Rule source on GitHub

DowngradePregUnmatchedAsNullConstantRector

Remove PREG_UNMATCHED_AS_NULL from preg_match and set null value on empty string matched on each match

 class SomeClass
 {
     public function run()
     {
-        preg_match('/(a)(b)*(c)/', 'ac', $matches, PREG_UNMATCHED_AS_NULL);
+        preg_match('/(a)(b)*(c)/', 'ac', $matches);
+
+        array_walk_recursive($matches, function (&$value) {
+            if ($value === '') {
+                $value = null;
+            }
+        });
     }
 }

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\DowngradePhp72\Rector\FuncCall\DowngradePregUnmatchedAsNullConstantRector;

return RectorConfig::configure()
    ->withRules([
        DowngradePregUnmatchedAsNullConstantRector::class,
    ]);