https://google.github.io/styleguide/cppguide.html

https://github.com/westes/flex

https://joycecoder.tistory.com/9

https://github.com/jarro2783/cxxopts

https://riptutorial.com/ebook/cplusplus

https://regexr.com/

https://www.geeksforgeeks.org/regex-regular-expression-in-c/

http://www.jiniya.net/ng/2017/11/regex/

https://www.sapphosound.com/archives/1653

https://blog.box.kr/?p=335

https://github.com/NTDLS/CMathParser

https://stackoverflow.com/questions/11703082/parsing-math-expression-in-c