title: What does double square brackets [[]] mean in a regex?
body: <p>I ran into this RegExp <code>/[[0]]/</code> in JavaScript, and have been wondering what it means.</p>
  
  <p>I know that the outer pair of <code>[]</code> should be use as a character class, but what about the inner pair? I searched in Google and found <a href="http://www.regular-expressions.info/posixbrackets.html#coll" rel="nofollow">this link</a>, but "Collating Sequences" doesn't seem to be it 'cuz I can't get <code>/[[.some.]]/</code> work as this page claims.</p>
  
  <p>Thanks in advance if anyone can give me a hint.</p>