title: How to check if a column only contains certain letters
body: <p>I have a dataframe and I want to check one column that only contains letter A for example.</p>
  
  <p>The column contains a lot of letters. It looks like:</p>
  
  <p>AAAAAAAAAAAAAAAA
  AAABBBBBDBBSBSBB</p>
  
  <p>I want to check if this column only contains letter A, or both letter A or B, but nothing else.</p>
  
  <p>Do you know which function I shall use?</p>