This benchmark measures the time it takes to validate JSON, i.e. to check whether it is well-formed. Validation was chosen as opposed to parsing, as parsing speed depends on the JSON data structure as well. Implementing an efficient JSON container is out of scope for lexy, so it would have a disadvantage over the specialized JSON libraries.

The average validation times for each input are shown in the boxplots below. Lower values are better.

The implementations

This simply adds all input characters of the JSON document without performing actual validation.


A JSON validator using the lexy grammar from the example.


A JSON validator using the PEGTL JSON grammar.


A JSON validator using JSON for Modern C++ implemented by nlohmann::json::accept().


A JSON validator using rapidjson implemented using a SAX parser with the rapidjson::BaseReaderHandler.


A JSON validator using Boost.JSON implemented using a custom parse handler.

The inputs

Contains lots of 2-element arrays holding floating-point coordinate pairs. Taken from


Big JSON file with some variety. Taken from


Some data from twitter’s API. Taken from

The Methodology

The input data is read using lexy::read_file(). The resulting buffer is then passed to the various implementations using their memory inputs. Benchmarking is done by nanobench on an AMD FX-6300.