What's happening here is that fd's `-e/--extension` flag requires that its parameter value be valid UTF-8. The only case where this doesn't work is if you want to filter by a file path whose extension is not valid UTF-8. Needless to say, I can't ever think of a case where you'd really want to do this.

But if you do, fd still supports it. You just can't use the `-e/--extension` convenience:

    $ touch $'a.\xFF'
    $ fd '.*\.(?-u:\xFF)'
    a.�

That is, `fd` requires that the regex patterns you give it be valid UTF-8, but that doesn't mean the patterns themselves are limited to only matching valid UTF-8. You can disable Unicode mode and match raw bytes via escape sequences (that's what `(?-u:\xFF)` is doing).

So as a matter of fact, the sibling comments get the analysis wrong here. `fd` doesn't assume all of its file paths are valid UTF-8. As demonstrated above, it handles non-UTF-8 paths just fine. But some conveniences, like specifying a file extension, do require valid UTF-8.