title: When does .str.count('\w') work and when doesn't it?
body: <p>This is a follow up question to <a href="https://stackoverflow.com/questions/52882843/regex-inside-findall-vs-regex-inside-count/52883329#52883329">Regex inside findall vs regex inside count</a></p>
  
  <p><code>.str.count('\w')</code> works for me when called on the column of a dataframe, but not when called on a Series.  </p>
  
  <p><code>X_train[0:7]</code> is a Series:</p>
  
  <p><code>872                       I'll text you when I drop x off
   831     Hi mate its RV did u hav a nice hol just a mes...
   1273    network operator. The service is free. For T &amp;...
   3314    FREE MESSAGE Activate your 500 FREE Text Messa...
   4929    Hi, the SEXYCHAT girls are waiting for you to ...
   4249                              How much for an eighth?
   3640    You can stop further club tones by replying \S...
   Name: text, dtype: object</code></p>
  
  <p><code>X_train[0:7].str.count('\w')</code>
  returns</p>
  
  <p><code>872     0
   831     0
   1273    0
   3314    0
   4929    0
   4249    0
   3640    1
   Name: text, dtype: int64)</code></p>
  
  <p>When called on the same Series, converted into a dataframe column:</p>
  
  <p><code>d = X_train[0:7]</code></p>
  
  <p><code>df = pd.DataFrame(data=d)</code></p>
  
  <p><code>df['col1'].str.count('\w')</code> returns:</p>
  
  <p><code>872      23
  831     101
  1273     50
  3314    120
  4929     98
  4249     18
  3640     98
  Name: col1, dtype: int64</code></p>
  
  <p>Why does it work on a dataframe column, but not on a series?  Grateful for your advice.</p>