Multiple keywords in dc:subject

I've been adding tagging/category support to my RSS aggregator and I've 
noticed that a lot of people use the dc:subject element for tagging entries 
in their RSS feeds (Atom too). The problem is that nobody does it 
consistently and I can't seem to come up with a safe way of parsing 
dc:subject that will work for everyone.

According to the Dublin Core Usage Guide when using multiple keywords you 
should "either separate terms with semi-colons or use separate iterations of 
the Subject element". [1]

I've never seen semi-colons used in an RSS feed, but multiple subject 
elements are quite common (e.g. Danny Ayers' blog [2]) and I've also seen 
commas used as a separator (e.g. Simon Willison's blog [3]). Both these 
cases are fairly straightforward to parse (as long as Danny doesn't include 
commas in his categories which I think is fairly safe if not guaranteed). 
However, the real problem arises when people include multiple keywords in a 
subject element separated only by a space (e.g. flickr[4] and 
del.icio.us[5]).

If I split subject elements into multiple keywords whenever there is a space 
I'll break Danny's feed and Simon's feed (they use tags such as "Semantic 
Web" and "Web Services"). However if I treat a subject element with no 
special separators as a single keyword/category then I'll break both flickr 
and del.icio.us (I would imagine two of the biggest providers of tagged 
feeds). I'm fairly sure flickr and del.icio.us are doing it wrong, but I'm 
not sure it would be very easy to get them to fix their code.

So what is the solution? Any suggestions would be greatly appreciated.

Regards
James

[1] http://dublincore.org/documents/usageguide/elements.shtml#subject
[2] http://dannyayers.com/feed/rdf/
[3] http://simon.incutio.com/syndicate/rss1.0
[4] http://www.flickr.com/groups/interestingness/pool/feed/?format=atom_03
[5] http://del.icio.us/rss/


 
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