title: Number of iems in an array containing a part of string
body: <p>consider </p>
  
  <pre><code>df['something'].unique()
  </code></pre>
  
  <p>This generates unique items in variable something. Lets consider it as array of unique items.</p>
  
  <pre><code>array(['aabb','aacc','aadd','bbcc']).
  </code></pre>
  
  <p>Now I want to check how any items in that array starts from 'aa'. </p>