title: Delete rows from DF with partial string matching
body: <p>From the following df I want to delete all rows with do not start with CBT_21 or CBT_TY1:</p>
  
  <pre><code>               code        date type  strike  settlement
  0    CBT_06_F2016_S  2015-01-02    P   240.0        2.45
  1    CBT_06_F2016_S  2015-01-02    P   360.0       48.60
  2    CBT_21_F2016_S  2015-01-02    P   210.0        0.80
  3    CBT_TY1_F2016_S 2015-01-02    P   320.0       23.20
  4    CBT_06_F2016_S  2015-01-02    C   430.0        3.70
  </code></pre>
  
  <p>desired output:</p>
  
  <pre><code>               code        date type  strike  settlement
  0    CBT_21_F2016_S  2015-01-02    P   210.0        0.80
  1    CBT_TY1_F2016_S 2015-01-02    P   320.0       23.20
  </code></pre>
  
  <p>What is the most efficient pythonic way of doing this please? I have a very large file to process.</p>