code: def validate(self, value, model=None, context=None):
          
          import re
  
          value = str(value)
          match = re.match(r'^\d+', value)
          if not match or value != match.group():
              return Error(self.not_digital)
  
          # success otherwise
          return Error()
docs: Validate
          Perform value validation and return result
  
          :param value:           value to check, cast to string
          :param model:           parent model being validated
          :param context:         object or None, validation context
          :return:                shiftschema.results.SimpleResult