title: Extract a substring according to a pattern
body: <p>Suppose I have a list of string:</p>
  
  <pre><code>string = c("G1:E001", "G2:E002", "G3:E003")
  </code></pre>
  
  <p>Now I hope to get a vector of string that contains only the parts after the colon ":", i.e <code>substring = c(E001,E002,E003)</code>.</p>
  
  <p>Is there a convenient way in R to do this? Using <code>substr</code>?</p>