Følgende er aftestet
$test = preg_replace("/\[url=([^\]]+)\]([^\[]+)\[\/url\]/i","<a href=\"\\1\" target=\"_blank\">\\2</a>",$test);
$test = preg_replace("/\[gbi\]([^\[]+)\[\/gbi\]/i","<a href=\"side.php?id=\\1\">se indlæg</a>",$test);
Og et opstillet test scenarie
<?php
/*
Konverteringer
linktekst -> <a href="
http://noget.dk" target="_blank">linktekst</a>
[gbi]20040219124230[/gbi] -> <a href="side.php?id=20040219124230">se indlæg</a>
*/
$test = 'abc
linktekst def [gbi]20040219124230[/gbi] ghi';
echo "Original:<br>";
echo $test."<br>";
$test = preg_replace("/\[url=([^\]]+)\]([^\[]+)\[\/url\]/i","<a href=\"\\1\" target=\"_blank\">\\2</a>",$test);
$test = preg_replace("/\[gbi\]([^\[]+)\[\/gbi\]/i","<a href=\"side.php?id=\\1\">se indlæg</a>",$test);
echo "<br>Ny:<br>";
echo $test."<br>";
?>