Can’t change username to an all-alphabetic word

Question

TLDR: The alphabetic check is erroneously checking UTF-16 code units not Unicode code points!

I attempted to change my username to the Gothic version, and I received the rather rudely rubicund retort of:

Now, the string in question was 𐌸𐍇𐍂𐌹𐍃𐍄, which as you can plainly see, is entirely composed of letters in the Gothic alphabet. In particular, those are:

• "\N{GOTHIC LETTER THIUTH}\N{GOTHIC LETTER IGGWS}\N{GOTHIC LETTER RAIDA}\N{GOTHIC LETTER EIS}\N{GOTHIC LETTER SAUIL}\N{GOTHIC LETTER TEIWS}"

Now, lest there be any doubt that those are all valid letters, notice:

$ perl -Mutf8 -le 'print "𐌸𐍇𐍂𐌹𐍃𐍄" =~ /^\p{Letter}+$/ ? 1 : 0'
1

$ perl -Mutf8 -le 'print "𐌸𐍇𐍂𐌹𐍃𐍄" =~ /^\w+$/ ? 1 : 0'
1

Which looked like this to me:

What’s almost certainly happening is that the verification code doesn’t actually deal with Unicode characters at all, which is highly disappointing. You see, that purely alphabetic string contains letters whose code points exceed 0xFFFF, and thus occupy two UTF-16 code units instead of one. The string in question is this one using hex escapes:

• "\x{10338}\x{10347}\x{10342}\x{10339}\x{10343}\x{10344}"

You can see how those are over 0xFFFF.

But that doesn’t matter, because they are legal alphabetic code points, and are even displayed correctly:

Please stop discriminating against valid Unicode letters and telling me I didn’t use letters when I did! 😢