|Extensible Markup Language|
|Mime:||application/xml, text/xml (deprecated)|
|Owner:||World Wide Web Consortium|
|Extended To:||XHTML, RSS, Atom, ...|
|Standard:||1.0 (Fifth Edition) 1.1 (Second Edition)|
The Extensible Markup Language (XML) is a general-purpose specification for creating custom markup languages. It is classified as an extensible language, because it allows the user to define the mark-up elements. XML's purpose is to aid information systems in sharing structured data, especially via the Internet,  to encode documents, and to serialize data; in the last context, it compares with text-based serialization languages such as JSON, YAML and S-Expressions. 
XML's set of tools helps developers in creating web pages but its usefulness goes well beyond that. XML, in combination with other standards, makes it possible to define the content of a document separately from its formatting, making it easy to reuse that content in other applications or for other presentation environments. Most importantly, XML provides a basic syntax that can be used to share information between different kinds of computers, different applications, and different organizations without needing to pass through many layers of conversion.
XML began as a simplified subset of the Standard Generalized Markup Language (SGML), meant to be readable by people via semantic constraints; application languages can be implemented in XML. These include XHTML, RSS, MathML, GraphML, Scalable Vector Graphics, MusicXML, and others. Moreover, XML is sometimes used as the specification language for such application languages.
An XML document has two correctness levels:
If only a well-formed element is required, XML is a generic framework for storing any amount of text or any data whose structure can be represented as a tree. The only indispensable syntactical requirement is that the document has exactly one root element (also known as the document element), i.e. the text must be enclosed between a root start-tag and a corresponding end-tag, known as a "well-formed" XML document:
<book>This is a book... </book>
The root element can be preceded by an optional XML declaration element stating what XML version is in use (normally 1.0); it might also contain character encoding and external dependencies information.
<?xml version="1.0" encoding="UTF-8" ?>
The specification requires that processors of XML support the pan-Unicode character encodings UTF-8 and UTF-16 (UTF-32 is not mandatory). The use of more limited encodings, e.g. those based on ISO/IEC 8859, is acknowledged, widely used, and supported.
Comments can be placed anywhere in the tree, including in the text if the content of the element is text or #PCDATA.
XML comments start with <!-- and end with -->. Two consecutive dashes (--) may not appear anywhere in the text of the comment.
<!-- This is a comment. -->
In any meaningful application, additional markup is used to structure the contents of the XML document. The text enclosed by the root tags may contain an arbitrary number of XML elements.The basic syntax for one element is:
<element_name attribute_name="attribute_value">Element Content</element_name>
<recipe name="bread" prep_time="5 mins" cook_time="3 hours"> <title>Basic bread</title> <ingredient amount="8" unit="dL">Flour</ingredient> <ingredient amount="10" unit="grams">Yeast</ingredient> <ingredient amount="4" unit="dL" state="warm">Water</ingredient> <ingredient amount="1" unit="teaspoon">Salt</ingredient> <instructions> <step>Mix all ingredients together.</step> <step>Knead thoroughly.</step> <step>Cover with a cloth, and leave for one hour in warm room.</step> <step>Knead again.</step> <step>Place in a bread baking tin.</step> <step>Cover with a cloth, and leave for one hour in warm room.</step> <step>Bake in the oven at 180(degrees)C for 30 minutes.</step> </instructions> </recipe>
Attribute values must always be quoted, using single or double quotes, and each attribute name may appear only once in any single element.
XML requires that elements be properly nested—elements may never overlap, and so must be closed in the order opposite to which they are opened. For example, this fragment of code below cannot be part of a well-formed XML document because the title and author elements are closed in the wrong order:
<!-- WRONG! NOT WELL-FORMED XML! --> <title>Book on Logic<author>Aristotle</title></author>
<!-- Correct: well-formed XML fragment. --> <title>Book on Logic</title> <author>Aristotle</author>
XML provides special syntax for representing an element with empty content. Instead of writing a start-tag followed immediately by an end-tag, a document may contain an empty-element tag. An empty-element tag resembles a start-tag but contains a slash just before the closing angle bracket. The following three examples are equivalent in XML:
<foo></foo> <foo /> <foo/>
<info author="John Smith" genre="science-fiction" date="2009-Jan-01" />
An entity in XML is a named body of data, usually text. Entities are often used to represent single characters that cannot easily be entered on the keyboard; they are also used to represent pieces of standard ("boilerplate") text that occur in many documents, especially if there is a need to allow such text to be changed in one place only.
