Looks good, you might compare to this implementation of Monkey with a bunch of extensions (such as "objects", regular expressions, and a standard-library written in the language itself):
https://github.com/skx/monkey/
A similar idea allows embedded usage of a scripting language, as a filter:
https://github.com/skx/evalfilter/
Of course it gets addictive writing little languages, so you'll probably want to have fun and try making more! I went with BASIC:
https://github.com/skx/gobasic
But then you start thinking about assembly language:
https://github.com/skx/math-compiler
