The URI 'abc://' isn't recognized as a valid URI.

'abc:' is schema.

'//': hier-part ::= "//" authority path-abempty

'': authority ::= [userinfo "@"] host [":" port]
'': host ::= IP-literal | IPv4address | reg-name
'': reg-name ::= *(...)
authority can be empty

'': path-abempty ::= *("/" segment)
can be empty too

(see RFC 3986)