How to parse JSON data using Lua patterns? - json

Question

I am integrating lua-module in nginx. I want to check parameter's value is empty or not. Whenever I use following code, I get true result against the JSON request
Request
{
"data": {
"user": {
"username": "Ethen",
"type": "PDF"
}
},
"passport": {
"user": "001"
},
}
Code
local arg = ngx.req.get_body_data();
L="return "..arg:gsub('("[^"]-"):','[%1]=')
T=loadstring(L)()
ngx.print(T.data.user.username)
if T.data.user.username == "" then
ngx.say("Empty username");
end
But when I use this request, I got error (attempt to call a nil value stack traceback)
Request
{
"reference" : "567456314",
"callback_url" : "http://www.example.com/",
"verification_mode" : "any",
"document" : {
"proof" : "data:image/png;base64,iVBOR=“,
"additional_proof": "data:image/png;base64,iVBORw0=",
"supported_types" : ["id_card","driving_license","passport"],
"expiry_date" : "",
"document_number" : ""
},
"address" : {
"proof" : "data:image/png;base64,iVBORw0KG=",
"supported_types" : ["id_card","bank_statement"],
"name" : "",
"issue_date":""
}
}
Code
local arg = ngx.req.get_body_data();
L="return "..arg:gsub('("[^"]-"):','[%1]=')
T=loadstring(L)()
ngx.print(T.document.proof)
if T.document.proof == "" then
ngx.say("Empty proof");
end
What's the problem and solution??....Thanks in advance!!