While that will change all the code to look for that URL.
The point of making this rewrite rule is to just affect certain URLs, not everyone. So instead of changing a piece of code that can affect thousands of clients, we just want to affect one client to call the new javascript code, instead of everyone, so thinking of solving this using mod_rewrite.
So you have to care about the cookie : this means you have to dynamically get the current cookie of the personne whose requests are going to be rewrite to the new *.v2 URL
so if you have the cookie of that personne in a variable $COOKIE, then :
sed 's|http://SUB.DOMAIN.COM/showAssignment/'"$COOKIE"'/jsmodule/Nevow.Athena|&.v2|'
Normally, we don't use SED for rewriting URLs via HTTP_HOST. We use Apache2 mod_rewrite, that is why the original poster requested a regex to a rewrite URL to another URL based on HTTP_HOST. isn't that right?
Will this work? Are you looking for a redirect? If not, I think it is better to rewrite to the substitution for Nevow.Athena.v2 rather than Nevow.Athena.v2.
Put this in the .htaccess file in your document root