I don't think there's really anything you can do in the preg_replace() statement to do this (I might be wrong), but you can accomplish this easily..
if (!ereg("http://", $sText) {
$sText = "http://" . $sText;
}
// Then your preg_replace() here...
What this does is checks the initial string for "http://"...if it does not include it, then it adds it at the beginning. This is going to reak havok on ftp:// and gopher:// addresses though 🙁