$re = '~\[img]([^\'\"&?]*?/([^\'\"&?/]*\.(?:gif|jpe?g|png|bmp)))\[\/img]~';
$str = 'style_Forever 21 [img]http://cdn9.lbstatic.nu/files/looks/medium/2013/08/15/3225332_IMG_20130810_2.jpg[/img][img]http://cdn11.lbstatic.nu/files/looks/medium/2013/08/15/3225294_1000340_10201946229836897_118758683_n.jpg[/img]';
$subst = "<img src=\"$1\" alt=\"$2\"/>";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php