Some questions, for example
Instead of using a correct URL, such as
http://meta.stackexchange.com/questions/79543/٩●̮̮̃-̃۶-forbid-unicode
are using
https://superuser.com/questions/516363/what-is-the-��-character
What is causing this behavior?
The Stack Exchange Title-to-URL-slug converter cannot handle characters outside the BMP (the Basic Multilingual Plane), running from U+0000 through to U+FFFF.
The character in question, U+1F60B, is encoded to UTF-16 using a surrogate pair instead, so you get U+D83D U+DE0B, and those are not 'displayable' (not valid Unicode) and get replaced by two U+FFFD characters on conversion to UTF-8.
The correct UTF-8 conversion would be F0 9F 98 8B, so the URL should be:
http://superuser.com/questions/516363/what-is-the-%f0%9f%98%8b-character
which my browser then happily displays as
http://superuser.com/questions/516363/what-is-the-😋-character
(if an Emoji is displayed depends on your font and OS).
http://meta.stackexchange.com/questions/79543/٩●̮̮̃-̃۶-forbid-unicodeis not a valid URL either. I'm not saying the replacement characters are any more valid, but anything outside the ASCII range would have to be encoded in a URL. Browsers may elect to display decoded characters though.http://superuser.com/questions/516363/what-is-the-%ef%bf%bd%ef%bf%bd-character. The Meta post URL ishttp://meta.stackexchange.com/questions/79543/%d9%a9%e2%97%8f%cc%ae%cc%ae%cc%83-%cc%83%db%b6-forbid-unicode.