title: Difference of two character vectors with substring
body: <p>I have two lists:</p>
  
  <pre><code>a &lt;- c("da", "ba", "cs", "dd", "ek")
  b &lt;- c("zyc", "ulk", "mae", "csh", "ddi", "dada")
  </code></pre>
  
  <p>I want to remove the elements from list b which would have a substring match with any of the values in a, e.g.</p>
  
  <pre><code>grepl("da","dada") # TRUE
  </code></pre>
  
  <p>How would you go about doing this efficiently?</p>