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Copyright © 2007 W3C ® ( MIT , ERCIM , Keio ), All Rights Reserved. W3C liability , trademark and document use rules apply.
The aim of this document is to outline a syntax for expressing URIs in a generic, abbreviated syntax. While it has been produced in conjunction with the HTML Working Group, it is not specifically targeted at use by XHTML Family Markup Languages. Note that the target audience for this document is Markup Language designers, not the users of those Markup Languages.
This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.
This document is a first public working draft, but is nearly complete. It is based upon work done in the definition of [ XHTML2 ], and work done by the RDF-in-HTML task force [RDFHTML] , a joint task force of the Semantic Web Best Practices and Deployment Working Group [SWBPD-WG] and HTML Working Group [HTML-WG] . It is not yet stable, but has had extensive review over the last 8 months. It is being released in a separate, stand-alone specification in order to speed its adoption and facilitiate its use in various specifications.
This document has been produced by the W3C HTML Working Group ( Members only ) as part of the HTML Activity . The goals of the HTML Working Group are discussed in the HTML Working Group charter .
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Publication as a Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.
Please report errors in this specification to www-html-editor@w3.org ( archive ). It is inappropriate to send discussion email to this address. Public discussion may take place on www-html@w3.org ( archive ).
This section is informative.
More and more grammars are expressing URIs in XML using QNames. Since QNames are invariably shorter than the URI that they express, this is obviously a very useful device. However, a major problem is that the origin of the notion of a QName [NAMESPACES-IN-XML-QNAMES] is such that it does not allow all possible URIs to be expressed. (For the definition of the XML Schema datatype for QNames see [XML-SCHEMA-QNAME] .)
A specific example of the problem this causes comes from the IPTC [IPTC] . They would like to be able to use attributes in their mark-up to carry metadata in their documents, and as a consequence sought to make extensive use of QNames to keep the amount of data being transferred as small as possible. In other words, instead of sending lots of long URIs, QNames were to be used to abbreviate them. However, the purpose of QNames in XML is to provide a way for XML elements that contain a colon to be interpreted as an element with a different name (see [NAMESPACES-IN-XML-QNAMES] ). For this reason, the definition is such that the part after the colon must be a valid element name, making an example such as the following invalid :
iptc:10112244
This is not a valid QName simply because '10112244' is not a valid element name. Yet, in the IPTC example given, the whole reason for using a QName was to abbreviate the URI, and not to create a namespace qualified element name. This gives rise to an interesting problem; the definition of a QName insists on the use of valid XML element names, but an increasingly common use of QNames is as a means to abbreviate URIs, and unfortunately the two are in conflict with each other.
This specification addresses the problem by creating a new data type whose purpose is specifically to allow for the abbreviation of URIs in exactly this way. This type is called a "CURIE" or a "Compact URI", and QNames are a subset of this.
Although they are not currently called CURIEs, the technique described here is in widespread usage. However, taken literally, QNames would not support many of the examples that we would find 'in the wild' — the fact that they do is mainly because systems and authors take a very lax approach to QNames.
In other words, the principle used in QNames — that of substituting a namespace prefix for a URI and thereby producing a longer URI — is widely used, but little checking is done on the element part to ensure that the string is a valid element name. However, this does mean that CURIEs can be easily used in a number of places, since there is already a large amount of 'mind-share'. Current uses include:
Many
Wikis
support
a
feature
where
a
prefix
like
isbn
can
be
substituted
for
something
like:
http://www.amazon.com/?isbn=
or:
http://www.barnesandnoble.com/?q=
When a Wiki author wants to make use of this, they can simply enter:
Go and buy T. V. Raman's [[isbn:0321154991][book on XForms]].
and the Wiki software will automatically generate:
Go and buy T. V. Raman's <a href="http://www.amazon.com/?isbn=0321154991">book on XForms</a>
This section is normative .
The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [ RFC2119 ].
A conforming user agent must support all of the features required in this specification.
This section is normative.
A
CURIE
is
comprised
of
two
components,
a
prefix
and
a
suffix
.
The
prefix
is
separated
from
the
suffix
by
a
colon
(
:
).
To
disambiguate
a
CURIE
when
it
appears
in
a
context
where
a
normal
[
URI
]
may
also
be
used,
the
entire
CURIE
is
permitted
to
be
enclosed
in
brackets
(
[
,
]
).
curie := ( prefix ':' suffix ) | ( '[' prefix ':' suffix ']' ) prefix := NCName suffix := irelative-ref (as defined in IRI)
When CURIES are used in an XML grammar, prefix values MUST be defined using the 'xmlns:' syntax specified in [ XMLNAMES ].
