1.1 A Note on Notation and Usage

WhereEMPHASIZED, the key wordsMUST,MUST NOT,REQUIRED,SHOULD,SHOULD NOT,MAYin this document are to be interpreted as described in[Keywords].

Note that many of thenonterminals in the productions in this specification are defined not here but inthe XML specification[XML].When nonterminals defined here have the same names as nonterminalsdefined in the XML specification, the productions herein all cases match a subset of the strings matched by the corresponding ones there.

In this document's productions, theNSC is a "Namespace Constraint", one of the rules that documents conforming to this specificationMUSTfollow.

2.1 Basic Concepts

[Definition: AnXML namespace is identified bya URI reference[RFC3986];element and attribute namesmay be placed in an XML namespace using the mechanisms describedin this specification.]

[Definition: Anexpanded nameis a pair consisting of anamespace nameand alocal name.][Definition: For a nameN in a namespace identifiedbya URII, thenamespace nameisI. For a nameN that is not in a namespace, thenamespace namehas no value.][Definition: In either case thelocal nameisN.]It is this combination of the universally managed URI namespacewith the vocabulary's local names that is effective in avoidingname clashes.

URIreferences can contain characters not allowed in names, and are ofteninconveniently long, so expanded names are not used directly to nameelements and attributes in XML documents. Insteadqualified namesare used.[Definition: Aqualified nameis a name subject to namespace interpretation.]In documents conforming to this specification,element and attribute names appear as qualified names.Syntactically, they are eitherprefixed names orunprefixed names.An attribute-based declaration syntax is provided to bind prefixes tonamespace names and to bind a default namespace that applies tounprefixed element names;these declarations are scoped by the elements on which they appear so thatdifferent bindings may apply in different parts of a document.Processors conforming to this specificationMUSTrecognize and act on these declarations and prefixes.

2.2 Use ofURIsas Namespace Names

The empty string, though it is a legalURIreference, cannot be used as a namespace name.

The use of relativeURIreferences,including same-document references, in namespace declarations isdeprecated.

2.3 ComparingURIReferences

URIreferences identifying namespaces are compared when determiningwhether a name belongs to a given namespace, and whether two namesbelong to the same namespace.[Definition: The twoURIsare treated as strings, and they areidenticalif and only if the strings are identical, that is, if theyare the same sequence of characters. ]The comparison is case-sensitive, and no %-escaping is done or undone.

A consequence of this is thatURIreferences which are not identicalin this sense may resolve to the same resource. Examples includeURIreferences which differ only in case or %-escaping, or which arein external entities which have different base URIs (but note thatrelativeURIsare deprecated as namespace names).

In a namespace declaration, theURIreference is thenormalized valueof the attribute, so replacement of XML character and entity referenceshas already been done before any comparison.

Examples:

TheURIreferences below are all different for the purposes of identifyingnamespaces, since they differ in case:

• http://www.example.org/wine

• http://www.Example.org/wine

• http://www.example.org/Wine

The URI references below are also all different for the purposes of identifyingnamespaces:

• http://www.example.org/~wilbur

• http://www.example.org/%7ewilbur

• http://www.example.org/%7Ewilbur

Because of the risk of confusion betweenURIsthat would be equivalentif dereferenced, the use of %-escaped characters in namespace names isstrongly discouraged.

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

Namespace constraint: Reserved Prefixes and Namespace Names

The prefixxml is by definition bound to the namespace namehttp://www.w3.org/XML/1998/namespace. ItMAY,but need not, bedeclared, andMUST NOTbebound to any other namespace name. Other prefixesMUST NOTbe bound to this namespacename,and itMUST NOTbe declared as the default namespace.

The prefixxmlns is used only to declare namespace bindings and is bydefinition bound to the namespace namehttp://www.w3.org/2000/xmlns/. ItMUST NOTbe declared.Other prefixesMUST NOTbe bound to this namespace name,and itMUST NOTbe declared as the default namespace.Element namesMUST NOThave the prefixxmlns.

All other prefixes beginning with the three-letter sequence x, m, l,in any case combination, are reserved. This means that:

• usersSHOULD NOT use them except as defined by later specifications

• processorsMUST NOTtreat them as fatal errors.

