over the Internet. Hundreds of document formats using XML syntax have been developed,^[12] including RSS, Atom, Office Open XML, OpenDocument, SVG, COLLADA, and XHTML. XML also provides the base language for communication protocols such as SOAP and XMPP. It is one of the message exchange formats used in the Asynchronous JavaScript and XML (AJAX) programming technique.

Many industry data standards, such as

One of the applications of XML in science is the representation of operational meteorology information based on IWXXM standards.^[13]

The material in this section is based on the XML

Character

An XML document is a string of characters. Every legal Unicode character (except Null) may appear in an (1.1) XML document (while some are discouraged).

Processor and application

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.

Markup and content

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 section, the delimiters <![CDATA[ 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.

Tag

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 />.

Element

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 />.

Attribute

An attribute is a markup construct consisting of a 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^[i] 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,^[ii] a space-delimited list can be used. An example with space as a delimiter is <div class="inner greeting-box">Welcome!</div>, where the attribute "class" both has the value "inner greeting-box" and also indicates the two CSS class names "inner" and "greeting-box".

XML declaration

XML documents may begin with an XML declaration that describes some information about themselves. An example is <?xml version="1.0" encoding="UTF-8"?>.

XML documents consist entirely of characters from the

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.

Unicode code points in the following ranges are valid in XML 1.0 documents:^[14]

• 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 noncharacters 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 noncharacters.

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.^[15] 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  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

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 (which the XML standard recommends using, without a BOM) and UTF-16.^[16] There are many other text encodings that predate Unicode, such as ASCII and various ISO/IEC 8859; their character repertoires are in every case 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.^[17] Encodings other than UTF-8 and UTF-16 are not necessarily recognized by every XML parser (and in some cases not even UTF-16, even though the standard mandates it to also be recognized).

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 section. It is allowed, but not recommended, to use "<" in XML entity values.^[18]
• 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
  Cyrillic capital letter A (А) "А" and the Latin capital letter A
  (A) "A".

There are five predefined entities:

• < represents "<";
• > represents ">";
• & represents "&";
• ' represents "'";
• " represents '"'.

All permitted Unicode characters may be represented with a numeric character reference. 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 中 or 中. Similarly, the string "I <3 Jörg" could be encoded for inclusion in an XML document as I <3 Jörg.

� is not permitted because the null character is one of the control characters excluded from XML, even when using a numeric character reference.^[19] An alternative encoding mechanism such as Base64 is needed to represent such characters.

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

This example contains Armenian text. Without proper rendering support, you may see question marks, boxes, or other symbols instead of Armenian letters.

XML 1.0 (Fifth Edition) and XML 1.1 support the direct use of almost any

<?xml version="1.0" encoding="UTF-8"?>
<俄语 լեզու="ռուսերեն">данные</俄语>

The XML specification defines an XML document as a well-formed text, meaning that it satisfies a list of syntax rules provided in the specification. Some key points 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.

XML processors are classified as validating or non-validating depending on whether or not they check XML documents for validity.[26] 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 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.

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
  namespaces
  .
• 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 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.

A newer schema language, described by the W3C as the successor of DTDs, is

<?xml version="1.0" encoding="UTF-8" ?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"></xs:schema>

• 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 (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.
• 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 above
  .

Some other specifications conceived as part of the "XML Core" have failed to find wide adoption, including XInclude, XLink, and XPointer.

The design goals of XML include, "It shall be easy to write programs which process XML documents."

• 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 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
  .

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.

Pull parsing treats the document as a series of items read in sequence using the 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.^[27] Examples of pull parsers include Data::Edit::Xml in Perl, StAX in the Java programming language, XMLPullParser in Smalltalk, XMLReader in PHP, ElementTree.iterparse in Python, SmartXML in Red, System.Xml.XmlReader in the .NET Framework, 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, 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.

The Document Object Model (DOM) is an interface that allows for navigation of the entire document as if it were a tree of node 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 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.

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.[22]^[33] 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 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.^[34]

XML was compiled by a

Other sources of technology for XML were the TEI (Text Encoding Initiative), which defined a profile of SGML for use as a "transfer syntax" and HTML. 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.

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;^[41] 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.

