[Esprima] (http://esprima.org/test/)
[PHP's Parser] (http://www.dsource.org/projects/leds/browser/trunk/src/language/php/Parser.d)
[How To Write A Calculator in 70 Python Lines] (http://blog.erezsh.com/how-to-write-a-calculator-in-70-python-lines-by-writing-a-recursive-descent-parser/)
[Khan Math Model] (https://github.com/Khan/khan-exercises/blob/master/utils/math-model.js)
[Ruby JSON Parser the Hard Way] (https://practicingruby.com/articles/parsing-json-the-hard-way?u=dc2ab0f9bb)