Special characters can be represented either using entity references, or by means of numeric character references. An example of a numeric character reference is "
&#x20AC;", which refers to the Euro symbol by means of its Unicode codepoint in hexadecimal.
An entity reference is a placeholder that represents that entity. It consists of the entity's name preceded by an ampersand ("
&") and followed by a semicolon ("
;"). XML has five predeclared entities:
&amp;(& or "ampersand")
&lt;(< or "less than")
&gt;(> or "greater than")
&apos;(' or "apostrophe")
&quot;(" or "quotation mark")
Here is an example using a predeclared XML entity to represent the ampersand in the name "AT&T":
<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE example [ <!ENTITY copy "©"> <!ENTITY copyright-notice "Copyright © 2009, XYZ Enterprises"> ]> <example> ©right-notice; </example>
When viewed in a suitable browser, the XML document above appears as:
Copyright © 2009, XYZ Enterprises
Numeric character references look like entity references, but instead of a name, they contain the "
[[number sign|#]]" character followed by a number. The number (in decimal or "
x"-prefixed hexadecimal) represents a Unicode code point. Unlike entity references, they are neither predeclared nor do they need to be declared in the document's DTD. They have typically been used to represent characters that are not easily encodable, such as an Arabic character in a document produced on a European computer. The ampersand in the "AT&T" example could also be escaped like this (decimal 38 and hexadecimal 26 both represent the Unicode code point for the "&" character):
Similarly, in the previous example, notice that "©" is used to generate the “©” symbol.
See also numeric character references.
In XML, a well-formed document must conform to the following rules, among others:
<IAmEmpty />. This is equal to
&apos;. You may also use the numeric character entity references (hex)
&#x0027;or their equivalent decimal notations
Element names are case-sensitive. For example, the following is a well-formed matching pair:
</Step>whereas these are not
By carefully choosing the names of the XML elements one may convey the meaning of the data in the markup. This increases human readability while retaining the rigor needed for software parsing.
Choosing meaningful names implies the semantics of elements and attributes to a human reader without reference to external documentation. However, this can lead to verbosity, which complicates authoring and increases file size.
It is relatively simple to verify that a document is well-formed or validated XML, because the rules of well-formedness and validation of XML are designed for portability of tools. The idea is that any tool designed to work with XML files will be able to work with XML files written in any XML language (or XML application). Here are some examples of ways to verify XML documents:
irb> require "rexml/document" irb> include REXML irb> doc = Document.new(File.new("test.xml")).root
By leaving the names, allowable hierarchy, and meanings of the elements and attributes open and definable by a customizable schema or DTD, XML provides a syntactic foundation for the creation of purpose-specific, XML-based markup languages. The general syntax of such languages is rigid - documents must adhere to the general rules of XML, ensuring that all XML-aware software can at least read and understand the relative arrangement of information within them. The schema merely supplements the syntax rules with a set of constraints. Schemas typically restrict element and attribute names and their allowable containment hierarchies, such as only allowing an element named 'birthday' to contain one element named 'month' and one element named 'day', each of which has to contain only character data. The constraints in a schema may also include data type assignments that affect how information is processed; for example, the 'month' element's character data may be defined as being a month according to a particular schema language's conventions, perhaps meaning that it must not only be formatted a certain way, but also must not be processed as if it were some other type of data.
An XML document that complies with a particular schema/DTD, in addition to being well-formed, is said to be valid.
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 constraints imposed by XML itself. A number of standard and proprietary XML schema languages have emerged for the purpose of formally expressing such schemas, and some of these languages are XML-based, themselves.
Before the advent of generalised data description languages such as SGML and XML, software designers had to define special file formats or small languages to share data between programs. This required writing detailed specifications and special-purpose parsers and writers.
XML's regular structure and strict parsing rules allow software designers to leave parsing to standard tools, and since XML provides a general, data model-oriented framework for the development of application-specific languages, software designers need only concentrate on the development of rules for their data, at relatively high levels of abstraction.
Well-tested tools exist to validate an XML document "against" a schema: the tool automatically verifies whether the document conforms to constraints expressed in the schema. Some of these validation tools are included in XML parsers, and some are packaged separately.
Other usages of schemas exist: XML editors, for instance, can use schemas to support the editing process (by suggesting valid elements and attributes names, etc).
See main article: Document Type Definition. The oldest schema format for XML is the Document Type Definition (DTD), inherited from SGML. While DTD support is ubiquitous due to its inclusion in the XML 1.0 standard, it is seen as limited for the following reasons:
DTD is still used in many applications because it is considered the easiest to read and write.