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.

The second (XML 1.1) was initially published on February 4, 2004, the same day as XML 1.0 Third Edition,^[42] 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.^[43] 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.^[44]

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 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. 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, Cham, or Phoenician scripts among many others added to Unicode since Unicode 3.2.^[43]

Almost any Unicode code point can be used in the character data and attribute values of an XML 1.0/1.1 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^[43]). Among the supported control characters in XML 1.1 are two line break codes that must be treated as whitespace characters, which are the only control codes that can be written directly.

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), which one of the original developers of XML has written,^[45] contains some proposals for what an XML 2.0 might look like, including elimination of DTDs from syntax, as well as integration of XML namespaces, XML Base and XML Information Set into the base standard.

In 2012, James Clark (technical lead of the XML Working Group) and John Cowan (editor of the XML 1.1 specification) formed the MicroXML Community Group within the W3C and published MicroXML, a specification for a significantly reduced subset of XML.^[46] MicroXML provides a much simpler core syntax by stripping away many features of full XML, such as document type declarations and CDATA sections,^[21] while ensuring XML namespace validity by disallowing names conflicting with namespace prefixing.

Due to the verbosity of textual XML, various binary formats have been proposed as compact representations for XML:

XML and its extensions have regularly been criticized for verbosity, complexity and redundancy.[47]

Mapping the basic tree model of XML to

Other criticisms attempt to refute the claim that XML is a

• AIDX
• Binary XML
• Comparison of data-serialization formats
• EBML
• Extensible programming
• List of XML markup languages
• List of types of XML schemas
• Simple XML
• WBXML
• XML Protocol

Bibliography

• Bray, T.; Paoli, J.; Sperberg-McQueen, C. M.; Maler, E.; Yergeau, F, eds. (26 November 2008). "Extensible Markup Language (XML) 1.0" (W3C Recommendation) (5th ed.). W3C.
• Bray, T.; Paoli, J.; Sperberg-McQueen, C. M.; Maler, E.; Yergeau, F; Cowan, J., eds. (16 August 2006). "Extensible Markup Language (XML) 1.1" (W3C Recommendation) (2nd ed.). W3C.
• Dykes, Lucinda (2005). XML for Dummies (4th ed.). Hoboken, N.J.: Wiley.
  ISBN 978-0-7645-8845-7
  .
• Harold, E. R.; Means, W. S. (2002). XML in a Nutshell (2nd ed.). Sebastopol, CA: O'Reilly.
  ISBN 978-0-5960-0292-3
  .

Further reading

• Annex A of ISO 8879:1986 (SGML)
• Cunningham, L. A. (2005). "Language, Deals and Standards: The Future of XML Contracts". Washington University Law Review. 84 (2): 313–373.
  SSRN 900616
  .
• Bosak, Jon; Bray, Tim (May 1999). "XML and the Second-Generation Web". Scientific American. 280 (5): 89.
  doi:10.1038/scientificamerican0599-89 (inactive 12 July 2025). Archived from the original on 1 October 2009.{{cite journal}}: CS1 maint: DOI inactive as of July 2025 (link
  )
• Kelly, Sean (February 6, 2006). "Making Mistakes with XML". Developer.com. Archived from the original on 13 April 2021.
• St. Laurent, Simon (February 12, 2003). "Five Years Later, XML." O'Reilly XML Blog. O'Reilly Media. Retrieved 26 October 2010.
• "W3C XML is Ten!". World Wide Web Consortium. 12 February 2008. Retrieved 26 October 2010.
• Geneves, Pierre (October 2012). "Introduction to XML" (PDF). Course Slides. Project WAM. Archived (PDF) from the original on 2015-10-16.

External links

Wikimedia Commons has media related to XML.

Wikibooks has a book on the topic of: Subject:XML

• Official website, World Wide Web Consortium (W3C)
• XML 1.0 Specification
• Retrospective on Extended Reference Concrete Syntax Archived 2019-11-18 at the Wayback Machine by Rick Jelliffe
• XML, Java and the Future of the Web (1997) by Jon Bosak
• The Official (W3C) Markup Validation Service
• The XML FAQ originally for the W3C's XML SIG by Peter Flynn