code: func isForbiddenString(b []byte) bool {
  	for len(b) == 0 {
  		return false
  	}
  
  	switch strings.ToLower(string(b)) {
  	case "n/a":
  		return true
  	}
  
  	for len(b) > 0 {
  		r, size := utf8.DecodeRune(b)
  		if !((r >= 32 && r <= 47) ||
  			(r >= 58 && r <= 64) ||
  			(r >= 91 && r <= 96) ||
  			(r >= 123 && r <= 126)) {
  			// non metadata character included in value
  			return false
  		}
  		b = b[size:]
  	}
  
  	return true
  }
docs: // The BR state that attributes MUST NOT contain metadata such as '.', '-', ' ',
  // this check implements a structure wide validation for values that indication
  // that the field is absent, incomplete, or not applicable.
  //
  // ASCII range of forbidden metadata characters are 32 - 47, 58 -64, 91 - 96,
  // 123 - 126, if the value does only contain metadata this value is forbidden.
  // This check does also detect double characters or any combination of metadata
  // characters.