Extensible Markup Language - SW (XML-SW)

2.1 Start-Tags, End-Tags, and Empty-Element Tags

[Definition: The beginning of every non-empty XML element is marked by a start-tag.]

Start-tag

The QName in the start- and end-tags gives the element's type. [Definition: The QName-AttValue pairs are referred to as the attribute specifications of the element], [Definition: with the QName in each pair referred to as the attribute name] and [Definition: the content of the AttValue (the text between the ' or " delimiters) as the attribute value.] Note that the order of attribute specifications in a start-tag or empty-element tag is not significant.

<termdef id="dt-dog" term="dog">

[Definition: The end of every element that begins with a start-tag must be marked by an end-tag containing a name that matches the element's type as given in the start-tag:]

End-tag

An example of an end-tag:

</termdef>

[Definition: The text between the start-tag and end-tag is called the element's content:]

Content of Elements

[Definition: An element with no content is said to be empty.] The representation of an empty element is either a start-tag immediately followed by an end-tag, or an empty-element tag. [Definition: An empty-element tag takes a special form:]

Tags for Empty Elements

Empty-element tags may be used for any element which has no content.

Examples of empty elements:

<IMG align="left"
 src="http://www.example.org/Icons/madonna" />
<br></br>
<br/>

2.2 XML Namespaces

The names that appear as element types and attribute names serve as labels for the logical components of an XML document. [Definition: Software modules are often designed to process a particular set of elements and attributes and their content, identifying them using these labels. Let us refer to such a set, understood by some software module, as a markup vocabulary.]

We envision applications of XML where a single XML document may contain elements and attributes from more than one markup vocabulary. One motivation for this is modularity; if such a markup vocabulary exists which is well-understood and for which there is useful software available, it is better to re-use this markup rather than re-invent it.

Such documents, containing multiple markup vocabularies, pose problems of recognition and collision. Software modules need to be able to recognize the elements and attributes which they are designed to process, even in the face of "collisions" occurring when markup intended for some other software package uses the same element type or attribute name.

These considerations require that document constructs should have universal names, whose scope extends beyond their containing document. This section describes a mechanism, XML namespaces, which accomplishes this.

[Definition: An XML namespace is identified by a URI reference [URIRef]; element types and attribute names may be placed in it using prefixing syntax described in this section.]

[Definition: URI references which identify namespaces are considered identical when they are exactly the same character-for-character.] Note that URI references which are not identical in this sense may in fact be functionally equivalent. Examples include URI references which differ only in case.

Names from XML namespaces may appear as qualified names, which contain a single colon, separating the name into a namespace prefix and a local part. The prefix, which is mapped to a URI reference, selects a namespace. The combination of the universally managed URI namespace and the document's own namespace produces identifiers that are universally unique. Mechanisms are provided for prefix scoping and defaulting.

URI references can contain characters not allowed in names, so cannot be used directly as namespace prefixes. Therefore, the namespace prefix serves as a proxy for a URI reference. An attribute-based syntax described below is used to declare the association of the namespace prefix with a URI reference.

<x xmlns:eg='http://example.com/schema'>
  <!-- the "eg" prefix is bound to http://example.com/schema
       for the "x" element and contents -->
</x>

<x xmlns:eg='http://example.com/schema'>
  <!-- the 'price' element's namespace is http://example.com/schema -->
  <eg:price units='Euro'>32.18</eg:price>
</x>

<x xmlns:eg='http://example.com/schema'>
  <!-- the 'taxClass' attribute's namespace is http://example.com/schema -->
  <lineItem eg:taxClass="exempt">Baby food</lineItem>
</x>

<?xml version="SW"?>
<!-- all elements here are explicitly in the HTML namespace -->
<html:html xmlns:html='http://www.w3.org/1999/xhtml'>
  <html:head><html:title>Frobnostication</html:title></html:head>
  <html:body><html:p>Moved to 
    <html:a href='http://frob.com'>here.</html:a></html:p></html:body>
</html:html>

<?xml version="SW"?>
<!-- both namespace prefixes are available throughout -->
<bk:book xmlns:bk="http://www.example.com/books/"
         xmlns:isbn='http://www.example.com/isbn/">
    <bk:title>Cheaper by the Dozen</bk:title>
    <isbn:number>1568491379</isbn:number>
</bk:book>

<?xml version="SW"?>
<!-- elements are in the HTML namespace, in this case by default -->
<html xmlns='http://www.w3.org/1999/xhtml'>
  <head><title>Frobnostication</title></head>
  <body><p>Moved to 
    <a href='http://example.com/frob/'>here</a>.</p></body>
</html>

