title: R, remove row if there is a certain character
body: <p>if my data frame looks like this, I want to remove the rows if there is "A" in the A column</p>
  
  <pre><code>df    
     A            B       C
  1 "4:10:05A"  James    Math
  2 "3:10:45"   Tony    Science
  3 "3:10:55"   Hong     Math
  4 "1:34:23A"  Kim      Math
  5 "2:34:13A:  Min     English
  </code></pre>
  
  <p>the result would look like this</p>
  
  <pre><code>df    
     A            B       C
  
  2 "3:10:45"   Tony    Science
  3 "3:10:55"   Hong     Math
  </code></pre>