title: rsplit on pandas series with regular expression not working
body: <p>rsplit on pandas series using regular expression not working. I want to split the series based on separator without removing separator.</p>
  
  <pre><code>df2= pd.Series(['Series of Class A','Series of Class B part of Class C','Class D','Class'])
  seperator='Class'
  data = df2.str.split(r'.(?='+seperator+')', n = 2, expand=True)
  </code></pre>
  
  <p>result is:</p>
  
  <pre><code> 0                1        2
  0  Series of          Class A     None
  1  Series of  Class B part of  Class C
  2    Class D             None     None
  3      Class             None     None
  </code></pre>
  
  <p>I want to do same thing using rsplit</p>
  
  <p>I tried </p>
  
  <pre><code>data = df2.str.rsplit(r'.(?='+seperator+')', n = 2, expand=True)
  </code></pre>
  
  <p>Expecting same result using rsplit</p>
  
  <pre><code> 0                1        2
  0  Series of          Class A     None
  1  Series of  Class B part of  Class C
  2    Class D             None     None
  3      Class             None     None
  </code></pre>