<?xml version="SW"?>
<!-- unprefixed element types are from "books" -->
<book xmlns="http://www.example.com/books/"
      xmlns:isbn="http://www.example.com/isbn/">
    <title>Cheaper by the Dozen</title>
    <isbn:number>1568491379</isbn:number>
</book>

<?xml version="SW"?>
<!-- initially, the default namespace is "books" -->
<book xmlns="http://www.example.com/books/"
      xmlns:isbn="http://www.example.com/isbn/">
    <title>Cheaper by the Dozen</title>
    <isbn:number>1568491379</isbn:number>
    <notes>
      <!-- make HTML the default namespace for some commentary -->
      <p xmlns=http://www.w3.org/1999/xhtml'>
          This is a <i>funny</i> book!
      </p>
    </notes>
</book>

<?xml version='SW'?>
<Beers>
  <!-- the default namespace is now that of HTML -->
  <table xmlns='http://www.w3.org/TR/REC-html40'>
   <th><td>Name</td><td>Origin</td><td>Description</td></th>
   <tr> 
     <!-- no default namespace inside table cells -->
     <td><brandName xmlns="">Huntsman</brandName></td>
     <td><origin xmlns="">Bath, UK</origin></td>
     <td>
       <details xmlns=""><class>Bitter</class><hop>Fuggles</hop>
         <pro>Wonderful hop, light alcohol, good summer beer</pro>
         <con>Fragile; excessive variance pub to pub</con>
         </details>
        </td>
      </tr>
    </table>
  </Beers>

<!-- http://www.example.com/ is bound to n1 and n2 -->
<x xmlns:n1="http://www.example.com/"
   xmlns:n2="http://www.example.com/" >
  <bad a="1"     a="2" />
  <bad n1:a="1"  n2:a="2" />
</x>

<!-- http://www.example.com/ is bound to n1 and is the default -->
<x xmlns:n1="http://www.example.com/"
   xmlns="http://www.example.com/" >
  <good a="1"     b="2" />
  <good a="1"     n1:a="2" />
</x>

2.3 Parent, Child, and Root Elements

An XML document matches the document production, which implies that:

1. It contains one or more elements.

2. [Definition: There is exactly one element, called the root, or document element, no part of which appears in the content of any other element.] For all other elements, if the start-tag is in the content of another element, the end-tag is in the content of the same element. More simply stated, the elements, delimited by start- and end-tags, nest properly within each other.

[Definition: As a consequence of this, for each non-root element C in the document, there is one other element P in the document such that C is in the content of P, but is not in the content of any other element that is in the content of P. P is referred to as the parent of C, and C as a child of P.]

Example of a root, parent and child elements.

<root>This root element is the parent of the "parent" element.
<parent>This parent element is a child of the "root" element and
parent of the "child" element.
<child>This child element is a child of the "parent" element.</child>
</parent>
</root>

2.4 Reserved Attributes

<div>
<p xml:space="default">In this paragraph,
  line breaks and indentation mean nothing.</p>
<p xml:space="preserve">Here, space matters:

      \o/
       |
      / \

</p></div>

<p xml:lang="en">The quick brown fox jumps over the lazy dog.</p>
<p xml:lang="en-GB">What colour is it?</p>
<p xml:lang="en-US">What color is it?</p>
<sp who="Faust" desc='leise' xml:lang="de">
  <l>Habe nun, ach! Philosophie,</l>
  <l>Juristerei, und Medizin</l>
  <l>und leider auch Theologie</l>
  <l>durchaus studiert mit heißem Bemüh'n.</l>
</sp>

<?xml version="SW"?>
<doc xml:base="http://example.org/today/"
     xmlns:xlink="http://www.w3.org/1999/xlink">
  <head>
    <title>Virtual Library</title>
  </head>
  <body>
    <paragraph>See <link xlink:type="simple" xlink:href="new.xml">what's
      new</link>!</paragraph>
    <paragraph>Check out the hot picks of the day!</paragraph>
    <olist xml:base="/hotpicks/">
      <item>
        <link xlink:type="simple" xlink:href="pick1.xml">Hot Pick #1</link>
      </item>
      <item>
        <link xlink:type="simple" xlink:href="pick2.xml">Hot Pick #2</link>
      </item>
      <item>
        <link xlink:type="simple" xlink:href="pick3.xml">Hot Pick #3</link>
      </item>
    </olist>
  </body>
</doc>

3.1 Prolog and Document Type Declaration

[Definition: XML documents should begin with an XML declaration which specifies the version of XML being used.] For example, the following is a complete XML document.

