title: Regular Expression in R with a negative lookbehind
body: <p>So I have the following data, let's say called "my_data":</p>
  
  <pre><code>Storm.Type
  TYPHOON
  SEVERE STORM
  TROPICAL STORM
  SNOWSTORM AND HIGH WINDS
  </code></pre>
  
  <p>What I want is to classify whether or not each element in my_data$Storm.Type is a storm, BUT I don't want to include tropical storms as storms (I'm going to classify them separately), such that I would have</p>
  
  <pre><code>Storm.Type                    Is.Storm
  TYPHOON                       0
  SEVERE STORM                  1
  TROPICAL STORM                0
  SNOWSTORM AND HIGH WINDS      1
  </code></pre>
  
  <p>I have written the following code:</p>
  
  <pre><code>my_data$Is.Storm  &lt;-  my_data[grep("(?&lt;!TROPICAL) (?i)STORM"), "Storm.Type"]
  </code></pre>
  
  <p>But this only returns the "SEVERE STORM" as a storm (but leaves out SNOWSTORM AND HIGH WINDS).  Thank you!</p>