Right, it is intentionally not POSIX compliant. This saves fairly significant development effort and also permits expanding the tool to be more user friendly, such as transparently searching UTF-16 encoded files.

ripgrep cannot be a drop-in replacement. Despite that, it can certainly replace grep in a wide variety of use cases. See: https://github.com/BurntSushi/ripgrep/blob/master/FAQ.md#pos...