Re: [PATCH v3 1/3] send-mail: improve checks for valid_fqdn

Aditya Garg [off-list ref] writes:

quoted

 * ... but I don't know if your use of negative lookaround
   assersions is correct.  Shouldn't the "a label cannot begin or
   end with dash" be applied not just to the first label but
   consistently to all of the dot-separated labels?

I think you are talking about this case:

someone.-example.com

No, its not valid.a

I was not talking about any "case"; I was talking more about your
regexp to catch invalid addresses.  From RFC 5321,

    helo       = "HELO" SP Domain CRLF
    Domain     = sub-domain *("." sub-domain)
    sub-domain = Let-dig [Ldh-str]
    Let-dig    = ALPHA / DIGIT
    Ldh-str    = *( ALPHA / DIGIT / "-" ) Let-dig

so the syntax for first "sub-domain" applies equally to the other
"sub-domain".  If "-example" cannot be the third-level subdomain,
then it equally cannot be the second-level, either, but IIRC, the
patch had regexp that treated the first level differently from the
rest.