It's great as a learning experience for the author, but it would be a good idea for the author to take a look at a JSON spec and see if the regular expressions used by the lexer support the whole spec and don't accept non-legal JSON. From a cursory glance, it seems like exponents in floating point numbers aren't supported, nor are escaped quotation marks in strings (`\"`). I think it supports string escapes that are valid in go, but not in JSON like `\xab` and 8-hexdigit unicode escapes `\U1234abcd`.
I just started learning about parsers and compilers and json is probably the easiest thing to parse, also json is used everwhere and i have used it in pretty much every project so why not