title: Extract part of string between two different patterns
body: <p>I try to use <code>stringr</code> package to extract part of a string, which is between two particular patterns.</p>
  
  <p>For example, I have: </p>
  
  <pre><code>my.string &lt;- "nanaqwertybaba"
  left.border  &lt;- "nana"
  right.border &lt;- "baba"
  </code></pre>
  
  <p>and by the use of <code>str_extract(string, pattern)</code> function (where pattern is <em>defined by a POSIX regular expression</em>) I would like to receive:</p>
  
  <pre><code>"qwerty"
  </code></pre>
  
  <p>Solutions from Google did not work.</p>