title: Extract the largest number from a dataframe column containing strings
body: <p>I have a pandas dataframe containing columns extracted from a csv file. One of the columns has strings which contain a particular number, I want to extract. It showed a <code>TypeError</code>, which I think was because of the object data type of the column, but neither does setting data type of the column during extraction work, nor does <code>astype</code> work on the particular column. Earlier, I extracted the same column from the Excel file, and the regex expression worked on it, no problem.</p>
  
  <p>The working head is as follows:</p>
  
  <pre><code>  Transaction Date                                        PARTICULARS DEPOSITS WITHDRAWALS Amount Dr/Cr  Calc_Amount  Calc RRN Number RRN-AMT
  0       2019-05-30              UPI/914923281641/UPI/raghu.m.v2016@o/        0       32.86  32.86    Dr        32.86     914923281641    0100
  1       2019-05-30              UPI/915000512028/UPI/hemanth1999kuma/        0        0.95   0.95    Dr         0.95     915000512028    0100
  2       2019-05-30          UPI/RVSL915000512028/UPI/hemanth1999kuma/     0.95           0   0.95    Cr        -0.95     915000512028    0100
  3       2019-05-30  UPI/914923451855/UPI/tpmanzoor55@okh/Federal Bank     1.19           0   1.19    Cr        -1.19     914923451855    0100
  4       2019-05-30              UPI/914923339262/UPI/ravimaurya8735@/        0        0.94   0.94    Dr         0.94     914923339262    0100
  </code></pre>
  
  <p>From this code:</p>
  
  <pre><code>for i, row in bank_statement_30May.iterrows():
      result = [e for e in re.split("[^0-9]",row[1]) if e != '']
      bank_statement_30May.loc[i,"Calc RRN Number"] = max(map(int,result))
  </code></pre>
  
  <p>This is the error from the second code:</p>
  
  <pre><code>    result = [e for e in re.split("[^0-9]",row[1]) if e != '']
    File "C:\Users\Suraj Joshi\AppData\Local\Programs\Python\Python37\lib\re.py", line 213, in split
      return _compile(pattern, flags).split(string, maxsplit)
  TypeError: cannot use a string pattern on a bytes-like object
  </code></pre>