title: Flutter cannot parse regex
body: <p>Flutter cannot parse <a href="https://regex101.com/r/H9NUV7/3" rel="nofollow noreferrer">this working regex</a>, and doesn't return any error or info.</p>
  
  <pre><code>(?&lt;=id=)[^&amp;]+
  </code></pre>
  
  <p>However, when I add it into my Flutter app:</p>
  
  <pre><code>print("before");
  new RegExp(r'(?&lt;=id=)[^&amp;]+');
  print("after");
  </code></pre>
  
  <p>It doesn't do anything, doesn't return any error. The <code>print("after");</code> never gets executed. It doesn't completly freeze the app, because it's in async.</p>