How to parse json in lua?

Question

Is there any lightweight, preferably pure lua, library for lua to parse json content? Basically I wanna augment my ngnix with a lua module that needs to verfiy some information from a json object I'm getting from Redis.

The object looks like as follow:

{
  "data": {
    "user": {
      "username": "username",
      "type": "TYPE"
    }
  },
  "passport": {
    "user": "uuid"
  },
}

In my lua code, I need to verify if the data.user.username exists. Then I can let the nginx continue with its redirection. Can anybody please show me an example of how can I achieve that?

The easy answer just pick library you like. There many of them :) — moteus
– moteus, Commented Feb 9, 2017 at 14:44

lhf · Accepted Answer · 2017-02-09 15:12:41Z

5

JSON data in that form is very close to Lua tables. So you can transform the JSON data into Lua code and run it, if you trust the JSON data.

J=[[
{
  "data": {
    "user": {
      "username": "username",
      "type": "TYPE"
    }
  },
  "passport": {
    "user": "uuid"
  },
}
]]
L="return "..J:gsub('("[^"]-"):','[%1]=')
T=loadstring(L)()
print(T.data.user.username)

If have any qualms about the JSON data, you may want to run the string in L in a sandbox.

answered Feb 9, 2017 at 15:12

lhf

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7 Comments

warspyking Over a year ago

Why would you use loadstring?

nobody Over a year ago

@warspyking Because among the "pure lua" ways, this is almost certainly the fastest. And it can be done in a safe way. (Unless there's a really ugly bug in the Lua parser, but that's quite unlikely.) Further, there's a difference in threat level between e.g. a web API exposed to unknown (and quite likely some malicious) data and a tool that you use to quickly load data generated out of some other trusted tool running locally (even if that tool interfaces with the 'net).

warspyking Over a year ago

I understand what it does, but loadstring for something so simple is really expensive don't you think?

lhf Over a year ago

@warspyking, I doubt that hand-written JOSN parsers in pure Lua will be faster or use less memory than the Lua parser, but I haven't measured anything.

moteus Over a year ago

This is works only for limited subset of JSON. And even in this example JSON is not valid but it parse without any error.

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