title: What's the technical reason for "lookbehind assertion MUST be fixed length" in regex?
body: <p>For example,the regex below will cause failure reporting <strong>lookbehind assertion is not fixed length</strong>:</p>
  <pre><code>#(?&lt;!(?:(?:src)|(?:href))=[&quot;\']?)((?:https?|ftp)://[^\s\'&quot;&lt;&gt;()]+)#S
  </code></pre>
  <p>Such kind of restriction doesn't exist for lookahead.</p>