title: find class name by regex
body: <p>I want to get all class that match the key by regex, so when we search for btn, we get something like this :</p>
  
  <pre><code>&lt;div class="btn"&gt; // match
      &lt;div class="span4"&gt; // not match
          &lt;div class='btnbtn-success'&gt;  // not match
  
          &lt;/div&gt;
          &lt;div class='   bbtn '&gt;  // not match
  
          &lt;/div&gt;
          &lt;div class='   bbtn '&gt;  // not match
  
          &lt;/div&gt;
          &lt;div class='   b  btn '&gt; / match
  
          &lt;/div&gt; 
          &lt;div class='cbbtn '&gt;  // not match
  
          &lt;/div&gt;
          &lt;div class='btn '&gt;  // match
  
          &lt;/div&gt;
          &lt;div class=' bbtn '&gt;  // not match
  
          &lt;/div&gt;
          &lt;div class='    btn'&gt;  // match
  
          &lt;/div&gt;
          &lt;div class=" btn "&gt;  // match
  
          &lt;/div&gt;
          &lt;div class='hello btn '&gt;  // match
  
          &lt;/div&gt;
          &lt;div class="  btn     "&gt;  // match
  
          &lt;/div&gt;
          &lt;div class=" btn     hello"&gt;  // match
  
          &lt;/div&gt;
          &lt;div class=" btnaa     hello"&gt;  // not match
  
          &lt;/div&gt;
  
          &lt;div class=" hello btn     hello"&gt; // match
  
          &lt;/div&gt;
          &lt;div class=" hello btn"&gt;  // match
  
          &lt;/div&gt;
          &lt;div class=" hello btn     hello"&gt; // match
  
          &lt;/div&gt;
  
          &lt;div class=" hello btn  "&gt;  // match
  
          &lt;/div&gt;
          &lt;div class=" btna "&gt;  // not match
  
          &lt;/div&gt;
          &lt;div class=" btna-success "&gt;  // not match
  
          &lt;/div&gt;
      &lt;/div&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>what is best regex to find <strong>btn</strong> class name in the code?</p>