<?php
$good="http://www.google.com/";
$bad="http://img.google.com/";
$r="#(?i)((?:f|ht)tps?://((?!img|jpe?g|gif|png|bmp))(?:([-a-z0-9()@:%_+.~\#?&;/=,])(?2))+(?!(?3)))#";
$rep="<a href=$1 target='_blank'><b>$1</b></a>"; 
echo preg_replace($r,$rep,$good); echo "\n";
echo preg_replace($r,$rep,$bad);
