title: Filter rows which contain a certain string
body: <p>I have to filter a data frame using as criterion those row in which is contained the string <code>RTB</code>.</p>
  <p>I'm using <code>dplyr</code>.</p>
  <pre><code>d.del &lt;- df %&gt;%
    group_by(TrackingPixel) %&gt;%
    summarise(MonthDelivery = as.integer(sum(Revenue))) %&gt;%
    arrange(desc(MonthDelivery))
  </code></pre>
  <p>I know I can use the function <code>filter</code> in <code>dplyr</code> but I don't exactly how to tell it to check for the content of a string.</p>
  <p>In particular I want to check the content in the column <code>TrackingPixel</code>. If the string contains the label <code>RTB</code> I want to remove the row from the result.</p>