title: regex lookahead assertion
body: <p>I'm new to python regex and am learning the lookahead assertion. </p>
  
  <p>I found the following strange. Could someone tell me how it works? </p>
  
  <pre><code>import regex as re
  re.search('(\d*)(?&lt;=a)(\.)','1a.')
  &lt;regex.Match object; span=(2, 3), match='.'&gt;
  
  re.search('(\d+)(?&lt;=a)(\.)','1a.')
  out put nothing
  </code></pre>
  
  <p>Why doesn't the second one match anything?</p>