Try this one:
db.testlogwiki.find({"line_text" : new RegExp(result) };
Both will work
db.testlogwiki.find({"line_text" : new RegExp(result, 'i') }; // to make this case insensitive
or
db.testlogwiki.find( { line_text: { $regex: result[, $options: 'i'] } } );
You can also use the $regex option.