Can I isolate an environment variable name in the expand() function

Question

In my shell, I can use curly braces to isolate the names of environment variables.

For example, if I have two env vars, ONE and TWO, equal to 1 and 2, then

> echo "$ONE_$TWO"
ONE_: Undefined variable
> echo "${ONE}_${TWO}"
1_2

I like to try something similar with Vim's expand() function, but the curly braces don't help.

:echo expand('$ONE_$TWO')
$ONE_$TWO
:echo expand('${ONE}_${TWO}')
$ONE_$TWO

Answer 1

If you put any character that is invalid in an environment variable name after the environment variable name, it will force the expansion. Example:

ec expand('$SHELL-$HOME')

Output:

/bin/zsh-/home/wilson

Then you can replace all instances of the invalid character with an underscore:

ec expand('$SHELL-$HOME')->substitute('-', '_', 'g')

Output:

/bin/zsh_/home/wilson

I recommend you define your own function in your vimrc for this

function! Exp(str)
    return expand(a:str)->substitute('-', '_', 'g')
endf

Then run ec Exp('$SHELL-$HOME') as needed.

Or you could even do:

function! Exp(str)
    return expand(a:str->substitute('_', '-', 'g'))->substitute('-', '_', 'g')
endf

and run it as ec Exp('$SHELL_$HOME'). The only concern is if the unexpanded string contains hyphens or if the values of the environment variables contain hyphens.

Of course, it doesn't have to be a hyphen, it could be anything other than an alphanumeric or underscore. You might come up with some weird character that will definitely never cause undesired results. You could even use a single closing curly brace and use expand('$SHELL}_$HOME')->substitute('}', '', 'g'), as long as the result will definitely not contain any } characters.