Missing the definition of `host` field in the message source

[jprjr]

Hi there, I noticed that RFC2812 specifies that the host portion of the source (formerly prefix) should be hostname, ipv4 address, or ipv6 address, I've copied the relevant parts here from RFC2812 as a reference:

prefix     =  servername / ( nickname [ [ "!" user ] "@" host ] )
host       =  hostname / hostaddr
hostname   =  shortname *( "." shortname )
shortname  =  ( letter / digit ) *( letter / digit / "-" ) *( letter / digit )
ip4addr    =  1*3digit "." 1*3digit "." 1*3digit "." 1*3digit
ip6addr    =  1*hexdigit 7( ":" 1*hexdigit )
ip6addr    =/ "0:0:0:0:0:" ( "0" / "FFFF" ) ":" ip4addr

However, a lot of IRC servers in the wild support "masking" the host portion, so the client will receive lines like:

:jprjr!~jprjr@user/jprjr JOIN #someroom

This doesn't validate, since that / character isn't allowed in a hostname.

Currently, the Modern IRC docs does not specify what values are allowed in the host portion of a source