Modify sh.vim to accept (@)_ as part of a variable name for Bash highlighting

Question

I have Bash file in which I have constructs that include (@)_ as part of the variable name. For example:

(@)_VariableName
${(@)_VariableName[@]}
${#(@)_VariableName[@]}
${!(@)_VariableName[@]}

The Bash files are pre-processed and the literal string (@)_ is replaced with characters so that the final result is valid Bash. The sequence (@)_, if present, will always be at the beginning of a variable name.

I'd like to modify the sh.vim syntax file so that variable names are considered to be valid in identifier highlighting and not shown with shDerefWordError highlighting. I've tried modifying:

syn match shDerefVar contained "{\@<=!\k\+" nextgroup=@shDerefVarList

to:

syn match shDerefVar contained "{\@<=\((@)_\)\?!\k\+" nextgroup=@shDerefVarList

but this has not worked.

Note that this allowance might reasonably be made conditional to a variable such as b:is_eggsh.

Can anyone suggest a solution?

Answer 1

You are close. The problem is that you took the syntax definition for Bash's special ${!varname}, and that doesn't match.

I would also define a separate syntax group, used exclusively for your placeholder variables. The sh.vim syntax script is extensive via syntax clusters, that's how you install it as an additional dereference:

syn match shDerefPlaceholder contained "{\@<=[!#]\?(@)_\k\+" nextgroup=@shDerefVarList
syn cluster shDerefList add=shDerefPlaceholder

Alternative

Integrating with an existing syntax can be messy... especially with an advanced one like shell (which supports several sub-modes). An alternative would be using :match Preproc /(@)_[a-zA-Z_][a-zA-Z0-9_]*/; this always applies over existing syntax highlighting. Only complication: this is window-local, so you'd need some :autocmds to install it, like this:

au BufNewFile,BufRead * if &syntax == 'sh' | match Preproc /(@)_[a-zA-Z_][a-zA-Z0-9_]*/ | endif