title: Google BigQuery extract string from column with regexp_extract
body: <p>I need to extract the app_id from the following string in the column below: </p>
  
  <pre><code>&amp;app_id=4.25.9&amp;
  </code></pre>
  
  <p>so anything that starts with <code>&amp;app_id=</code> and ends with <code>&amp;</code></p>
  
  <p>Any ideas on how to write this example? The number of characters and symbols may differ.</p>