title: Negative regexp in logstash configuration
body: <p>I cannot get negative regexp expressions working within LogStash (as described in <a href="http://logstash.net/docs/1.3.3/configuration#conditionals" rel="nofollow">the docs</a>)</p>
  
  <p>Consider the following positive regex which works correctly to detect fields that have been assigned a value:</p>
  
  <pre><code>if [remote_ip] =~ /(.+)/ {
      mutate { add_tag =&gt; ["ip"] }
  }
  </code></pre>
  
  <p>However, the negative expression seems to return false even when the field is blank:</p>
  
  <pre><code>if [remote_ip] !~ /(.+)/ {
      mutate { add_tag =&gt; ["no_ip"] }
  }
  </code></pre>
  
  <p>Am I misunderstanding the usage?</p>
  
  <p><strong>Update</strong> - this was fuzzy thinking on my part. There were issues with my config file. If the rest of your config file is sane, the above should work.</p>