True. But if that specific case is dealing in, let's say, URLs, then all of the content should be ASCII - either as a direct character representation, or encoded, again to ASCII.

Never did I see the parent mention "this is sufficient for humans", or just "...for English" - even that would be a naïve assumption (see? ;)).

If your definition of "URLs" includes "IRIs" (a term nobody really uses but which encompasses the idea that you can use Unicode in URLs), then this isn't a good assumption to make.

I would rather pass around a link to https://ja.wikipedia.org/wiki/メインページ than to https://ja.wikipedia.org/wiki/%E3%83%A1%E3%82%A4%E3%83%B3%E3... , and lots of software will support both. And if you want all URLs to be ASCII, you'll need to convert the first into the second, not just delete the Japanese characters.

If the URL is "on the wire" it had better be 7-bit ASCII. Actually even more restrictive than that. Because that's the spec. https://tools.ietf.org/html/rfc3986

In user interaction with a browser or wherever else it seems that anything goes.

That's the spec but this is also the spec: https://www.ietf.org/rfc/rfc3987.txt

It's true that protocols such as HTTP only use ASCII URIs on the wire. If you are implementing an HTTP client yourself, you will need to implement percent-encoding.

Which is different from saying "well, I know URLs should be ASCII on the wire, so I can safely delete all the non-ASCII characters from a URL." That's not true.

As soon as you add or remove to the string, it no longer points to the same resource; I considered this too obvious to mention. OTOH, for _validation_, this is useful: "you have a ZWJ character in an URL, that's unlikely". And yes, I understand that there are protocols that allow you to pass around the full Unicode or aunt Matilda or whatever - I should have been more specific.

The first version is much more readable and less hacky; alas, unless you are positive that all the software you're interfacing with can handle Unicode (never have I ever been in such a glorious situation), fallback to URL-encoding it is.