Vim syntax file: Adding certain characters to iskeyword isn't working

Question

I'm making a Vim syntax file, and I need to add some characters to the iskeyword option. I've had trouble figuring out how it works from the Vim help files, but this is what I've currently written:

set iskeyword+=_,.,-,64,133,!,?,@,172,8743,8744,8853,8592,8593,8594,8595,8596

#64 and #133 are the at sign and the ellipsis. The last few characters are ¬, ∧, ∨, ⊕, →, ←, ↓, ↑, and ↔.

Of all these characters, only _, ., -, and ¬ (#172) seem to be recognized as keyword characters.

Is there something else I have to do? Or does Vim simply not support keyword characters beyond a certain point in Unicode (but the at sign isn't being recognized either, and that's definitely within even ASCII)?

Thanks for your help!

Answer 1

The help for iskeyword points to the help for isfname which answers all your questions:

[…]
Multi-byte characters 256 and above are always included, only the
characters up to 255 are specified with this option.
For UTF-8 the characters 0xa0 to 0xff are included as well.
[…]
The format of this option is a list of parts, separated with commas.
Each part can be a single character number or a range.  A range is two
character numbers with '-' in between.  A character number can be a
decimal number between 0 and 255 or the ASCII character itself (does
not work for digits).  Example:
[…]
If the character is '@', all characters where isalpha() returns TRUE
are included.  Normally these are the characters a to z and A to Z,
plus accented characters.  To include '@' itself use "@-@".  Examples:
    "@,^a-z"    All alphabetic characters, excluding lower
            case ASCII letters.
    "a-z,A-Z,@-@"   All letters plus the '@' character.
[…]

1. "only the characters up to 255 are specified with this option."
2. "A character number can be a decimal number between 0 and 255"
3. "To include '@' itself use "@-@""

Including multi-byte characters in iskeyword has already been discussed in many lists, like this one.