title: Extract substring between two colons / special characters
body: <p>I want to extract "SUBSTRING" with sub() from the following string:</p>
  
  <pre><code>attribute &lt;- "S4Q7b1_t1_r1: SUBSTRING: some explanation: some explanation - ..."
  </code></pre>
  
  <p>I used the following code, but unfortunately it didn't work:</p>
  
  <pre><code>sub(".*: (.*) : .*", "\\1", attribute) 
  </code></pre>
  
  <p>Does anyone know an answer for that?</p>