Description:
------------
The preg_replace_callback manual-entry says:

 If matches are found, the new subject will be returned, otherwise subject will be returned unchanged.

However; I have a testcase where no input is returned; and my callback isn't called either. This seems to be caused by a required pcre.backtrack_limit of 10000000

Shouldn't PHP return the input or an error giving some pointers about this limit being reached?

Reproduce code:
---------------
<?PHP
$input = '{?value==input.value}';
$regexp = '~\{\?(?:\(?!?.+(==)?.+\)?)+\}{\1\/}~sU';

var_dump(preg_replace_callback($regexp, 'callback', $input));

function callback(){
        die('called');
}


Expected result:
----------------
I expect an error to be raised when pcre.backtrack_limit is reached

Actual result:
--------------
NULL is var_dumped