on https://github.com/mongodb/mongo-tools/blob/96e9b570a394675917b9b7da7614b158f7ff3a56/common/bsonutil/bsonutil.go#L372 the extjson parser in the tools repo is validating regex flags, but after discussion with drivers https://mongodb.slack.com/archives/C0UD1CGUF/p1625258821139000 which flags are valid was removed from the extjson spec. I would recommend to remove this flag validation and leave it up to the server to throw an error or alternatively we could bring this set of flags inline with what the server currently supports (imsx)