The language concept is a bit more fundamental than config files. Rebol (and Ren-C and cousin Red) is a fully homoiconic language that itself has tools for parsing and interpreting DSLs ('Dialects' to use the language's nomenclature) that use the same language rules. In essence you do indeed get the lexer for free.

Would recommend a wee read of: http://blog.hostilefork.com/why-rebol-red-parse-cool/

I've argued that Ren is somewhat redundant as it fills the same space that Rebol does though Rebol lacks a formal specification at this time.