<?xml version="SW"?> <greeting>Hello, world!</greeting> 

The version number "SW" should be used to indicate conformance to this version of this specification; it is an error for a document to use the value "SW" if it does not conform to this version of this specification. Processors may signal an error if they receive documents labeled with versions they do not support.

Prolog

[Definition: For compatibility with XML 1.0, a document type declaration may appear in an XML document before the first element.]

Document Type Declaration

An example of an XML document with a document type declaration:

<?xml version="SW"?> 
<!DOCTYPE greeting SYSTEM "hello.dtd"> 
<greeting>Hello, world!</greeting> 

3.2 Comments

[Definition: Comments may appear anywhere in a document outside other markup; For compatibility, the string "--" (double-hyphen) must not occur within comments.]

Comments

An example of a comment:

<!-- declarations for <head> & <body> -->

Note that the grammar does not allow a comment ending in --->. The following example is not well-formed.

<!-- B+, B, or B--->

3.3 Processing Instructions

[Definition: Processing instructions (PIs) allow documents to contain instructions for applications.]

Processing Instructions

PIs are not part of the document's character data, but must be passed through to the application. [Definition: The PI begins with a target (PITarget) used to identify the application to which the instruction is directed.] The target names "XML", "xml", and so on are reserved for standardization in this or future versions of this specification.

3.4 CDATA Sections

[Definition: CDATA sections may occur anywhere character data may occur; they are used to escape blocks of text containing characters which would otherwise be recognized as markup. CDATA sections begin with the string "<![CDATA[" and end with the string "]]>":]

CDATA Sections

Within a CDATA section, only the CDEnd string is recognized as markup, so that left angle brackets and ampersands may occur in their literal form; they need not (and cannot) be escaped using "<" and "&". CDATA sections cannot nest.

An example of a CDATA section, in which "<greeting>" and "</greeting>" are recognized as character data, not markup:

<![CDATA[<greeting>Hello, world!</greeting>]]> 

4.1 Characters

[Definition: XML documents contain text, a sequence of characters, which may represent markup or character data.] [Definition: A character is an atomic unit of text as specified by ISO/IEC 10646 [ISO/IEC 10646] (see also [ISO/IEC 10646-2000]). Legal characters are tab, carriage return, line feed, and the legal characters of Unicode and ISO/IEC 10646. The versions of these standards cited in A.1 Normative References were current at the time this document was prepared. New characters may be added to these standards by amendments or new editions. Consequently, XML processors must accept any character in the range specified for Char. The use of "compatibility characters", as defined in section 6.8 of [Unicode] (see also D21 in section 3.6 of [Unicode3]), is discouraged.]

Character Range

The same encoding must be used for for all the characters in an XML document. All XML processors must accept the UTF-8 and UTF-16 encodings of 10646; the mechanisms for signaling which of the two is in use, or for bringing other encodings into play, are discussed later, in 4.5 Character Encoding in XML Documents.

4.2 Character References

[Definition: A character reference in an XML document stands for a specific character in the ISO/IEC 10646 character set, for example one not directly accessible from available input devices.]

Character Reference

If the character reference begins with "&#x", the digits and letters up to the terminating ";" provide a hexadecimal representation of the character's code point in ISO/IEC 10646. If it begins just with "&#", the digits up to the terminating ";" provide a decimal representation of the character's code point.

[Definition: For readability, a set of predefined patterns is also provided for the purpose of escaping XML's delimiter characters: & for &, < for <, > for >, ' for ', and " for ". This has exactly the same effect as using character references: < for <, & for & and so on.]

4.3 Character Data and Markup

Text consists of intermingled character data and markup. [Definition: Markup takes the form of start-tags, end-tags, empty-element tags, character references, comments, CDATA section delimiters, document type declarations, processing instructions, XML declarations, and any white space that is in the document, outside the document element, and not inside any other markup.]

[Definition: All text that is not markup constitutes the character data of the document.]

The ampersand character (&) and the left angle bracket (<) may appear in their literal form only when used as markup delimiters, or within a comment, a processing instruction, or a CDATA section. If they are needed elsewhere, they must be escaped using either numeric character references or the strings "&" and "<" respectively. The right angle bracket (>) may be represented using the string ">", and must, for compatibility, be escaped using ">" or a character reference when it appears in the string "]]>" in content, when that string is not marking the end of a CDATA section.

In the content of elements, character data is any string of characters which does not contain the start-delimiter of any markup. In a CDATA section, character data is any string of characters not including the CDATA-section-close delimiter, "]]>".

To allow attribute values to contain both single and double quotes, the apostrophe or single-quote character (') may be represented as "'", and the double-quote character (") as """.

