code: func (p *FilepathSpec) Set(value string) error {
  	var filter []filepathPat
  	for _, pat := range strings.Split(value, ",") {
  		pattern, v, err := parseFilter(pat)
  		if err != nil {
  			return err
  		}
  		if v == 0 {
  			continue
  		}
  		re, err := regexp.Compile(pattern)
  		if err != nil {
  			return fmt.Errorf("failed to compile %s as an regexp: %s", pattern, err)
  		}
  		// TODO: check syntax of filter?
  		filter = append(filter, filepathPat{re, pattern, Level(v)})
  	}
  	p.filter = filter
  	return nil
  }
docs: // Syntax: foo/bar=2,foo/bar/.*=1,f*=3