title: Pandas: count empty strings in a column
body: <p>I tried to find the number of cells in a column that only contain empty string <code>''</code>. The <code>df</code> looks like:</p>
  
  <pre><code>currency
  USD
  EUR
  ILS
  HKD
  </code></pre>
  
  <p>The code is:</p>
  
  <pre><code>df['currency'].str.contains(r'\s*')
  </code></pre>
  
  <p>but the code also recognizes cells with actual string values as containing empty strings.</p>
  
  <p>I am wondering how to fix this issue that it only detects cells that only contains empty strings.</p>