Description:
------------
The preg_replace function returns null when a certain string is passed as 
haystack. Removing some lines from the string fixes the issue. The search 
pattern is not even found in the problematic string.

This problem occurs with all PHP 5 versions I tested (5.2.0, 5.2.4, 5.2.5). It does 
not occur with PHP 4.

(The search pattern uses back-references in case that matters.)

Reproduce code:
---------------
Run this script: http://dfoerster.de/misc/preg_replace_bug

It prints the string length before and after the matching. With PHP 5 the length after the matching is 0, with PHP 4 it's the same as before as no replacement takes places.

Expected result:
----------------
strlen returns the same length as before the matching.

Actual result:
--------------
strlen returns 0 after matching.