From what I can tell, your best bet would be to build the generated parser (which will generate some C code) and then build a small interface in C. From there I would just use R/Python's foreign function interface to call your C wrapper.
Alternatively for python you could use a parse combinator like parsec. Depending on how complicated your file can get, using parsec could very likely be the better choice as it has first class support in python and seems powerful enough to handle your use case.
There do seem to be a number of parse combinators in R but none of them seem as well established as parsec for python (which is based on the much more well known parsec for haskell).
A quick look around seems to show that python will be your best bet for parsing as it has some decent tooling. Here is an article on some of the different parse generators/parse combinators in the space.
Alternatively for python you could use a parse combinator like parsec. Depending on how complicated your file can get, using parsec could very likely be the better choice as it has first class support in python and seems powerful enough to handle your use case.
There do seem to be a number of parse combinators in R but none of them seem as well established as parsec for python (which is based on the much more well known parsec for haskell).
A quick look around seems to show that python will be your best bet for parsing as it has some decent tooling. Here is an article on some of the different parse generators/parse combinators in the space.
https://tomassetti.me/parsing-in-python/