Description:
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We had this bug on our production server happening on matching Regex on a unicode string decoded from JSON.

I've taken the exact production code and removed everything that is not necessary for demonstrating the problem. Removing any other part from the snippet bellow fixes the crash.

It seems the issue only appeared in PHP 7.4: https://3v4l.org/DqL76

Test script:
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// if "’" string is used directly without json_decode, 
// the issue does not reproduce
$text = json_decode('"’"'); 

$pattern = '/\b/u';

// it has to be exact two calls to preg_match(), 
// with the second call offsetting after the tick symbol
preg_match($pattern, $text, $matches, 0, 0);
preg_match($pattern, $text, $matches, 0, 1);

echo 'OK';

Expected result:
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"OK" is output

Actual result:
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Segmentation fault (core dumped)