Btw, @mgrachev do you think it's ok just to allow all these chars?

#776 syntax still has some rules, while we may encounter something like: FOO=${SOME_VALUE--$$++???_END} - which is invalid and moreover is buggy. It will substitute SOME_VALUE and preserve remaining part

I don't think it's really good idea. Allowing alternative syntax, we also marks invalid ones as valid.
Proper solution would require some kind of parser here

You're right, we definitely shouldn't skip such values: FOO=${SOME_VALUE--$$++???_END}.

Perhaps we can check such values using a regular expression of the form \:[\+\=\?\-]??

Regex may do the trick.

However, need to keep in mind that we also need to ensure it's only 1 time occurrence per substitution, and not something like:
${SOME_VALUE:+one:-two:?three}

It still possible ofc, but probably we can just go with simpler solution:

• pre-define the list of allowed sequences, e.g: [":+", ":-", ":=", ":?"]
• ensure there is only one occurrence within the value

Also, need to ensure it will work with multiple substitutions correctly, e.g:

• positive test: FOO=${NODE_ENV:development}${CI:+only}
• negative test: FOO=${NODE_ENV:development}${CI:+only:?IS_UNSET:other}`

Good idea 👍