XML

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The Extensible Markup Language (XML) is a general-purpose specification for creating custom markup languages.^[1] It is classified as an extensible language because it allows its users to define their own elements. Its primary purpose is to help information systems share structured data, particularly via the Internet,^[2] and it is used both to encode documents and to serialize data. In the latter context, it is comparable with other text-based serialization languages such as JSON and YAML.^[3]

It started as a simplified subset of the Standard Generalized Markup Language (SGML), and is designed to be relatively human-legible. By adding semantic constraints, application languages can be implemented in XML. These include XHTML,^[4] RSS, MathML, GraphML, Scalable Vector Graphics, MusicXML, and thousands of others. Moreover, XML is sometimes used as the specification language for such application languages.

XML is recommended by the World Wide Web Consortium (W3C). It is a fee-free open standard. The recommendation specifies both the lexical grammar and the requirements for parsing.

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There are two levels of correctness of an XML document:

• Well-formed. A well-formed document conforms to all of XML's syntax rules. For example, if a start-tag appears without a corresponding end-tag, it is not well-formed. A document that is not well-formed is not considered to be XML; a conforming parser is not allowed to process it.
• Valid. A valid document additionally conforms to some semantic rules. These rules are either user-defined, or included as an XML schema, especially DTD. For example, if a document contains an undefined element, then it is not valid; a validating parser is not allowed to process it.

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As long as only well-formedness 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 (alternatively called the document element). This means that the text must be enclosed between a root start-tag and a corresponding end-tag. The following is a "well-formed" XML document:

<book>This is a book.... </book>

The root element can be preceded by an optional XML declaration. This element states what version of XML is in use (normally 1.0); it may also contain information about character encoding and external dependencies.

<?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, such as those based on ISO/IEC 8859, is acknowledged and is 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 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>

The two instances of »name« are referred to as the start-tag and end-tag, respectively. Here, »content« is some text which may again contain XML elements. So, a generic XML document contains a tree-based data structure. Here is an example of a structured XML document:

 <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 must appear only once in any element.

XML requires that elements be properly nested — elements may never overlap, and so must be closed in the opposite order 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>

One way of writing the same information in a way which could be incorporated into a well-formed XML document is as follows:

<!-- 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/>

An empty-element may contain attributes:

<info author="John Smith" genre="science-fiction" date="2009-Jan-01" />