I also think this string format is the right default choice, since most of the time I'm building parsers that don't change or need to be built programmatically.

I also think this string format is the right default choice, since most of the time I'm building parsers that don't change or need to be built programmatically. But Instaparse has exposed all the functionality as an "in the language" DSL as well using parser combinators in a map structure (near the bottom of the README). You can also turn a string specification into a grammar map/combinator specification, which I have used when a part of my parser was generated at run time.