Description:
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I found a situation where preg_replace returns NULL instead of the expected string. The regexp I use is (perhaps) erroneous: "/^\s*(.*?)\s*$/s"
But when the string size is less than 49997 characters, the function returns the string properly. Over that size, the function returns NULL.

Reproduce code:
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$a = str_repeat('a', 49998);
$a = preg_replace('/^\s*(.*?)\s*$/s', '$1', $a);


Expected result:
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$a

Actual result:
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NULL