code: def pattern(self):
          
          if self.dynamic_version_of is not None:
              return self.dynamic_version_of.pattern
  
          if not self._pattern:
              self._pattern = re.compile('^%s_(.+)$' % self.name)
          return self._pattern
docs: Return the pattern used to check if a field name can be accepted by this
          dynamic field. Use a default one ('^fieldname_(.+)$') if not set when
          the field was initialized