libbson currently provides no mechanism that would allow parsing a $regex query operator in an Extended JSON document, as it always interprets it as legacy BSON gegex type.

Quoting Parsers from the Extended JSON spec:

A parser that accepts Legacy Extended JSON MUST be configurable such that a JSON text of a MongoDB query filter containing the regex query operator can be parsed, e.g.:

{ "$regex": {
    "$regularExpression" : { "pattern": "foo*", "options": "" }
  },
  "$options" : "ix"
}

or:

{ "$regex": {
    "$regularExpression" : { "pattern": "foo*", "options": "" }
  }
}

This is also discussed in:

• Q&A ("How can implementations which require backwards compatibility...)
• Of Canonical Type Wrapper Formats: RegularExpression