>perl -wMstrict -le
"my $invalid = qr{ (?<! :) (?: : | :{3,}) (?! :) }xms;
 print '------ output ------';
 for my $s (@ARGV) {
   print $s =~ $invalid ? 'NO' : 'OK', qq{ '$s'};
   }
"
"sip:userid@example.com"   "sip::userid@example status::code"
"::"   "::a::"   ":::bad_example::"   ":"   ":::"   "::::"
------ output ------
NO 'sip:userid@example.com'
OK 'sip::userid@example status::code'
OK '::'
OK '::a::'
NO ':::bad_example::'
NO ':'
NO ':::'
NO '::::'