I have noticed some strange behavior of debugger.

I debugged simple file :

<?php
$subject = "abcdef";
$pattern = '/def/';
preg_match($pattern, $subject, $matches);
print_r($matches);
?>

When run by RUN mode it outputs source :

Array ( [0] => def )

But when debugging it outputs source:

Array ( )

It seems to "forget" the values of array at the very end of debugging process and thinks that the array to output is empty.

I'd suggest you to contact support and report this as a bug.