title: javascript regexp negative lookbehind doesn't match in some case
body: <p>i use node 10.15</p>
  
  <p>i want the expression to match only line2 but not line 1 and line 3</p>
  
  <p><a href="https://i.stack.imgur.com/13LAX.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/13LAX.jpg" alt="enter image description here"></a></p>
  
  <p>why this negative lookbehind failed?</p>
  
  <pre><code>(?&lt;!月)\d+日(?!后)
  </code></pre>