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Re: Semantic URI Syntax

From: Martin Hepp <martin.hepp@ebusiness-unibw.org>
Date: Wed, 27 Jun 2012 11:26:34 +0200
Cc: Semantic Web <semantic-web@w3.org>
Message-Id: <D5BD75F1-8BE6-4B72-819C-41F3A89342A3@ebusiness-unibw.org>
To: David Sheets <sheets@alum.mit.edu>

On Jun 26, 2012, at 8:54 PM, David Sheets wrote:

> Hello,
> I am working on a semanticization of WebGL shaders [1] and I have a
> few questions regarding URI syntax conventions.
> 1. Let <URI> be an absolute URI without fragment. Is (<URI> == <URI>#)
> true?
No. URIs as identifiers are equivalent only when they are the same, character by character. Only very few encoding variants may be treated as equal by some RDF implementations.
> Is this scheme-dependent?
From the top of my head, the hash fragment is defined for HTTP and HTTPS URIs only.

> What level of normalization is this?
This I don't understand.
> Is this question answered somewhere online already?

For URIs in the context of HTTP, RFC 2616, Section 3.2.3 (http://www.ietf.org/rfc/rfc2616.txt) is the authoritative spec:


3.2.3 URI Comparison

   When comparing two URIs to decide if they match or not, a client
   SHOULD use a case-sensitive octet-by-octet comparison of the entire
   URIs, with these exceptions:

      - A port that is empty or not given is equivalent to the default
        port for that URI-reference;

        - Comparisons of host names MUST be case-insensitive;

        - Comparisons of scheme names MUST be case-insensitive;

        - An empty abs_path is equivalent to an abs_path of "/".

   Characters other than those in the "reserved" and "unsafe" sets (see
   RFC 2396 [42]) are equivalent to their ""%" HEX HEX" encoding.

   For example, the following three URIs are equivalent:


This is for using URIs as locators. Note that for the use of identifiers in RDF, even those limited transformations will not be carried out, thus creating different identifiers.

A comprehensive discussion on URI comparison in the Semantic Web is e.g. in the mailing list archive, starting with




martin hepp
e-business & web science research group
universitaet der bundeswehr muenchen

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Received on Wednesday, 27 June 2012 09:27:08 UTC

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