title: Positive lookbehind vs match reset (\K) regex feature
body: <p>I just learned about the <a href="http://ruby-doc.org/core/Regexp.html#class-Regexp-label-Anchors" rel="noreferrer">apparently undocumented</a> <code>\K</code> behavior in Ruby regex (thanks to <a href="https://stackoverflow.com/a/35092157/513951">this answer</a> by <a href="https://stackoverflow.com/users/548225/anubhava">anubhava</a>). This feature (possibly named <em>Keep</em>?) also exists in PHP, Perl, and Python regex flavors. It is described elsewhere as "<em>drops what was matched so far from the match to be returned</em>."</p>
  
  <pre><code>"abc".match(/ab\Kc/)     # matches "c"
  </code></pre>
  
  <p>Is this behavior identical to the positive lookbehind marker as used below?</p>
  
  <pre><code>"abc".match(/(?&lt;=ab)c/)  # matches "c"
  </code></pre>
  
  <p>If not, what differences do the two exhibit?</p>