bug#40582: Valid URIs are rejected

Hi,

Using (web uri), I was trying to parse "uri://a/c". Reading RFC3986, it should be a valid URI (see rule for reg-name in 3.2.2). However, passing it to string->uri results in #f. I've tracked this down to valid-host? which returns #f for "a".

The reason is that the regexp checking if the host is an ipv6 matches "a", which shouldn't happen because a is not an ipv6 address. Indeed, when I try (string->uri "uri://g/b"), I get the expected result.