title: NiFi: GetHTTP Processor Regular Expression Invalid Error
body: <p>I have a simple NiFi flow, with <code>GetHTTP</code> and <code>PutFile</code> processors. I am trying to connect the <code>GetHTTP</code> processor to the DC Metro data API, with this link:</p>
  
  <pre><code>https://api.wmata.com/TrainPositions/TrainPositions?contentType={contentType}
  </code></pre>
  
  <p>(The website can be <a href="https://developer.wmata.com/" rel="nofollow noreferrer">found here</a>)</p>
  
  <p>I can get this error:</p>
  
  <p><a href="https://i.stack.imgur.com/8C2BX.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/8C2BX.png" alt="enter image description here"></a></p>
  
  <p>I can't debug this error in the log, since it has not run yet. I also cannot find any other examples of this error. I put the link above in the <code>URL</code> part of the configuration, and gave it a sample <code>Filename</code> of <code>wmata_data.json</code>. Thanks. </p>