title: Mask a part of creditcard number using logstash grok filter
body: <p>Trying to mask out a part (6-10th digit) of the 16 digit credit card.</p>
  
  <p><code>1234567898763456</code> to <code>123456######3456</code></p>
  
  <p>I can think of using gsub filter, but I can't find a way to match the exact section of numbers.</p>
  
  <p>The option would be to split in various sections and then gsub the whole field to '#' and then join back, seems too much work. Any suggestion would be welcome.</p>
  
  <p>Below example would mask all characters, how do I mask selective pattern, from 6th to 10th digit, to get a result like <code>123456######3456</code>.</p>
  
  <pre><code>filter
  {
    mutate {
      gsub =&gt; [
        "message","[0-9]{16}","################"   
      ]
    }
  }
  </code></pre>