title: grep with special characters
body: <p>I want to find the elements that contains star character in the following vector.</p>
  
  <pre><code>s &lt;- c("A","B","C*","D","E*")
  grep("*",s)
  
  [1] 1 2 3 4 5
  </code></pre>
  
  <p>This is not working. I can understand that since it is a special character.<br>
  When I read <a href="http://www.regular-expressions.info/characters.html" rel="noreferrer">here</a>, I decided to use "\" before star character. But that gave me an error:</p>
  
  <pre><code>grep("\*",s)
  Error: '\*' is an unrecognized escape in character string starting ""\*"
  </code></pre>
  
  <p>What am I doing wrong?</p>