Description:
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Capturing subpatterns that don't match anything still show an empty string in the match array.

Since empty string is a legitimate value, the match array should have null or false instead.

The workaround to detect a capturing subpattern not matching anything is to use PREG_OFFSET_CAPTURE and look for offset -1 for the particular subpattern.

Test script:
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<?php

preg_match('%(a)|(b)%','b',$m);
var_dump($m);

//$m[1] === "" as if the first capturing parentheses matched ""
//$m[1] === null would be more appropriate (javascript uses "undefined")