XML

XML

  Extensible Markup Language is a markup language and file format for storing, transmitting, and reconstructing arbitrary data. It defines a set of rules for encoding documents in a format that is both human-readable and machine-readable. The World Wide Web Consortium's XML 1.0 Specification of 1998 and several other related specifications—all of them free open standards—define XML.

The design goals of XML emphasize simplicity, generality, and usability across the Internet. It is a textual data format with strong support via Unicode for different human languages. Although the design of XML focuses on documents, the language is widely used for the representation of arbitrary data structures such as those used in web services.

Several schema systems exist to aid in the definition of XML-based languages, while programmers have developed many application programming interfaces to aid the processing of XML data.

Overview
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. XML is analogous to a lingua franca for representing information.
As a markup language, XML labels, categorizes, and structurally organizes information.

including RSS, Atom, Office Open XML, OpenDocument, SVG, and XHTML. XML has also provides the base language for communication protocols such as SOAP and XMPP.

It is the message exchange format for the Asynchronous JavaScript and XML programming technique.
Many industry data standards, such as Health Level 7, OpenTravel Alliance, FpML, MISMO, and National Information Exchange Model are based on XML and the rich features of the XML schema specification. In publishing, Darwin Information Typing Architecture is an XML industry data standard.

XML is used extensively to underpin various publishing formats.
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 every legal 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 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 section, the delimiters <!> 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: : 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 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 repertoire. Except for a small number of specifically excluded control characters, 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, U+000A, U+000D : 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 ;
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, U+000A, U+000D, and U+0085 by requiring them to be written in escaped form. 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 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 and UTF-16. There are many other text encodings that predate Unicode, such as ASCII and ISO/IEC 8859; 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 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 that cannot be visually distinguished from other characters, such as the non-breaking space " " and the space " ", and the Cyrillic capital letter A "А" and the Latin capital letter 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.

An alternative encoding mechanism such as Base64 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 string "--" 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 and XML 1.1 support the direct use of almost any Unicode character in element names, attributes, comments, character data, and processing instructions. The following is a well-formed XML document including Chinese, Armenian and Cyrillic characters:
данные
Syntactical correctness and error-handling
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 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…