Character Data

4.4 Common Syntactic Constructs

S (white space) consists of one or more space (#x20) characters, carriage returns, line feeds, or tabs.

White Space

Characters are classified for convenience as letters, digits, or other characters. A letter consists of an alphabetic or syllabic base character or an ideographic character. Full definitions of the specific characters in each class are given in B Character Classes.

[Definition: A Name is a token beginning with a letter or one of a few punctuation characters, and continuing with letters, digits, hyphens, underscores, colons, or full stops, together known as name characters.] Names beginning with the string "xml", or any string which would match (('X'|'x') ('M'|'m') ('L'|'l')), are reserved for standardization in this or future versions of this specification.

Names

To support XML namespaces (see 2.2 XML Namespaces), it is necessary to give element types and attribute names as a quoted pair of labels; the Qualified Name (QName) and No-colon Name (NCName) nonterminals support this.

Qualified Name

Literal data is any quoted string not containing the quotation mark used as a delimiter for that string. Literals are used for specifying the values of attributes (AttValue), and certain components of the document type declaration.

Literals

4.5 Character Encoding in XML Documents

XML documents encoded in UTF-16 must begin with the Byte Order Mark described by Annex F of [ISO/IEC 10646], Annex H of [ISO/IEC 10646-2000], section 2.4 of [Unicode], and section 2.7 of [Unicode3] (the ZERO WIDTH NO-BREAK SPACE character, #xFEFF). This is an encoding signature, not part of either the markup or the character data of the XML document. XML processors must be able to use this character to differentiate between UTF-8 and UTF-16 encoded documents.

Although an XML processor is required to read only XML documents in the UTF-8 and UTF-16 encodings, it is recognized that other encodings are used around the world, and it may be desired for XML processors to read XML documents that use them. In the absence of external character encoding information (such as MIME headers), XML documents which are stored in an encoding other than UTF-8 or UTF-16 must begin with an XML declaration containing an encoding declaration:

Encoding Declaration

The EncName is the name of the encoding used.

In an encoding declaration, the values "UTF-8", "UTF-16", "ISO-10646-UCS-2", and "ISO-10646-UCS-4" should be used for the various encodings and transformations of Unicode / ISO/IEC 10646, the values "ISO-8859-1", "ISO-8859-2", ... "ISO-8859-n" (where n is the part number) should be used for the parts of ISO 8859, and the values "ISO-2022-JP", "Shift_JIS", and "EUC-JP" should be used for the various encoded forms of JIS X-0208-1997. It is recommended that character encodings registered (as charsets) with the Internet Assigned Numbers Authority [IANA-CHARSETS], other than those just listed, be referred to using their registered names; other encodings should use names starting with an "x-" prefix. XML processors should match character encoding names in a case-insensitive way and should either interpret an IANA-registered name as the encoding registered at IANA for that name or treat it as unknown (processors are, of course, not required to support all IANA-registered encodings).

In the absence of information provided by an external transport protocol (e.g. HTTP or MIME), it is an error for an XML document including an encoding declaration to be presented to the XML processor in an encoding other than that named in the declaration, or for an XML document which begins with neither a Byte Order Mark nor an encoding declaration to use an encoding other than UTF-8. Note that since ASCII is a subset of UTF-8, ordinary ASCII XML documents do not strictly need an encoding declaration.

It is a fatal error when an XML processor encounters a data object with an encoding that it is unable to process. It is a fatal error if an XML document is determined (via default, encoding declaration, or higher-level protocol) to be in a certain encoding but contains octet sequences that are not legal in that encoding. It is also a fatal error if an XML document contains no encoding declaration and its content is not legal UTF-8 or UTF-16.

Examples of XML declarations containing encoding declarations:

<?xml version='SW' encoding='UTF-8'?>
<?xml version="SW" encoding='EUC-JP'?>

5.1 Base URI

Several information items have a "base URI" property. These are computed as specified in 2.4.3 Base URI Specification. Note that retrieval of a resource may involve redirection at the parser level (for example, in an entity resolver) or below; in this case the base URI is the final URI used to retrieve the resource after all redirection.

The value of these properties does not reflect any URI escaping that may be required for retrieval of the resource, but it may include escaped characters if these were specified in the XML document, or returned by a server in the case of redirection.

In some cases (such as a document read from a string or a pipe) the rules in 2.4.3 Base URI Specification may result in a base URI being application dependent. In these cases this specification does not define the value of the "base URI" property.

When resolving relative URIs the "base URI" property should be used in preference to the values of xml:base attributes; they may be inconsistent in the case of Synthetic Infosets.