See main article: XML Schema (W3C). A newer XML schema language, described by the W3C as the successor of DTDs, is XML Schema, or more informally referred to by the initialism 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, allow for more detailed constraints on an XML document's logical structure, and must be processed in a more robust validation framework. XSDs also use an XML-based format, which makes it possible to use ordinary XML tools to help process them, although XSD implementations require much more than just the ability to read XML.
See main article: RELAX NG. Another popular schema language for XML is RELAX NG. Initially specified by OASIS, RELAX NG is now also an ISO international standard (as part of DSDL). It has two formats: an XML based syntax and a non-XML compact syntax. The compact syntax aims to increase readability and writability but, since there is a well-defined way to translate the compact syntax to the XML syntax and back again by means of James Clark's Trang conversion tool, the advantage of using standard XML tools is not lost. 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 framework plug-ins; a RELAX NG schema author, for example, can require values in an XML document to conform to definitions in XML Schema Datatypes.
The ISO DSDL (Document Schema Description Languages) standard brings together a comprehensive set of small schema languages, each targeted at specific problems. DSDL includes RELAX NG full and compact syntax, Schematron assertion language, and languages for defining datatypes, character repertoire constraints, renaming and entity expansion, and namespace-based 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.
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 attribute defaults. RELAX NG and Schematron intentionally do not provide these; for example the infoset augmentation facility.
XML supports the direct use of almost any 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 open corner bracket, "<"). Therefore, the following is a well-formed XML document, even though it includes both Chinese and Cyrillic characters:
<?xml version="1.0" encoding="UTF-8"?> <俄語>Китайский</俄語>
Generally, generic XML documents do not carry information about how to display the data. Without using CSS or XSLT, a generic XML document is rendered as raw XML text by most web browsers. Some display it with 'handles' (e.g. + and - signs in the margin) that allow parts of the structure to be expanded or collapsed with mouse-clicks.
<?xml-stylesheet type="text/css" href="myStyleSheet.css"?>
Note that this is different from specifying such a stylesheet in HTML, which uses the
XSLT (XSL Transformations) can be used to alter the format of XML data, either into HTML or other formats that are suitable for a browser to display.
<?xml-stylesheet type="text/xml" href="myTransform.xslt"?>
Client-side XSLT is supported by many web browsers. Alternatively, one may use XSLT to convert XML into a displayable format on the server rather than being dependent on the end-user's browser capabilities. The end-user is not aware of what has gone on 'behind the scenes'; all they see is well-formatted, displayable data.
See the XSLT article for examples of XSLT in action.
XML files may be served with a variety of Media types. RFC 3023 defines the types "application/xml" and "text/xml", which say only that the data is in XML, and nothing about its semantics. The use of "text/xml" has been criticized as a potential source of encoding problems but is now in the process of being deprecated. RFC 3023 also recommends that XML-based languages be given media types beginning in "application/" and ending in "+xml"; for example "application/atom+xml" for Atom. This page discusses further XML and MIME.
Three traditional techniques for processing XML files are:
More recent and emerging techniques for processing XML files are:
SAX is a lexical, event-driven interface in which a document is read serially and its contents are reported as "callbacks" to various methods on a 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.
DOM (Document Object Model) is an interface-oriented Application Programming 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.
Pull parsing treats the document as a series of items which are 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 methods performing the parsing, or passed down (as method parameters) into lower-level methods, or returned (as method return values) to higher-level methods. Examples of pull parsers include StAX in the Java programming language, SimpleXML in PHP and System.Xml.XmlReader in .NET.
A pull parser creates an iterator that sequentially visits the various elements, attributes, and data in an XML document. Code which uses this 'iterator' can test the current item (to tell, for example, whether it is a start or end element, 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 which are parent elements of the element being parsed. Pull-parsing code can be more straightforward to understand and maintain than SAX parsing code.
Non-extractive XML Processing API is a new and emerging category of parsers that aim to overcome the fundamental limitations of DOM and SAX. The most representative is VTD-XML, which abolishes the object-oriented modeling of XML hierarchy and instead uses 64-bit Virtual Token Descriptors (encoding offsets, lengths, depths, and types) of XML tokens. VTD-XML's approach enables a number of interesting features/enhancements, such as high performance, low memory usage  , ASIC implementation  , incremental update  , and native XML indexing   .
