CURIE Syntax 1.0

A syntax for expressing Compact URIs

W3C Editor's Draft 05 September 2007

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Mark Birbeck, x-port.net Ltd. <Mark.Birbeck@x-port.net>
Shane McCarron, Applied Testing and Technology, Inc.

This document is also available in these non-normative formats: PostScript version, PDF version.

The English version of this specification is the only normative version. Non-normative translations may also be available.


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 Language designers, not the users of those Languages.

Status of this Document

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 [XHTML2WG]. 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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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).

Table of Contents

1. Introduction

This section is informative.

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.

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:




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>

2. Conformance Requirements

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].

2.1. User Agent Conformance

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

3. Syntax

This section is normative.

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 (:). 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 ([, ]).

    safe_curie  :=   '[' basic_curie ']'

    curie       :=   [ prefix [ ':' ] ] reference

    prefix      :=   NCName

    reference   :=   irelative-ref (as defined in IRI)

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

This section is informative.

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.


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

PREFIX foaf:   <http://xmlns.com/foaf/0.1/> 
SELECT ?x ?name 
WHERE  { ?x foaf:name ?name }

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:

        <title>An HTML document using RDFa</title>
        <meta scheme="prefix" name="myPrefix" content="http://www.example.com/myPrefix/" >
        <p about="http://www.example.com/something" rel="myPrefix:reference">
            some content

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"
        <title>An HTML document using RDFa</title>
        <link property="prefix" content="myPrefix" href="http://www.example.com/myPrefix/" >
        <p about="http://www.example.com/something" rel="myPrefix:reference">
            some content was written by <span property="dc:creator">some author</span>

5. Usage

This section is informative.

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 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 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:

Why not <a href="mailto: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:wp="http://en.wikipedia.org/wiki/">
            Find out more about <a href="[wp:Thales]">Thales</a>.


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/">
            Thales had a profound influence on other Greek thinkers and therefore on Western history.
            Some believe <a href="[wp:Anaximander]">Anaximander</a> was a pupil of Thales. Early
            sources report that one of Anaximander's more famous pupils,
            <a href="[wp: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.

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="..."
        <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>

A. References

This appendix is informative.

XML Schema Part 2: Datatypes Second Edition: Section 3.2.18 QName (See http://www.w3.org/TR/xmlschema-2/#QName.)
Namespaces in XML: Section 3: Qualified Names (See http://www.w3.org/TR/REC-xml-names/#dt-qualname.)
RDF-in-HTML Task Force (See http://w3.org/2001/sw/BestPractices/HTML/.)
Semantic Web Best Practices and Deployment Working Group (See http://w3.org/2001/sw/BestPractices/.)
XHTML 2 Working Group (See http://w3.org/MarkUp/.)
"Internationalized Resource Identifiers (IRI)", RFC 3987, M.Duerst, M. Suignard January 2005.
Available at: http://www.ietf.org/rfc/rfc3987.txt
"Key words for use in RFCs to indicate requirement levels", RFC 2119, S. Bradner, March 1997.
Available at: http://www.ietf.org/rfc/rfc2119.txt
"Uniform Resource Identifiers (URI): Generic Syntax", RFC 3986, T. Berners-Lee et al., January 2005.
Available at: http://www.rfc-editor.org/rfc/rfc3986.txt. This RFC updates RFC 1738 [URI] and obsoletes RFC 2732, 2396 and 1808.
"XHTML™ 2.0". J. Axelsson et al., 26 July 2006.
Available at: http://www.w3.org/TR/2006/WD-xhtml2-20060726
The latest version is available at: http://www.w3.org/TR/xhtml2
"Extensible Markup Language (XML) 1.0 (Third Edition)", W3C Recommendation, T. Bray, J. Paoli, C. M. Sperberg-McQueen, E. Maler, F. Yergeau, eds., 2 February 2004.
Available at: http://www.w3.org/TR/2004/REC-xml-20040204
"Namespaces in XML", W3C Recommendation, T. Bray, D. Hollander, A. Layman, eds., 14 January 1999.
Available at: http://www.w3.org/TR/1999/REC-xml-names-19990114