URIについて

えと…すみません考察してないメモ書きです。
んと。発端は「domain名だけのURIのおけつに/(スラッシュ)は必須なのか？」って話。
いわゆる「ちょいと口うるさい方」が割とこの辺を突っ込むのですが(URIでdomainのみのケース 例えばhttp://domain.com でも、最後に/がないのはおかしい)。
RFCを見てみると………なんか違うような。

RFC 3986のABNFを見てみると

URI = scheme ":" hier-part [ "?" query ] [ "#" fragment ]

hier-part = "//" authority path-abempty
/ path-absolute
/ path-rootless
/ path-empty

URI-reference = URI / relative-ref

absolute-URI = scheme ":" hier-part [ "?" query ]

relative-ref = relative-part [ "?" query ] [ "#" fragment ]

relative-part = "//" authority path-abempty
/ path-absolute
/ path-noscheme
/ path-empty

scheme = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )

authority = [ userinfo "@" ] host [ ":" port ]
userinfo = *( unreserved / pct-encoded / sub-delims / ":" )
host = IP-literal / IPv4address / reg-name
port = *DIGIT

IP-literal = "[" ( IPv6address / IPvFuture ) "]"

IPvFuture = "v" 1*HEXDIG "." 1*( unreserved / sub-delims / ":" )

IPv6address = 6( h16 ":" ) ls32
/ "::" 5( h16 ":" ) ls32
/ [ h16 ] "::" 4( h16 ":" ) ls32
/ [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32
/ [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32
/ [ *3( h16 ":" ) h16 ] "::" h16 ":" ls32
/ [ *4( h16 ":" ) h16 ] "::" ls32
/ [ *5( h16 ":" ) h16 ] "::" h16
/ [ *6( h16 ":" ) h16 ] "::"

h16 = 1*4HEXDIG
ls32 = ( h16 ":" h16 ) / IPv4address
IPv4address = dec-octet "." dec-octet "." dec-octet "." dec-octet

dec-octet = DIGIT ; 0-9
/ %x31-39 DIGIT ; 10-99
/ "1" 2DIGIT ; 100-199
/ "2" %x30-34 DIGIT ; 200-249
/ "25" %x30-35 ; 250-255

reg-name = *( unreserved / pct-encoded / sub-delims )

path = path-abempty ; begins with "/" or is empty
/ path-absolute ; begins with "/" but not "//"
/ path-noscheme ; begins with a non-colon segment
/ path-rootless ; begins with a segment
/ path-empty ; zero characters

path-abempty = *( "/" segment )
path-absolute = "/" [ segment-nz *( "/" segment ) ]
path-noscheme = segment-nz-nc *( "/" segment )
path-rootless = segment-nz *( "/" segment )
path-empty = 0

segment = *pchar
segment-nz = 1*pchar
segment-nz-nc = 1*( unreserved / pct-encoded / sub-delims / "@" )
; non-zero-length segment without any colon ":"

pchar = unreserved / pct-encoded / sub-delims / ":" / "@"

query = *( pchar / "/" / "?" )

fragment = *( pchar / "/" / "?" )

pct-encoded = "%" HEXDIG HEXDIG

unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~"
reserved = gen-delims / sub-delims
gen-delims = ":" / "/" / "?" / "#" / "[" / "]" / "@"
sub-delims = "!" / "$" / "&" / "'" / "(" / ")"
/ "*" / "+" / "," / ";" / "="

とありまして。
…きっちり紐解いてはいないのですが。

ド端的に、
hier-part = "//" authority path-abempty
authority = [ userinfo "@" ] host [ ":" port ]
path-abempty = *( "/" segment )
から見るに。
path-abemptyがemptyなら、必然的に先頭にある/(スラッシュ)も無くなるんじゃないかなぁと思うのですが。
つまり「http://domain.com」は「RFC的に正しい」と思われるのですが…なんかABNF読み間違えてますかねぇ？

最近割と気になってるので、ちと「後で考察しよう」と思いつつ、memoり。