Negative Look Ahead Regex - Code Mistake or Bug?

Hello, I¹ve run into a problem with a regex and need help determining if this is my mistake or a bug. The regex is for inserting a SID into every link in the buffer before it¹s flushed, but only when each link doesn¹t already have a SID. An example of the code is here: $buffer = preg_replace('/"http\:\/\/www\.domain\.com([\/\w\.-]*)(?!\?PHPSESSID\=2u0cca ffoh6jaeapkke35qpp87;?)/', '"http://www.domain.com$1?PHPSESSID=t9gksvpdcuobsnqt98qloe6lg4;', $buffer); The code works correctly in most cases except when a SID already exists and there is a path after the .com such as: http://www.domain.com/path1/path2.php?PHPSESSID=2u0ccaffoh6jaeapkke35qpp87; When this URL is processed by the regex, it becomes: http://www.domain.com/path1/path2.ph?PHPSESSID=t9gksvpdcuobsnqt98qloe6lg4;p? PHPSESSID=2u0ccaffoh6jaeapkke35qpp87; So the problem is the regex not only falsely matches, it's inserting the SID in between 2nd and 3rd letters of the extension. You can see the code in action here: http://kottmann.com/test.php The full source for the test code is also available when viewing the source of this page. I've tested this on PHP v5.1.6 and PHP v5.2.6. TIA, Cameron