When
CURIES
are
used
in
a
non-XML
grammar,
the
grammar
MUST
provide
a
mechanism
for
defining
the
mapping
from
the
prefix
to
an
IRI.
The
concatenation
of
the
namespace
associated
with
a
CURIE
and
its
suffix
MUST
be
an
IRI
[
IRI
]
.
The namespace prefix '_' has special meaning when CURIEs are used for RDF serialisations. In order to provide support for BNodes , the namespace prefix '_' is reserved and RDF processors are free to generate anonymous URIs as they see fit. For this reason, namespace declarations using '_' SHOULD be avoided by authors.
Grammars MAY define additional constraints on these syntax rules when CURIES are used in the context of those grammars. Grammars MUST NOT relax the constraints defined this specification.
This section is informative.
CURIEs can be used in exactly the same way that QNames are, with the modification that the format of the strings before and after the colon are looser. In all cases a parsed CURIE will produce a IRI. However, the process of parsing involves substituting the namespace represented by the namespace prefix for the prefix itself, and then simply appending the part after the colon.
All of the following are valid CURIEs — even though they are not valid QNames — and they take advantage of the fact that the part after the colon no longer needs to conform to the rules for element names:
home:#start joseki: google:xforms+or+'xml+forms'
There
will
be
situations
in
the
design
of
a
language
where
it
is
desirable
for
an
attribute
that
can
take
a
URI
to
also
be
able
to
contain
a
CURIE.
For
example,
in
XHTML
the
href
attribute
allows
a
URI
to
be
specified
that
will
be
navigated
on
user
action,
but
it
would
also
be
useful
to
be
able
to
abbreviate
this
URI,
using
the
compact
syntax.
However,
the
problem
is
that
it
is
not
possible
for
the
language
parser
to
be
completely
sure
whether
it
has
located
a
CURIE
or
a
URI.
For
example,
a
link
to
an
email
address
can
be
expressed
like
this:
Why not <a href="smtp:contactus@example.com">drop us a line</a>.
There is no way to be sure that this is a normal URI, or a CURIE. Therefore the syntax for carrying a CURIE when there is any possibility of ambiguity is to enclose the CURIE in square brackets, as in the following example:
<html xmlns:wiki="http://en.wikipedia.org/wiki/"> <head>...</head> <body> <p> Find out more about <a href="[wiki:Thales]">Thales</a>. </p> </body> </html>
Note:
Not only does this abbreviate the URI, but it also makes it possible to change a whole group of URIs to point to some other source, simply by changing the namespace definition. For example, consider the following mark-up:
<html xmlns:wiki="http://en.wikipedia.org/wiki/"> <head>...</head> <body> <p> Thales had a profound influence on other Greek thinkers and therefore on Western history. Some believe <a href="[wiki:Anaximander]">Anaximander</a> was a pupil of Thales. Early sources report that one of Anaximander's more famous pupils, <a href="[wiki:Pythagoras]">Pythagoras</a>, visited Thales as a young man, and that Thales advised him to travel to Egypt to further his philosophical and mathematical studies. </p> </body> </html>
Given
that
all
references
to
Wiki
entries
are
based
on
the
namespace
defined
in
xmlns:wiki
,
then
simply
changing
this
namespace
changes
the
base
for
all
Wiki
references
within
the
document.
It
is
not
difficult
to
see
how,
by
extending
this
principle
a
user
can
begin
to
get
control
of
their
own
browsing
experience.
For
example,
a
document
might
contain
a
reference
to
a
company,
with
links
to
news
about
the
company,
financial
information
and
details
on
key
directors.
By
using
CURIEs
to
express
those
links
it
is
possible
to
use
different
sources
for
the
information,
event
to
the
extent
that
they
could
be
overridden
the
user:
<html xmlns:finance="..." xmlns:news="..." xmlns:people="..."> <head>...</head> <body> <p>We hear from people in the know that the great thinker Bullwinkle is being recruited by <b>Google</b> (nasdaq: <a href="[finance:GOOG]" class="maintkrlink">GOOG</a> - <a href="[news:GOOG]">news</a> - <a href="[people:GOOG]">people</a>) was an "unconfirmed rumor", but that the search engine behemoth is indeed keen to expand its cartoon presence.</p> </body> </html>
This appendix is normative.