Namespaces (XML in a Nutshell, 2nd Edition)

11.5. Namespaces

Since XPointers may appear in places that are not XML documents (HTML documents, database fields, magazine pages, etc.), they require their own mechanism for binding namespace prefixes to namespace URIs. This is done by placing one or more xmlns parts before the xpointer part. The syntax is xmlns(prefix=URI). For example, this XPointer maps the svg prefix to the http://www.w3.org/2000/svg namespace and then searches out all rect elements in that namespace:

xmlns(svg=http://www.w3.org/2000/svg) xpointer(//svg:rect)

As with most other uses of namespaces, only the URI matters in an XPointer, not the prefix. The previous XPointer finds all rect elements in the http://www.w3.org/2000/svg namespace regardless of what prefix they use or whether they're in the default namespace.

There is no way to define a default, unprefixed namespace for an XPointer. However, prefixed names in an XPointer can refer to unprefixed but namespace-qualified elements in the targeted document. For example, this XPointer finds the third div element in an XHTML document:

xmlns(html=http://www.w3.org/1999/xhtml) xpointer(//html:div[3])

It uses the prefix html to identify the XHTML namespace, even though XHTML documents never use prefixes themselves.

More than one namespace prefix can be used simply by adding extra xmlns parts. For example, this XPointer seeks out svg elements in XHTML documents by declaring one prefix each for the SVG and XHTML namespaces:

xmlns(svg=http://www.w3.org/2000/svg)
xmlns(h=http://www.w3.org/1999/xhtml) xpointer(/h:html//svg:svg)

If an XPointer is included in an XML document, the namespace bindings established by that document do not apply to the XPointer. Only the bindings established by the xmlns parts apply to the XPointer. If the xpointer parts contain XPath expressions that refer to elements or attributes in a namespace, then they must be preceded by xmlns parts declaring the namespaces.