title: Syntax for multiple positive lookaheads in JavaScript regex
body: <p>I'm trying to include two positive lookaheads in one regex. Here's the problem I'm working on as an example.</p>
  
  <pre><code>(?=[a-zA-Z])(?=[0-9])[a-zA-Z0-9]{0,20}
  </code></pre>
  
  <p>This is what I'm trying to match:</p>
  
  <ul>
  <li>0-20 characters</li>
  <li>one or more letter anywhere</li>
  <li>one or more number anywhere</li>
  <li>only letters and numbers allowed</li>
  </ul>
  
  <p>When I do this with only one lookahead, it works, but as soon as I add the other, it breaks. What's the correct syntax for two lookaheads?</p>