title: Logstash conditional to check if tag exists?
body: <p>Is there any way in logstash to use a conditional to check if a specific tag exists?</p>
  
  <p>For example,</p>
  
  <pre><code>grok {
  match =&gt; [
  "message", "Some expression to match|%{GREEDYDATA:NOMATCHES}"
  ]
  </code></pre>
  
  <p>if NOMATCHES exists Do something.</p>
  
  <p>How do I verify if NOMATCHES tag exists or not?</p>
  
  <p>Thanks.</p>