XML
In computing, Extensible Markup Language(XML) is a markup language that defines a set of rules for encoding documents in a formatthat is both human-readable and machine-readable. The W3C's XML 1.0 Specification[2]and several other related specifications[3]—all of them free open standards—define XML.[4]
The design goals of XML emphasize simplicity, generality, and usability across theInternet.[5] It is a textual data format with strong support via Unicode for differenthuman languages. Although the design of XML focuses on documents, the language is widely used for the representation of arbitrarydata structures[6] such as those used in web services.
Several schema systems exist to aid in the definition of XML-based languages, while programmers have developed manyapplication programming interfaces (APIs) to aid the processing of XML data.
Applications of XML
The essence of why extensible markup languages are necessary is explained atMarkup language (for example, see Markup language § XML) and at Standard Generalized Markup Language.
Hundreds of document formats using XML syntax have been developed,[7] including RSS,Atom, SOAP, SVG, and XHTML. XML-based formats have become the default for many office-productivity tools, including Microsoft Office (Office Open XML), OpenOffice.org andLibreOffice (OpenDocument), and Apple'siWork[citation needed]. XML has also provided the base language for communication protocolssuch as XMPP. Applications for the Microsoft.NET Framework use XML files for configuration. Apple has an implementation of a registry based on XML.[8]
Most industry data standards, e.g. HL7, OTA, NDC, FpML, MISMO etc. 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, DITA is an XML industry data standard. XML is used extensively to underpin various publishing formats.
XML is widely used in a Services Oriented Architecture (SOA). Disparate systems communicate with each other by exchanging XML messages. The message exchange format is standardised as an XML schema (XSD). This is also referred to as the canonical schema.
XML has come into common use for the interchange of data over the Internet. IETFRFC:3023, now superseded by RFC:7303, gave rules for the construction of Internet Media Types for use when sending XML.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. The use of text/xml has been criticized[i] as a potential source of encoding problems and it has been suggested that it should be deprecated.[9]
RFC 7303 also recommends that XML-based languages be given media types ending in
+xml; for example image/svg+xmlfor 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.
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.
- Character
- An XML document is a string of characters. Almost every legal Unicode character may appear in an XML document.
- Processor and application
- The processor analyzes the markup and passes structured information to anapplication. 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 andcontent, 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>. Tags come in three flavors:- start-tag, such as
<section>; - end-tag, such as
</section>; - empty-element tag, such as
<line-break />.
- start-tag, such as
- 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[ii] 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,[iii] 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 twoCSS 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"?>.
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 theencoding 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:[10]
- 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 (not all) 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[11] 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
 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 includeUTF-8 and UTF-16.[12] There are many other text encodings that predate Unicode, such asASCII 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.[13] 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.[14]
- 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) "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, however, because the null character is one of the control characters excluded from XML, even when using a numeric character reference.[15] An alternative encoding mechanism such asBase64 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 "--" (double-hyphen) is not allowed inside comments;[16] 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 Unicodecharacter 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 includingChinese, Armenian and Cyrillic characters:
<?xml version="1.0" encoding="UTF-8"?>
<俄语 լեզու="ռուսերեն">данные</俄语>
Well-formedness 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 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 error handling," stands in notable contrast to the behavior of programs that process HTML, which are designed to produce a reasonable result even in the presence of severe markup errors.[17] XML's policy in this area has been criticized as a violation of Postel's law ("Be conservative in what you send; be liberal in what you accept").[18]
The XML specification defines a valid XML document as a well-formed XML documentwhich also conforms to the rules of aDocument Type Definition (DTD).[19][20]
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