environ8r

#!/usr/bin/awk -f
BEGIN { w = "[a-zA-Z_][a-zA-Z0-9_]*"; var = sprintf("\\$(%s|{%s})", w, w) }
{
    str = ""
    tail = $0
    while (match(tail, var)) {
        head = substr(tail, 1, RSTART - 1) # text before match
        repl = substr(tail, RSTART, RLENGTH) # match to replace
        tail = substr(tail, RSTART + RLENGTH) # text after match
        if ((match(head, /\\+/) ? RLENGTH + 1 : 1) % 2 == 1) {
            sub(/^\$/, "", repl)
            gsub(/(^{|}$)/, "", repl)
            repl = ENVIRON[repl]
        }
        str = str head repl
    }
    str = str tail
    print str
}