  <!-- the 'price' element's namespace is http://ecommerce.example.org/schema -->  <edi:price xmlns:edi='http://ecommerce.example.org/schema' units='Euro'>32.18</edi:price>

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

Namespace constraint: Prefix Declared

The namespace prefix, unless it isxmlorxmlns,MUSThave beendeclared in anamespace declarationattribute in either the start-tag of the element where the prefix is used or in an ancestor element (i.e., an element in whosecontent theprefixed markup occurs).

Namespace constraint: No Prefix Undeclaring

In anamespace declaration for aprefix (i.e., where theNSAttName is aPrefixedAttName), theattribute valueMUST NOT be empty.

6.1 Namespace Scoping

The scope of a namespace declaration declaring a prefix extends fromthe beginning of the start-tag in which it appears to the end of thecorresponding end-tag, excluding the scope of any inner declarationswith the same NSAttName part.In the case of an empty tag, the scope is the tag itself.

Such a namespace declaration applies to all element and attributenames within its scope whose prefix matches that specified in thedeclaration.

Theexpanded namecorresponding to a prefixed element or attribute name has theURIto which theprefixis bound as itsnamespace name,and thelocal partas itslocal name.

<?xml version="1.0"?><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.example.com'>here.</html:a></html:p></html:body></html:html>

Multiple namespace prefixes can be declared as attributes of a single element, as shown in this example:

<?xml version="1.0"?><!-- both namespace prefixes are available throughout --><bk:book xmlns:bk='urn:loc.gov:books'         xmlns:isbn='urn:ISBN:0-395-36341-6'>    <bk:title>Cheaper by the Dozen</bk:title>    <isbn:number>1568491379</isbn:number></bk:book>

6.2 Namespace Defaulting

The scope of adefault namespace declarationextends from the beginning of thestart-tag in which it appears to the end of the corresponding end-tag,excluding the scope of any inner default namespace declarations.In the case of an empty tag, the scope is the tag itself.

A default namespace declaration applies to all unprefixed element nameswithin its scope.Default namespace declarations do not apply directly to attribute names;the interpretation of unprefixed attributes isdetermined by the element on which they appear.

If there is a default namespace declaration in scope, theexpanded namecorresponding to an unprefixed element name has theURIof thedefault namespaceas itsnamespace name.If there is no default namespace declaration in scope, thenamespace name has no value.The namespace name for an unprefixed attribute name always has no value.In all cases, thelocal name islocal part(which is of course the same as the unprefixed name itself).

<?xml version="1.0"?><!-- 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://frob.example.com'>here</a>.</p></body></html>

<?xml version="1.0"?><!-- unprefixed element types are from "books" --><book xmlns='urn:loc.gov:books'      xmlns:isbn='urn:ISBN:0-395-36341-6'>    <title>Cheaper by the Dozen</title>    <isbn:number>1568491379</isbn:number></book>

A larger example of namespace scoping:

<?xml version="1.0"?><!-- initially, the default namespace is "books" --><book xmlns='urn:loc.gov:books'      xmlns:isbn='urn:ISBN:0-395-36341-6'>    <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>

The attribute value in a default namespace declarationMAYbe empty.This has the sameeffect, within the scope of the declaration, of there being no defaultnamespace.

<?xml version='1.0'?><Beers>  <!--the default namespace inside tables is that of HTML -->  <table xmlns='http://www.w3.org/1999/xhtml'>   <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>

6.3 Uniqueness of Attributes

This constraint is equivalent to requiring that no element have twoattributes with the sameexpanded name.

For example, each of thebad empty-element tags is illegal in thefollowing:

<!-- http://www.w3.org is bound to n1 and n2 --><x xmlns:n1="http://www.w3.org"    xmlns:n2="http://www.w3.org" >  <bad a="1"     a="2" />  <bad n1:a="1"  n2:a="2" /></x>

However, each of the following is legal, the second because the default namespace does not apply to attribute names:

<!-- http://www.w3.org is bound to n1 and is the default --><x xmlns:n1="http://www.w3.org"    xmlns="http://www.w3.org" >  <good a="1"     b="2" />  <good a="1"     n1:a="2" /></x>

Note:

ProductionNC-NCNameStartChar takes advantage of the fact that a single-character NCName is necessarily an NCNameStartChar, and works by subtracting from the set of NCNames of all lengths the set of all strings of two or more characters, leaving only the NCNames which are one character long.