title: pandas equivalent for grep
body: <p>I'm new comer for pandas,
  For dataframe like:</p>
  
  <pre><code>N  Chem    Val
  A  Sodium  9
  B  Sodium  10
  A  Chlorid 7
  B  Chlorid 10
  A  Sodium  17
  </code></pre>
  
  <p>I'd like to do like <code>grep</code>in bash, to select lines containing <code>'A'</code> in 1st column and <code>'Sodium'</code> in 3rd column :</p>
  
  <pre><code>A  Sodium  9
  A  Sodium  17
  </code></pre>
  
  <p>How should I do? I guess I need to use <code>df[].str.contains()</code>?
  Thx</p>