Description:
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preg_replace returns null instead of a string. 
The input is an html file I was parsing. 
The bug disapears when I change the file in many ways. 
I managed to reduce the html file's size a bit, but it is still too long to paste hear, and the posting might remove caracters that might be involved in the bug.

I put the html file here: http://marcel.demosphere.fr/php-preg-bug.txt



Reproduce code:
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var_dump(preg_replace('@^.*</head>@s','',file_get_contents("bug")));

Expected result:
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preg_replace should return a string with all of the input.

Actual result:
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preg_replace returns null