Are URIs case-insensitive?

Question

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 th

Answer 1

As mentioned in answer by Remy Lebeau, the rules are set for client side. Actually, this means that client software should not try to make arbitrary case modifications to all parts of URIs, except for specifically stated parts. So, when a browser e.g. sees a relative URL in a page anchor, is should not convert it to lowercase before checking if it is already cached in its cache; neither should it use the URI lowercased to post to server. Also, it should not decide that two URIs that differ in case only point to same resource (thus possibly wrongly skipping a transaction and returning cached result instead).

This means that client should not assume how servers treat the URIs. It does require servers to treat some parts case-insensitive: e.g., scheme and host. But otherwise, it's up to server to decide if two URIs that differ in case point to the same resource, or not. Standard does not impose any restrictions on servers in this regards, there's nothing "server should" or "server should not" besides directly prescribed. If server decides that its URIs are case-insensitive, that's absolutely fine. If they are case-sensitive, that's fine, too.