Work-around 5.3 default value bug

Apparently in 5.3 you cannot provide "null" as a default value; this appears to have been corrected as of PHP 5.4.8: http://php.net/manual/en/function.mb-substr.php#116272

src/Util/RegexHelper.php

@@ -192,7 +192,7 @@ public function getHRuleRegex()
     public static function matchAt($regex, $string, $offset)
     {
         $matches = array();
-        $string = mb_substr($string, $offset, null, 'utf-8');
+        $string = mb_substr($string, $offset, mb_strlen($string), 'utf-8');
         if (!preg_match($regex, $string, $matches, PREG_OFFSET_CAPTURE)) {
             return null;
         }