Is it possible to make “???” a keyword that can be highlighted?

Question

Is it possible to make ??? (3 question marks) a keyword, and highlight it?

I have tried the following in a syntax file. Other strings on the line are highlighted, but not ???. \?\?\? does not work either.

syntax keyword textTodos TODO ??? XXX LLL
highlight default link textTodos Todo

Answer 1

I would recommend to do it with a syntax match like this:

syntax match TQM '???' containedin=ALL
highlight link TQM Todo

If you want to have it always available, you could put the following in your vimrc:

augroup my_syntax_autocmd
    autocmd!
    autocmd Syntax * syntax match TQM '???' containedin=ALL |  highlight link TQM Todo
augroup END

Answer 2

syntax keywords are supposed to be - well - keywords. Initially, this meant, it must match the definition of the 'iskeyword' option, which usually does not include the quotation mark. Some years later, the iskeyword option has been de-coupled from the syntax highlighting option and now one can use the :syn iskeyword command to specify the iskeyword option only for syntax highlighting (which falls back to the iskeyword) option, if it is not specified.

To make this possible, you can add the ? to the syntax definition of a keyword by using :syn iskeyword=<c-r>=&iskeyword<cr>,?

This uses the expression-register = to get the current value of the iskeyword option and adds the ? to it.

Note however, this can have unwanted consequences to your syntax highlighting, depending on the patterns used in the syntax definitions.