Why ü is an ASCII charatcter?

That is definitely surprising. The use of the u flag does change the meaning of some POSIX character classes but there’s no mention of [:ascii:] changing in those circumstances as you noted.

In my unicode_set library I build the regex in the way you expect - just in case that lib helps you in some way.

iex> regex = Unicode.Regex.compile! "[[:ascii:]]"
~r/[\x{0}-\x{7F}]/u
iex> Unicode.Regex.match? regex, "ü"             
false

I also tried it without the u flag and got the same result.

iex(1)> Regex.match?( ~r/^[[:ascii:]]+$/, "ü")
true

Yes, which is even more weird since in non-unicode mode the comparison is meant to be byte-wise. Since << 195, 188 >> = "ü", neither of which is 0 < ascii < 128.

I believe :ascii: works for extended ASCII table which is 0-255. So something is wrong in docs.

Probably something inherited from the erlang regex implementation:

nix shell nixpkgs#erlang -c erl
Erlang/OTP 22 [erts-10.7] [source] [64-bit] [smp:2:2] [ds:2:2:10] [async-threads:1] [hipe]

Eshell V10.7  (abort with ^G)
1> re:run(<<"ü">>, "[[:ascii:]]").
{match,[{0,1}]}
2> re:run(<<"ü"/utf8>>, "[[:ascii:]]").
{match,[{0,1}]}
3> re:run("ü", "[[:ascii:]]").         
{match,[{0,1}]}

looking at the docs I would not expect this:

EDIT:

iex(1)> match_char = fn c -> Regex.match?( ~r/^[[:ascii:]]+$/u, List.to_string([c])) end
#Function<44.79398840/1 in :erl_eval.expr/5>
iex(2)> length(for c <- 0..255, match_char.(c), do: c)                                  
256

Well, Regex is just wrapper over re, so this is pretty obvious.

@AdiletAbylov I would check whether it isn’t bug in PCRE which is used by Erlang (with few patches).

I tried this in several online regex checkers, each of which claims to use PCRE. And none of them return the same result as :re. So seems to be either a bug or a doc error.
.

I opened an issue on the erlang OTP repository.

It is definitely a bug in the character class.

The character is: 𑩄 U+11A44 ZANABAZAR SQUARE MARK LONG TSHEG

iex> Regex.match?( ~r/^[[:ascii:]]+$/, <<72260::utf8>>)   
true

The challenge is to find a character that will return false