title: Converting strings to a lower case in pandas
body: <p>I have a data that contains domain names: </p>
  
  <pre><code> url          var1
  www.CNN.com   xsd
  www.Nbc.com   wer
  www.BBc.com   xyz
  www.fOX.com   zyx
  ....
  </code></pre>
  
  <p>The data is of the Series type. I am using the following to convert url variable to lower case:</p>
  
  <pre><code>df.apply(lambda x: x.astype(str).str.lower())
  </code></pre>
  
  <p>However, they remain the same. </p>
  
  <p>What am I doing wrong? </p>