preg_replace_callback and brackets

I'm kind of confused right now... when I call preg_replace_callback with a regular expression with brackets, sometimes I get an array back with multiple elements like array(3) { [0]=> string(19) "http://www.aol.com " [1]=> string(0) "" [2]=> string(19) "http://www.aol.com " } and I have no idea why... Why does the middle one have nothing in it? the regular expression was $dumb_body = preg_replace_callback ( '/' //Case spaceURLspace . '([\s]http:\/\/[^\s]+[\s])' . '|' //Case URLspace . '(^http:\/\/[^\s]+[\s])' . '|' //Case spaceURL . '([\s]http:\/\/[^\s]+$)' . '|' //Case URL . '(^http:\/\/[^\s]+$)' . '/i' , array( &$this, "storeUrlCallback" ), $dumb_body ); I am very confused at the point of the array in this function period... can anyone help? Josh.