This is correct. According to this article[0], posted in another comment[1], iOS 9&#x27;s content blocker regular expressions are limited to a subset of regex for which matching can be done in linear time:<p><i>&quot;Extremely powerful, regular expressions can surprisingly be done in linear time if you use a strict set of regular expression characters through Thompson’s NFA algorithm. Safari’s Content Blocker implementation in fact imposes this strict regular expression limitation ...&quot;</i><p>0. <a href="http:&#x2F;&#x2F;blog.appgrounds.com&#x2F;content-blockers-beat-adblocking&#x2F;" rel="nofollow">http:&#x2F;&#x2F;blog.appgrounds.com&#x2F;content-blockers-beat-adblocking&#x2F;</a><p>1. <a href="https:&#x2F;&#x2F;news.ycombinator.com&#x2F;item?id=10192562" rel="nofollow">https:&#x2F;&#x2F;news.ycombinator.com&#x2F;item?id=10192562</a>