Another form of XML Processing API is data binding, where XML data is made available as a hierarchy of custom, strongly typed classes, in contrast to the generic objects created by a Document Object Model parser. This approach simplifies code development, and in many cases allows problems to be identified at compile time rather than run-time. Example data binding systems include the Java Architecture for XML Binding (JAXB) , Liquid XML Data Binder for C++, Java, VB & .Net  and CodeSynthesis XSD for C++  .
The native file format of OpenOffice.org, AbiWord, and Apple's iWork applications is XML. Some parts of Microsoft Office 2007 are also able to edit XML files with a user-supplied schema (but not a DTD), and Microsoft has released a file format compatibility kit for Office 2003 that allows previous versions of Office to save in the new XML based format. There are dozens of other XML editors available.
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 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 an (approximately) 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 served as Technical Lead of the Working Group, notably contributing the empty-element "
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 twenty weeks of intense work between July 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.
XML 1.0 achieved the Working Group's goals of Internet usability, general-purpose usability, SGML compatibility, facilitation of easy development of processing software, minimization of optional features, legibility, formality, conciseness, and ease of authoring.Like its antecedent SGML, XML allows for some redundant syntactic constructs and includes repetition of element identifiers. In these respects, terseness was not considered essential in its structure.
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. Removed were the SGML Declaration (XML has a fixed delimiter set and adopts Unicode as the document character set).
Other sources of technology for XML were the Text Encoding Initiative (TEI), which defined a profile of SGML for use as a 'transfer syntax'; HTML, in which elements were synchronous with their resource, the separation of document character set from resource encoding, the xml:lang attribute, and the HTTP notion that metadata accompanied the resource rather than being needed at the declaration of a link; and the Extended Reference Concrete Syntax (ERCS), from which XML 1.0's naming rules were taken, and which had introduced hexadecimal numeric character references and the concept of references to make available all Unicode characters.
Ideas that developed during discussion which were novel in XML, were 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.
There are two current versions of XML. 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, 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 - mainly enabling the use of line-ending characters used on EBCDIC platforms, and the use of scripts and characters absent from Unicode 2.0. XML 1.1 is not very widely implemented and is recommended for use only by those who need its unique features. 
Prior to the fifth edition of XML 1.0, it and XML 1.1 differed in the requirements of characters used for element and attribute names: the first four editions of XML 1.0 only allowed characters which are defined in Unicode 2.0, which includes most world scripts, but excludes those which were added in later Unicode versions. Among the excluded scripts are Mongolian, Cambodian, Amharic, Burmese, and others.
Almost any Unicode character can be used in the character data and attribute values of an XML 1.1 document, even if the character is not defined, aside from having a code point, in the current version of Unicode. The approach in XML 1.1 is that only certain characters are forbidden, and everything else is allowed, whereas in older editions of XML 1.0, only certain characters were explicitly allowed, and thus prior to its fifth edition XML 1.0 could not accommodate the addition of characters in future versions 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. 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.
There are also discussions on an XML 2.0, although it seems unlikely if such will ever come about. XML-SW (SW for skunk works), 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 namespaces, XML Base and XML Information Set (infoset) into the base standard.
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 the XML infoset. 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 | ISO/IEC 24824-1).
In October 2005 the small company Scientigo publicly asserted that two of its patents, and , apply to the use of XML. The patents cover the "modeling, storage and transfer [of data] in a particular non-hierarchical, non-integrated neutral form", according to their applications, which were filed in 1997 and 1999. Scientigo CEO Doyal Bryant expressed a desire to "monetize" the patents but stated that the company was "not interested in having us against the world." He said that Scientigo was discussing the patents with several large corporations.
XML users and independent experts responded to Scientigo's claims with widespread skepticism and criticism. Some derided the company as a patent troll. Tim Bray described any claims that the patents covered XML as "ridiculous on the face of it".
Commentators have offered various critiques of XML, suggesting circumstances where XML provides both advantages and potential disadvantages.
XBRL (eXtensible Business Reporting Language), which is based on XML, is widely used in the financial world. Due to increased demand for financial transparency, XBRL technology helps facilitate the processes of transferring data and reporting financial results. Notably, the SEC requires some filings in XBRL,  and Japanese banks are said to use XBRL in their daily business.
Web publishing uses XML to have a single source for creating and updating all content, which significantly saves time and reduces costs for companies when printing documents. In addition, some governments use XML for large documentation and printing maps, and airplane and car manufactures may use XML technology to print maintenance booklets.
In addition to the ISO standards mentioned above, other related document include: