Meta Index / Home Page / Developer's notes / Parameter specification file XML: escape sequences In text and attribute values, you need to escape ASCII characters like the. Jun 02, 2018 XML escape characters. There are only five:' " ' ' > & & Escaping characters depends on where the special character is used. The examples can be validated at W3C Markup Validation Service. The safe way is to escape all five characters in text, however, the three characters ', ' and needn't be escaped in text.
Filename extension | .xml |
---|---|
Internet media type | |
Uniform Type Identifier (UTI) | public.xml |
UTI conformation | public.text |
Developed by | World Wide Web Consortium |
Type of format | Markup language |
Extended from | SGML |
Extended to |
|
Standard |
|
Open format? | Yes |
Status | Published |
---|---|
Year started | 1996; 23 years ago |
Editors |
|
Related standards | XML Schema |
Domain | Data serialization |
Abbreviation | XML |
Website | XML 1.0 |
Extensible Markup Language (XML) is a markup language that defines a set of rules for encoding documents in a format that 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 the Internet.[5] 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[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 many application programming interfaces (APIs) to aid the processing of XML data.
- 3Characters and escaping
- 5Schemas and validation
- 7Programming interfaces
- 8History
Applications[edit]
The essence of why extensible markup languages are necessary is explained at Markup 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 and LibreOffice (OpenDocument), and Apple's iWork[citation needed]. XML has also provided the base language for communication protocols such as XMPP. Applications for the Microsoft.NET Framework use XML files for configuration, and property lists are an implementation of configuration storage built on XML.[8]
Many industry data standards, e.g. HL7, OTA, FpML, MISMO, NIEM, 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.RFC 7303 also 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.
Key terminology[edit]
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 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.
<
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 />
.
- 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.<div>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 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[edit]
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[edit]
Unicode code points in the following ranges are valid in XML 1.0 documents:[9]
- 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 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 extends the set of allowed characters to include all the above, plus the remaining characters in the range U+0001–U+001F.[10] 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[edit]
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.[11] 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.[12] Encodings other than UTF-8 and UTF-16 are not necessarily recognized by every XML parser.
Escaping[edit]
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.[13]
- 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.[14] An alternative encoding mechanism such as Base64 is needed to represent such characters.Comments[edit]
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;[15] 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[edit]
XML 1.0 (Fifth Edition) and XML 1.1 support 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 less-than sign, '<'). The following is a well-formed XML document including Chinese, Armenian and Cyrillic characters:
Well-formedness and error-handling[edit]
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.[16] 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').[17]
The XML specification defines a valid XML document as a well-formed XML document which also conforms to the rules of a Document Type Definition (DTD).[18][19]
Schemas and validation[edit]
In addition to being well-formed, an XML document may be valid. This means that it contains a reference to a Document Type Definition (DTD), and that its elements and attributes are declared in that DTD and follow the grammatical rules for them that the DTD specifies.
XML processors are classified as validating or non-validating depending on whether or not they check XML documents for validity. 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.
Document type definition[edit]
![Xml escape characters python Xml escape characters python](/uploads/1/2/5/7/125761419/896848794.png)
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 features of XML, most importantly 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 and/or markup that the XML processor inserts in the DTD itself and in the XML document wherever they are referenced, like character escapes.
![Xml escape characters java Xml escape characters java](/uploads/1/2/5/7/125761419/285794354.png)
DTD technology is still used in many applications because of its ubiquity.
Schema[edit]
A newer schema language, described by the W3C as the successor of DTDs, is XML Schema, often 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 and allow for more detailed constraints on an XML document's logical structure. XSDs also use an XML-based format, which makes it possible to use ordinary XML tools to help process them.
xs:schema element that defines a schema:
RELAX NG[edit]
RELAX NG (Regular Language for XML Next Generation) was initially specified by OASIS and is now a standard (Part 2: Regular-grammar-based validation of ISO/IEC 19757 - DSDL). RELAX NG schemas may be written in either an XML based syntax or a more compact non-XML syntax; the two syntaxes are isomorphic and James Clark's conversion tool—Trang—can convert between them without loss of information. 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.
Schematron[edit]
Schematron is a language for making assertions about the presence or absence of patterns in an XML document. It typically uses XPath expressions. Schematron is now a standard (Part 3: Rule-based validation of ISO/IEC 19757 - DSDL).
DSDL and other schema languages[edit]
DSDL (Document Schema Definition Languages) is a multi-part ISO/IEC standard (ISO/IEC 19757) that 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 the infoset augmentation facility and attribute defaults. RELAX NG and Schematron intentionally do not provide these.
Related specifications[edit]
A cluster of specifications closely related to XML have been developed, starting soon after the initial publication of XML 1.0. It is frequently the case that the term 'XML' is used to refer to XML together with one or more of these other technologies that have come to be seen as part of the XML core.
- 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.
- XML Encryption defines syntax and processing rules for 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.
Programming interfaces[edit]
The design goals of XML include, 'It shall be easy to write programs which process XML documents.'[5] Despite this, the XML specification contains almost no information about how programmers might go about doing such processing. The XML Infoset specification provides a vocabulary to refer to the constructs within an XML document, but does not provide any guidance on how to access this information. A variety of APIs for accessing XML have been developed and used, and some have been standardized.
Existing APIs for XML processing tend to fall into these categories:
- 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.
Stream-oriented facilities require less memory and, for certain tasks based on a linear traversal of an XML document, are faster and simpler than other alternatives. Tree-traversal and. Other names that had been put forward for consideration included 'MAGMA' (Minimal Architecture for Generalized Markup Applications), 'SLIM' (Structured Language for Internet Markup) and 'MGML' (Minimal Generalized Markup Language). The co-editors of the specification were originally Tim Bray and Michael Sperberg-McQueen. Halfway through the project Bray accepted a consulting engagement with Netscape, provoking vociferous protests from Microsoft. Bray was temporarily asked to resign the editorship. This led to intense dispute in the Working Group, eventually solved by the appointment of Microsoft's Jean Paoli as a third co-editor.
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 a short burst of intense work between August and November 1996,[31] when the first Working Draft of an XML specification was published.[32] Further design work continued through 1997, and XML 1.0 became a W3C Recommendation on February 10, 1998.
Sources[edit]
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 TEI (Text Encoding Initiative), which defined a profile of SGML for use as a 'transfer syntax'; and HTML, in which elements were synchronous with their resource, document character sets were separate from resource encoding, the
xml:lang
attribute was invented, and (like HTTP) metadata accompanied the resource rather than being needed at the declaration of a link. 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. The XML header followed that of ISO HyTime.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;[33] 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.
Versions[edit]
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,[34] 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.[35] 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.[36]
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.[35]
Almost any Unicode code point can be used in the character data and attribute values of an XML 1.0 or 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[37]). 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 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), written by one of the original developers of XML,[38] 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 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 XML Information Set. 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 and ISO/IEC 24824-1).
Criticism[edit]
XML and its extensions have regularly been criticized for verbosity, complexity and redundancy.[39] Mapping the basic tree model of XML to type systems of programming languages or databases can be difficult, especially when XML is used for exchanging highly structured data between applications, which was not its primary design goal. However, XML data binding systems allow applications to access XML data directly from objects representing a data structure of the data in the programming language used, which ensures type safety, rather than using the DOM or SAX to retrieve data from a direct representation of the XML itself. This is accomplished by automatically creating a mapping between elements of the XML schema XSD of the document and members of a class to be represented in memory. Other criticisms attempt to refute the claim that XML is a self-describing language[40] (though the XML specification itself makes no such claim). JSON, YAML, and S-Expressions are frequently proposed as simpler alternatives (see Comparison of data serialization formats);[41] that focus on representing highly structured data rather than documents, which may contain both highly structured and relatively unstructured content. However, W3C standardized XML schema specifications offer a broader range of structured XSD data types compared to simpler serialization formats and offer modularity and reuse through XML namespace.
See also[edit]
Notes[edit]
- ^i.e., embedded quote characters would be a problem
- ^A common example of this is CSS class or identifier names.
References[edit]
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- ^'Resource Description Framework (RDF): Concepts and Abstract Syntax'. W3.org. Retrieved 22 August 2010.
- ^'ISO/IEC 19757-3'. ISO/IEC. 1 June 2006: vi.
- ^Bray, Tim (February 2005). 'A conversation with Tim Bray: Searching for ways to tame the world's vast stores of information'. Association for Computing Machinery's 'Queue site'. Retrieved 16 April 2006.
- ^edited by Sueann Ambron; Kristina Hooper & foreword by John Sculley. (1988). 'Publishers, multimedia, and interactivity'. Interactive multimedia. Cobb Group. ISBN1-55615-124-1.CS1 maint: Extra text: authors list (link)
- ^Eliot Kimber (2006). 'XML is 10'. Drmacros-xml-rants.blogspot.com. Retrieved 16 November 2017.
- ^The working group was originally called the 'Editorial Review Board.' The original members and seven who were added before the first edition was complete, are listed at the end of the first edition of the XML Recommendation, at http://www.w3.org/TR/1998/REC-xml-19980210.
- ^'Reports From the W3C SGML ERB to the SGML WG And from the W3C XML ERB to the XML SIG'. W3.org. Retrieved 31 July 2009.
- ^'Oracle Technology Network for Java Developers - Oracle Technology Network - Oracle'. Java.sun.com. Retrieved 16 November 2017.
- ^'Extensible Markup Language (XML)'. W3.org. 1996-11-14. Retrieved 31 July 2009.
- ^Jon Bosak; Sun Microsystems (2006-12-07). 'Closing Keynote, XML 2006'. 2006.xmlconference.org. Archived from the original on 2007-07-11. Retrieved 31 July 2009.
- ^'Extensible Markup Language (XML) 1.0 (Third Edition)'. W3.org. Retrieved 22 August 2010.
- ^ ab'Extensible Markup Language (XML) 1.1 (Second Edition) , Rationale and list of changes for XML 1.1'. W3.org. Retrieved 20 January 2012.
- ^Harold, Elliotte Rusty (2004). Effective XML. Addison-Wesley. pp. 10–19. ISBN0-321-15040-6.
- ^'Extensible Markup Language (XML) 1.1 (Second Edition)'. W3.org. Retrieved 22 August 2010.
- ^Tim Bray: Extensible Markup Language, SW (XML-SW). 2002-02-10
- ^'XML: The Angle Bracket Tax'. Codinghorror.com. Retrieved 16 November 2017.
- ^'The Myth of Self-Describing XML'(PDF). Workflow.healthbase.info. September 2003. Retrieved 16 November 2017.
- ^'What usable alternatives to XML syntax do you know?'. Stackoverflow.com. Retrieved 16 November 2017.
Further reading[edit]
- Annex A of ISO 8879:1986 (SGML)
- Lawrence A. Cunningham (2005). 'Language, Deals and Standards: The Future of XML Contracts'. Washington University Law Review. SSRN900616.
- Bosak, Jon; Bray, Tim (May 1999). 'XML and the Second-Generation Web'. Scientific American. Archived from the original on 1 October 2009.
- Kelly, Sean (February 6, 2006). 'Making Mistakes with XML'. Developer.com. Retrieved 26 October 2010.
- 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.
- 'Introduction to XML'(PDF). Course Slides. Pierre Geneves. October 2012. Archived from the original on 2015-10-16.CS1 maint: BOT: original-url status unknown (link)
External links[edit]
Wikimedia Commons has media related to XML. |
Wikibooks has a book on the topic of: Subject:XML |
- Retrospective on Extended Reference Concrete Syntax by Rick Jelliffe
- XML, Java and the Future of the Web (1997) by Jon Bosak
- http://validator.w3.org/ The Official [W3C] Markup Validation Service
- The XML FAQ originally for the W3C's XML SIG by Peter Flynn
- XML Formatter W3C's XML Formatter
Retrieved from 'https://en.wikipedia.org/w/index.php?title=XML&oldid=901173109'
HTML |
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Comparisons |
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HTML (Hypertext Markup Language) has been in use since 1991, but HTML 4.0 (December 1997) was the first standardized version where international characters were given reasonably complete treatment. When an HTML document includes special characters outside the range of seven-bit ASCII, two goals are worth considering: the information's integrity, and universal browser display.
- 2Character references
Specifying the document's character encoding[edit]
There are several ways to specify which character encoding is used in the document. First, the web server can include the character encoding or '
charset
' in the Hypertext Transfer Protocol (HTTP) Content-Type
header, which would typically look like this:[1]This method gives the HTTP server a convenient way to alter document's encoding according to content negotiation; certain HTTP server software can do it, for example Apache with the module
mod_charset_lite
.[2]For HTML it is possible to include this information inside the
head
element near the top of the document:[3]HTML5 also allows the following syntax to mean exactly the same:[3]
XHTML documents have a third option: to express the character encoding via XML declaration, as follows:[4]
Note that as the character encoding cannot be known until this declaration is parsed, there can be a problem knowing which encoding is used for the declaration itself. The main principle is that the declaration shall be encoded in pure ASCII, and therefore (if the declaration is inside the file) the encoding needs to be an ASCII extension. In order to allow encodings not backwards compatible with ASCII, browsers must be able to parse declarations in such encodings. Examples of such encodings are UTF-16BE and UTF-16LE.
As of HTML5 the recommended charset is UTF-8.[3] An 'encoding sniffing algorithm' is defined in the specification to determine the character encoding of the document based on multiple sources of input, including:
- Explicit user instruction
- An explicit meta tag within the first 1024 bytes of the document
- A Byte order mark within the first three bytes of the document
- The HTTP Content-Type or other transport layer information
- Analysis of the document bytes looking for specific sequences or ranges of byte values,[5] and other tentative detection mechanisms.
For ASCII-compatible character encodings the consequence of choosing incorrectly is that characters outside the printable ASCII range (32 to 126) usually appear incorrectly. This presents few problems for English-speaking users, but other languages regularly—in some cases, always—require characters outside that range. In CJK environments where there are several different multi-byte encodings in use, auto-detection is also often employed. Finally, browsers usually permit the user to override incorrect charset label manually as well.
It is increasingly common for multilingual websites and websites in non-Western languages to use UTF-8, which allows use of the same encoding for all languages. UTF-16 or UTF-32, which can be used for all languages as well, are less widely used because they can be harder to handle in programming languages that assume a byte-oriented ASCII superset encoding, and they are less efficient for text with a high frequency of ASCII characters, which is usually the case for HTML documents.
Successful viewing of a page is not necessarily an indication that its encoding is specified correctly. If the page's creator and reader are both assuming some platform-specific character encoding, and the server does not send any identifying information, then the reader will nonetheless see the page as the creator intended, but other readers on different platforms or with different native languages will not see the page as intended.
Character references[edit]
In addition to native character encodings, characters can also be encoded as character references, which can be numeric character references (decimal or hexadecimal) or character entity references. Character entity references are also sometimes referred to as named entities, or HTML entities for HTML. HTML's usage of character references derives from SGML.
HTML character references[edit]
A numeric character reference in HTML refers to a character by its Universal Character Set/Unicodecode point, and uses the format
&#nnnn;
or
&#xhhhh;
where nnnn is the code point in decimal form, and hhhh is the code point in hexadecimal form. The x must be lowercase in XML documents. The nnnn or hhhh may be any number of digits and may include leading zeros. The hhhh may mix uppercase and lowercase, though uppercase is the usual style.
Not all web browsers or email clients used by receivers of HTML documents, or text editors used by authors of HTML documents, will be able to render all HTML characters. Most modern software is able to display most or all of the characters for the user's language, and will draw a box or other clear indicator for characters they cannot render.
For codes from 0 to 127, the original 7-bit ASCII standard set, most of these characters can be used without a character reference. Codes from 160 to 255 can all be created using character entity names. Only a few higher-numbered codes can be created using entity names, but all can be created by decimal number character reference.
Character entity references can also have the format
&name;
where name is a case-sensitive alphanumeric string. For example, 'λ' can also be encoded as λ
in an HTML document. The character entity references <
, >
, "
and &
are predefined in HTML and SGML, because <
, >
, '
and &
are already used to delimit markup. This notably does not include XML's '
(') entity. For a list of all named HTML character entity references (about 250), see List of XML and HTML character entity references.Unnecessary use of HTML character references may significantly reduce HTML readability. If the character encoding for a web page is chosen appropriately, then HTML character references are usually only required for markup delimiting characters as mentioned above, and for a few special characters (or none at all if a native Unicode encoding like UTF-8 is used). Incorrect HTML entity escaping may also open up security vulnerabilities for injection attacks such as cross-site scripting. If HTML attributes are left unquoted, certain characters, most importantly whitespace, such as space and tab, must be escaped using entities. Other languages related to HTML have their own methods of escaping characters.
XML character references[edit]
Unlike traditional HTML with its large range of character entity references, in XML there are only five predefined character entity references. These are used to escape characters that are markup sensitive in certain contexts:[6]
&
→ & (ampersand, U+0026)<
→ < (less-than sign, U+003C)>
→ > (greater-than sign, U+003E)"
→ ' (quotation mark, U+0022)'
→ ' (apostrophe, U+0027)
All other character entity references have to be defined before they can be used. For example, use of
é
(which gives é, Latin lower-case E with acute accent, U+00E9 in Unicode) in an XML document will generate an error unless the entity has already been defined. XML also requires that the x
in hexadecimal numeric references be in lowercase: for example ਛ
rather than ਛ
. XHTML, which is an XML application, supports the HTML entity set, along with XML's predefined entities.See also[edit]
- Charset sniffing – used by many browsers when character encoding metadata is not available
References[edit]
- ^'Content-Type', Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content, IETF, June 2014, retrieved 30 July 2014
- ^Apache Module mod_charset_lite
- ^ abc'Specifying the document's character encoding', HTML5, World Wide Web Consortium, 14 December 2017, retrieved 28 May 2018
- ^Bray, T.; Paoli, J.; Sperberg-McQueen, C.; Maler, E.; Yergeau, F. (26 November 2008), 'Prolog and Document Type Declaration', XML, W3C, retrieved 8 March 2010
- ^HTML5 prescan a byte stream to determine its encoding
- ^Bray, T.; Paoli, J.; Sperberg-McQueen, C.; Maler, E.; Yergeau, F. (26 November 2008), 'Character and Entity References', XML, W3C, retrieved 8 March 2010
External links[edit]
Retrieved from 'https://en.wikipedia.org/w/index.php?title=Character_encodings_in_HTML&oldid=891084769'