console.output

obj:  { title: /.*@followbtcnews.*/i,
  '$or': [ { body: /.*@followbtcnews.*/i } ] }
stringify:  {"title":{},"$or":[{"body":{}}]}
parsed:  { title: {}, '$or': [ { body: {} } ] }
{ title: /.*@followbtcnews.*/i,
  '$or': [ { body: /.*@followbtcnews.*/i } ] }

server.js

app.post('/mentions', function(req,res){
	// console.log(req.body);

	var titleRegex = ".*@"+req.body.username+".*";

	var titleRegexObj = new RegExp(titleRegex, 'i');


	var query = {
		"title": titleRegexObj,
		$or: [
			{
				"body": titleRegexObj
			}
		]
	};

	console.log("obj: ", query);
	console.log("stringify: ", JSON.stringify(query));
	console.log("parsed: ", JSON.parse(JSON.stringify(query)));

	console.log(query);

	res.json(query);
});