title: Pandas Data Frame Partial String Replace
body: <p>Given this data frame:</p>
  
  <pre><code>import pandas as pd
  d=pd.DataFrame({'A':['a','b',99],'B':[1,2,'99'],'C':['abcd99',4,5]})
  d
  
      A   B   C
  0   a   1   abcd*
  1   b   2   4
  2   99  99  5
  </code></pre>
  
  <p>I want to replace all 99s in the entire data frame with asterisks.
  I've tried this:</p>
  
  <pre><code>d.replace('99','*')
  </code></pre>
  
  <p>...but it only worked in the case of the string 99 in column B.</p>
  
  <p>Thanks in advance!</p>