title: Nifi regex on attributes
body: <p>I have flowfiles with filename like that:
  <code>xxx2019xxx.txt</code></p>
  <p>Where <code>xxx</code> are letters, I want to extract the year (<code>2019</code> or whatever looks like a 4 digits numbers) within the filename. It seems to me that the Expression Language´s regex functions like <code>matches(...)</code> just return a boolean value. Any ideas how to extract the year?</p>
  <p>Thank you and best regards.</p>