code: def convert(self, value, view):
          
          if isinstance(value, BASESTRING):
              if self.pattern and not self.regex.match(value):
                  self.fail(
                      u"must match the pattern {0}".format(self.pattern),
                      view
                  )
              return value
          else:
              self.fail(u'must be a string', view, True)
docs: Check that the value is a string and matches the pattern.