Description:
------------
We have a regular expression that removed nested comments from an HTML string. It was working OK in PHP 5.3 and 5.5 but crashes in PHP 5.6.20 and 5.6.22

The included test script is a very cut down version of string being processed. The number of characters seems to be important, removing some characters from the class names will make the script run but I can't figure the logic of it.

PHP crashes without outputting any errors, I only have the Windows Event Log output:

Faulting application name: httpd.exe, version: 2.4.12.0, time stamp: 0x550eaef9
Faulting module name: php5ts.dll, version: 5.6.22.0, time stamp: 0x57468b87
Exception code: 0xc00000fd
Fault offset: 0x0011ddd2
Faulting process id: 0x1600
Faulting application start time: 0x01d1c0c7b01411c2
Faulting application path: C:\Apache24\bin\httpd.exe
Faulting module path: C:\php-5.6.22\php5ts.dll
Report Id: 23060e48-35e8-41c0-96e7-f75628bdf8f9
Faulting package full name: 
Faulting package-relative application ID: 

Test script:
---------------
<?php
$html = <<<EOT
    <div>
            <p>Hello World</p>
            <!-- 
            <table class="123456789132456789123456789123456789123456789">
                    <tr class="">
                        <!--<td></td>-->
                        <!--<td></td>-->
                        <td class="123456789123456789123456789123456789123456789">FooBar</td>
                    </tr>
            </table>
            -->
    </div>
EOT;

do {
    $html = preg_replace("/<!--(?(?!<!--).)*-->/sU", '', $html, -1, $count);
} while ($count);

echo $html;

Expected result:
----------------
    <div>
        <p>Hello World</p>
        
    </div>

Actual result:
--------------
Site can't be reached error