title: extract word and fillna in specific range, between two points in pandas
body: <p>my df:</p>
  
  <pre><code>A,B
  hello my world, adam
  i like my turbo1, nan
  with love,nan
  good morning, john
  enev one,nan
  turbo2,nan
  good to you,nan
  man too,emily
  </code></pre>
  
  <p>I want to extract words <code>turbo1</code> and <code>turbo2</code> to B column, and then fill all nans of those words but only until any word appears in B column in each way up and down</p>
  
  <p>expected output:</p>
  
  <pre><code>A,B
  hello my world, adam
  i like my turbo1, turbo1
  with love,turbo1
  good morning, john
  enev one,turbo2
  turbo2,turbo2
  goon to you,turbo2
  man too,emily
  </code></pre>
  
  <p>my code:</p>
  
  <pre><code>df['B']=df['B'].str.extract(r'(turbo1|turbo2)').fillna(method='bfill').fillna(method='ffill')
  </code></pre>
  
  <p>problem i have is that i cannot fill nans only in this range between already existing words.</p>