5.2 "Unknown" and "No Value"

5.3 Synthetic Infosets

5.4 End-of-Line Handling

To simplify the tasks of applications, the information set items corresponding to line ends in the character data and attribute values of an XML document appear in a form "normalized" as follows: all appearances of either the literal two-character sequence "#xD#xA" or a standalone literal #xD are normalized into a single character information item for a #A character.

5.5 Information Items

a="

xyz"

6.1 Syntax Checking

Conforming XML processors must detect and report violations of this specification's grammar and well-formedness constraints in the content of data objects (which, if such violations exist, are by definition not XML documents).

When any such violation or any other fatal error is encountered, the XML processor may continue processing the data to search for further errors and may report such errors to the application. In order to support correction of errors, the processor may make unprocessed data from the document (with intermingled character data and markup) available to the application. Once a fatal error is detected, however, the processor must not continue normal processing - i.e. it must not continue making Infoset items available to the application.

6.2 Use of the XML Information Set by Other Specifications

One of the purposes of the Information Set (see 5 The Information Set) is to provide a set of definitions for use by other specifications.

Specifications conformant to this specification, when referring to the Infoset, must:

• Indicate the information items and properties that are needed to implement the specification.

• Specify how other information items and properties are treated (for example, they might be passed through unchanged).

• Note any information required from an XML document that is not defined by the Infoset.

• Note any difference in the use of terms defined by the Infoset (this should be avoided).

6.3 XML Processors and the XML Information Set

Conforming XML processors must provide a mechanism to make the information items from the information set available to applications with the characteristics described in 5 The Information Set.

7.1 Notation

symbol ::= expression

#xN

where N is a hexadecimal integer, the expression matches the character in ISO/IEC 10646 whose canonical (UCS-4) code value, when interpreted as an unsigned binary number, has the value indicated. The number of leading zeros in the #xN form is insignificant; the number of leading zeros in the corresponding code value is governed by the character encoding in use and is not significant for XML.

[a-zA-Z], [#xN-#xN]

matches any Char with a value in the range(s) indicated (inclusive).

[abc], [#xN#xN#xN]

matches any Char with a value among the characters enumerated. Enumerations and ranges can be mixed in one set of brackets.

[^a-z], [^#xN-#xN]

matches any Charwith a value outside the range indicated.

[^abc], [^#xN#xN#xN]

matches any Char with a value not among the characters given. Enumerations and ranges of forbidden values can be mixed in one set of brackets.

"string"

matches a literal string matching that given inside the double quotes.

'string'

matches a literal string matching that given inside the single quotes.

These symbols may be combined to match more complex patterns as follows, where A and B represent simple expressions:

(expression)

expression is treated as a unit and may be combined as described in this list.

A?

matches A or nothing; optional A.

A B

matches A followed by B. This operator has higher precedence than alternation; thus A B | C D is identical to (A B) | (C D).

A | B

matches A or B but not both.

A - B

matches any string that matches A but does not match B.

A+

matches one or more occurrences of A. Concatenation has higher precedence than alternation; thus A+ | B+ is identical to (A+) | (B+).

A*

matches zero or more occurrences of A. Concatenation has higher precedence than alternation; thus A* | B* is identical to (A*) | (B*).

Other notations used in the productions are:

/* ... */

comment.

[ wfc: ... ]

well-formedness constraint; this identifies by name a constraint associated with a grammar production, violation of which is a fatal error.

7.2 Terminology

may

[Definition: Conforming documents and XML processors are permitted to but need not behave as described.]

must

[Definition: Conforming documents and XML processors are required to behave as described; otherwise they are in error.]

error

[Definition: A violation of the rules of this specification; results are undefined. Conforming software may detect and report an error and may recover from it.]

fatal error

[Definition: An error which a conforming XML processor must detect and report to the application. ]

at user option

[Definition: Conforming software may or must (depending on the modal verb in the sentence) behave as described; if it does, it must provide users a means to enable or disable the behavior described.]

well-formedness constraint

[Definition: A rule which applies to all XML documents. Violations of well-formedness constraints are fatal errors.]

match

[Definition: (Of strings or names:) Two strings or names being compared must be identical. Characters with multiple possible representations in ISO/IEC 10646 (e.g. characters with both precomposed and base+diacritic forms) match only if they have the same representation in both strings. No case folding is performed. (Of strings and rules in the grammar:) A string matches a grammatical production if it belongs to the language generated by that production.]

for compatibility

[Definition: Marks a sentence describing a feature of XML included solely to ensure that XML remains compatible with SGML.]