title: XlsxWriter with Pandas Error
body: <p>I'm using pandas.ExcelWriter to add conditional formatting to a spreadsheet which im creating using df.to_excel.  When I hard code the cell range, as seen below, the code works fine.  </p>
  
  <pre><code>worksheet.conditional_format('A3:N361386', {'type' : 'no_blanks', 'format' : worksheet_format})
  </code></pre>
  
  <p>But when i parameterize it, like the below, i get an error. Any ideas?</p>
  
  <pre><code>Data_Range = "'" + 'A3:' + str(chr(len(THIS_proposals_data.columns)+96)).upper() + str(len(THIS_proposals_data.index)) + "'"
  
  worksheet.conditional_format(str(Data_Range), {'type' : 'no_blanks', 'format' : worksheet_format})
  </code></pre>
  
  <p>Error Message:</p>
  
  <pre><code>Traceback (most recent call last):
  
    File "&lt;ipython-input-9-25e65d1fb126&gt;", line 1, in &lt;module&gt;
      runfile('C:/Users/EHARTMAN/Desktop/untitled1.py', wdir='C:/Users/EHARTMAN/Desktop')
  
    File "C:\Anaconda3\envs\py27\lib\site-packages\spyderlib\widgets\externalshell\sitecustomize.py", line 699, in runfile
      execfile(filename, namespace)
  
    File "C:\Anaconda3\envs\py27\lib\site-packages\spyderlib\widgets\externalshell\sitecustomize.py", line 74, in execfile
      exec(compile(scripttext, filename, 'exec'), glob, loc)
  
    File "C:/Users/EHARTMAN/Desktop/untitled1.py", line 64, in &lt;module&gt;
      worksheet.conditional_format(Data_Range, {'type' : 'no_blanks', 'format' : worksheet_format})
  
    File "C:\Anaconda3\envs\py27\lib\site-packages\xlsxwriter\worksheet.py", line 85, in cell_wrapper
      row_1, col_1 = xl_cell_to_rowcol(cell_1)
  
    File "C:\Anaconda3\envs\py27\lib\site-packages\xlsxwriter\utility.py", line 108, in xl_cell_to_rowcol
      col_str = match.group(2)
  
  AttributeError: 'NoneType' object has no attribute 'group'
  </code></pre>