nginx + lua, stripping the leading / in the $uri

nginx.conf

map $uri $uri_stripped {
default null;
~^/(?P<s>.*)$ $s;
}

location / {
  set $json $uri_stripped-json;           # STILL 200 OK with this line commented out
  if ($json ~ "null-json|^-json") {       # STILL 200 OK with this line commented out
  return 404;  }                          # STILL 200 OK with this line commented out
  content_by_lua '
    -- ngx.say("Requested: ", ngx.var.json)   ## SENDING HEADER BEFORE THE 404 SO ALWAYS 200 OK...
    local redis, err = require "resty.redis"
    if not redis then
      ngx.say("could not load resty redis module")
    end
    local red = redis:new()
    red:set_timeout(500) 
    local ok, err = red:connect("127.0.0.1", 6379)
    if not ok then
      ngx.say("failed to connect: ", err)
      return
    end

    local res, err = red:get(ngx.var.json)
    if not res then
      ngx.say("failed to get key:", err)
      return
    end

    if res == ngx.null then
      ngx.status = ngx.HTTP_NOT_FOUD
      ngx.say("key not found.")   #  <- IF THIS LINE COMMENTED OUT = 500 Internal Error
      ngx.exit(404)
    end

    -- ngx.say("result: ", res) # REMOVED FOR SAFETY
    local ok, err = red:close()
    if not ok then
      ngx.say("failed to close: ", err)
      return
    end
    ';
}