CURIE Syntax 1.0

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This document is an updated working draft based upon comments received since the last draft. Originally this document was 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 XHTML 2 Working Group [XHTML2WG]. It is not yet stable, but has had extensive review and some use in other W3C documents. 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 XHTML 2 Working Group as part of the HTML Activity. The goals of the XHTML 2 Working Group are discussed in the XHTML 2 Working Group charter.

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1. Introduction

More and more languages 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 attempting to use QNames to keep the amount of data being transferred as small as possible. In other words, instead of sending lots of long URIs, QNames are sometimes 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:

This is not a valid QName simply because '0321154991' is not a valid element name. Yet, in the 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.

Note that this specification is targeted at markup language designers, not document authors. Any language designer considering the use of QNames in attribute values should consider instead using CURIEs, since CURIEs are designed for this purpose, while QNames are not.

1.1. Existing Uses of CURIEs

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:

1.1.1. Wikis

Many Wikis support a feature where a prefix like isbn can be substituted for something like:

2. Conformance Requirements

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].

2.1. User Agent Conformance

A conforming user agent must support all of the features required in this specification.

3. Syntax

A CURIE is by definition a superset of a QName. It is comprised of two components, a prefix and a reference. The prefix is separated from the reference by a colon (:). It is possible to omit both the prefix and the colon, or to omit just the prefix and leave the 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 ([, ]).

When CURIES are used in an XML-based host language, prefix values MUST be able to be defined using the 'xmlns:' syntax specified in [XMLNAMES]. Such host languages MAY also provide additional prefix mapping definition mechanisms.

When CURIES are used in a non-XML host language, the host language MUST provide a mechanism for defining the mapping from the prefix to an IRI.

A host language MAY provide a mechanism for defining a default prefix value. In such a host language, if the prefix is omitted from a CURIE, the default prefix value is used.

The concatenation of the prefix associated with a CURIE and its reference MUST be an IRI [IRI].

The CURIE prefix '_' is reserved. For this reason, prefix declarations using '_' SHOULD be avoided by authors.

Host languages MAY define additional constraints on these syntax rules when CURIES are used in the context of those host languages. Host languages MUST NOT relax the constraints defined this specification.

4. Incorporating CURIEs into Markup Languages

Each host language that incorporates CURIEs supplies a mechanism for defining prefix mappings. In the case of XML-based host languages, one such mechanism is required to be xmlns. This section illustrates some possible alternative mapping mechanisms available in various existing languages.

NOTE: There are a number of different situations where QNames are currently used. There will be more illustrations in the next draft.

4.1. SPARQL

The [SPARQL] language provides a PREFIX keyword for defining the prefix used in their CURIE-like identifiers.

4.2. HTML 4.01

HTML 4.01 does not currently employ CURIEs. An extension to HTML 4.01 to support RDFa, however, has been discussed. Such an extension would need to define a prefix mapping mechanism in order to support the use of CURIEs in the RDFa attributes. For example:

4.3. XHTML 2

XHTML 2 incorporates RDFa. Since XHTML 2 is an XML-based markup language, documents annotated with RDFa could use the xmlns mechanism to define prefixes. However XHTML 2 also defines a special "prefix" value for the property attribute. So, in XHTML 2 the following would work:

<html xmlns="http://www.w3.org/1999/xhtml"
      xmlns:dc="http://purl.org/dc/elements/1.1/">
    <head>
        <title>An HTML document using RDFa</title>
        <link property="prefix" content="myPrefix" href="http://www.example.com/myPrefix/" >
    </head>
    <body>
        <p about="http://www.example.com/something" rel="myPrefix:reference">
            some content was written by <span property="dc:creator">some author</span>
        </p>
    </body>
</html>

5. Usage

CURIEs can be used in exactly the same way that QNames have been used in attribute values, with the modification that the format of the strings after the colon are looser. In all cases a parsed CURIE will produce an IRI. However, the process of parsing involves substituting the value represented by the prefix for the prefix itself, and then simply appending the part after the colon.

5.1. Examples

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:

5.2. Ambiguities Between CURIEs and URIs

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:

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:

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 prefix definition. For example, consider the following mark-up:

<html xmlns:wp="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 
            <span resource="[wp:Anaximander]">Anaximander</span> 
            was a pupil of Thales. Early sources report that one of 
            Anaximander's more famous pupils,
            <span resource="[wp:Pythagoras]">Pythagoras</span>, 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 Wikipedia entries in this example are based on the prefix defined in xmlns:wp, then simply changing this prefix changes the base for all Wikipedia 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, even 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: <span resource="[finance:GOOG]" class="maintkrlink">GOOG</span>
        - <span resource="[news:GOOG]">news</span> 
        - <span resource="[people:GOOG]">people</span>) 
        was an "unconfirmed rumor", but that the search engine behemoth is 
        indeed keen to expand its cartoon presence.</p>
    </body>
</html>