in reply to Regex Validation

The magic you need is to look around. Consider:

use strict;
use warnings;

my @tests = (
    'sip:userid@example.com',
    'sip::userid@example status::code',
    '::',
    '::a::',
    ':::bad_example::',
);

print /^([^:]|(?<!:)::(?!:))+$/ ? "OK $_\n" : "NO $_\n" for @tests;

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BTW, the flack you got was for not (obviously) showing what you had tried. Although you did actually show [^:]+, because you didn't bother with code tags the square brackets turned the ^: into a link. There is a strong requirement here for supplicants to demonstrate that they have actually had a go at solving the problem themselves by showing the code they have tried.

True laziness is hard work

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Re^2: Regex Validation
by AnomalousMonk (Archbishop) on Jul 30, 2009 at 03:18 UTC
    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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