// It operates in one of two modes:
//   - strict
//   - non-strict
//
// In strict mode, parse only accepts ETags that
// are AWS S3 compatible. In particular, an AWS
// S3 ETag always consists of a 128 bit checksum
// value and an optional -<part-number> suffix.
// Therefore, s must have the following form in
// strict mode:  <32-hex-characters>[-<integer>]
//