Ripped straight from vB:
Code:
$bbcode=eregi_replace("\\[url\\]www.([^\\[]*)\\[/url\\]","<a href=\"http://www.\\1\" target=_blank>\\1</a>",$bbcode);
$bbcode=eregi_replace("\\[url\\]([^\\[]*)\\[/url\\]","<a href=\"\\1\" target=_blank>\\1</a>",$bbcode);
// do yyy
$bbcode=eregi_replace("\\[url=\"([^\"]*)\"\\]([^\\[]*)\\[\\/url\\]","<a href=\"\\1\" target=_blank>\\2</a>",$bbcode);
$bbcode=eregi_replace("\\[url=([^\"]*)\\]([^\\[]*)\\[\\/url\\]","<a href=\"\\1\" target=_blank>\\2</a>",$bbcode);
it's a regular expression that should take care of your problem