code: func Substitute(r, replacement string) Filter {
  	return FilterFunc(func(arg Arg) error {
  		re, err := regexp.Compile(r)
  		if err != nil {
  			return err
  		}
  		for s := range arg.In {
  			arg.Out <- re.ReplaceAllString(s, replacement)
  		}
  		return nil
  	})
  }
docs: // Substitute replaces all occurrences of the regular expression r in
  // an input item with replacement.  The replacement string can contain
  // $1, $2, etc. which represent submatches of r.