title: string Validation with xlsxwriter python
body: <p>I am trying to put validations on excel file. I field should only accept characters or character with '_' symbol. The following code is only redistricting user to enter values more than length 10.</p>
  
  <pre><code>import xlsxwriter
  wb = xlsxwriter.Workbook('staff.xlsx')
  ws = workbook.add_worksheet()
  
  ws.data_validation(1, 1, 10, 0,
                     {'validate': 'length',
                      'input_title': 'Enter value',
                      'criteria': '&lt;',
                      'value': firstname_max_length,
                      'error_message': 'Max Length is{0}'.format(10)})
  </code></pre>
  
  <p>Help me to validate values that should only except characters. </p>
  
  <p>Thanks.</p>