The scheme delimiter is [always `:` for all protocols](http://www.ietf.org/rfc/rfc1738.txt):

> In general, URLs are written as follows:
>
>     <scheme>:<scheme-specific-part>

e.g. it's `mailto:foo@example.com`, not `mailto://foo@example.com`.

The `//` syntax is not part of the general URI syntax, it belongs to HTTP-scheme-specific part.