My thought was "the same, but backwards": since everything seems OK except a group of either one or three or more colons (delimited by non-colons), look for that pattern and reject if found. 
>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 '::::'

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In reply to Re^2: Regex Validation by AnomalousMonk
in thread Regex Validation by jimb

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