#!/usr/bin/perl

use strict;
use warnings;

my $text=<<EOT;
[youtube][/youtube]
[youtube]1234_Abc[/youtube]
[youtube]http://www.youtube.de/?v=1234_Abc[/youtube]
[youtube]http://www.youtube.de/embed/1234_Abc[/youtube]
[youtube]foo bar bam 123[/youtube]
EOT

$text =~ s#\Q[youtube]\E\s*(?:\Qhttp://www.youtube.\E.+?(?:v=(\w+)|embed/(\w+))|(\w+)|.*?)\s*\Q[/youtube]\E#
    my $val=$1 || $2 || $3;
    $_=$val?qq(<iframe width="400" height="300" src="http://www.youtube.com/embed/$val" frameborder="0" allowfullscreen></iframe>):'';
  #isge;

print $text;