code: func grepper(restr string) func(string) string {
  	re := regexp.MustCompile(restr)
  	return func(s string) string {
  		matches := re.FindAllStringSubmatch(s, 1)
  		if len(matches) != 1 {
  			log.Fatalf("'%s' should match '%s' exactly once", restr, s)
  		}
  		groups := matches[0]
  		if len(groups) != 2 {
  			log.Fatalf("'%s' should have exactly one group (found %d)", restr, len(groups))
  		}
  		return groups[1]
  	}
  }
docs: //====================================================
  //meta helpers