When I started using react in 2014 I was using Sublime Text as my editor. JSX was pretty new at the time so there were no syntax definitions for it.
I decided to jump in and see how hard it was to write my own.
In my first attempt I took the html syntax and the javascript and pretty much just concatenated them together.
It turned out okay but there were lots of rough edges and the nesting of js inside attributes and markup didn't really work.
Eventually I decided to re-write it from scratch and learned a lot about regexes and how much language definitions fight with color schemes. (CSS is not alone with specificity struggles)