XPointers provide a number of convenient extensions to
XPath. One of the simplest is the shorthand
pointer . A shorthand pointer is similar to an HTML named
anchor; that is, a shorthand pointer identifies the element it's
pointing to by that element's ID. The ID is supplied by an ID type attribute of the
element being pointed at rather than by a special a element with a name attribute. To link to an element with a
shorthand pointer, append the usual fragment separator
# to the URL followed by the ID of the element to
which you're linking. For example,
http://www.w3.org/TR/1999/REC-xpath-19991116.xml#NT-AbsoluteLocationPath
links to the element in the XPath 1.0 specification that has an ID
type attribute with the value
NT-AbsoluteLocationPath.
The ID attribute is an attribute declared to have an ID type in
the document's DTD. It does not have to be named ID or id.
Shorthand pointers cannot be used to link to elements in documents
that don't have DTDs because such a document cannot have any ID type
attributes.
The inability to use IDs in documents without DTDs is a major
shortcoming of XML. Work is ongoing to attempt to remedy this,
perhaps by defining a generic ID attribute such as xml:id or by defining a namespace that
identifies ID type attributes.
For example, suppose you wanted to link to the Motivation and
Summary section of the Namespaces in XML
recommendation at http://www.w3.org/TR/1999/REC-xml-names-19990114/xml-names.xml.
A quick peek at the source code of this document reveals that it has
an id attribute with the value
sec-intro and that indeed this
attribute is declared to have an ID type in the associated DTD. Its
start-tag looks like this:
<div1 id='sec-intro'>
Thus,
http://www.w3.org/TR/1999/REC-xml-names-19990114/xml-names.xml#sec-intro
is a URL that points to this section. The name does not need to be
(and indeed should not be) enclosed in xpointer( ) to make this work. Just the ID
value is sufficient. This is basically just a convenient shorthand for
an XPointer built around an XPath expression using the id( ) function. The same URL could have been written as
http://www.w3.org/TR/1999/REC-xml-names-19990114/xml-names.xml#xpointer(id(sec-intro)).