- Updates codes based on Taut's comments

lib/Newsletter/Shortcodes/Shortcodes.php

@@ -4,7 +4,6 @@ namespace MailPoet\Newsletter\Shortcodes;
 class Shortcodes {
   public $newsletter;
   public $subscriber;
-  static $shortcodes_regex = '/\[(?:\w+):.*?\]/ism';
 
   function __construct(
     $newsletter = false,
@@ -15,7 +14,7 @@ class Shortcodes {
   }
 
   function extract($text) {
-    preg_match_all(self::$shortcodes_regex, $text, $shortcodes);
+    preg_match_all('/\[(?:\w+):.*?\]/ism', $text, $shortcodes);
     return array_unique($shortcodes[0]);
   }