  |
Uniform Resource Identifier (URI):一般的構文
https://triple-underscore.github.io/rfc-others/RFC3986-ja.html#section-3
URI =scheme ":" hier-part [ "?" query ] [ "#" fragment ] hier-part = "//"authoritypath-abempty /path-absolute /path-rootless /path-empty
: はscheme と hier-part を区切るために必要
7