title: delete Pandas df row using mask, based on the last letter of a string column
body: <p>For example, in the pandas dataframe below with 3 rows, all of which are strings.
  I wanna drop based on condition <code>if str[-1] == '-':</code></p>
  
  <pre><code>df = pd.DataFrame({'a': ["123-","123-1","123-2"]})
  </code></pre>
  
  <p>But if I do </p>
  
  <pre><code>df[df['a'][-1]=='-']
  </code></pre>
  
  <p>It will return an error. I know it's possible to do it with df.apply function. But I am just wondering if it can be done with mask.</p>