So it infinite loops if the input has \r\n ?

Not exactly. Infinite loops if the document has \r\n and the document otherwise has a parse error.

I think #"$" might be dicey because it will detect the end of a line, not consume it (because it parses zero characters), meaning you could parse infinite empty lines in a row

Ahhh… I’m trying to match end of input as the same as an end of line

Also, $ detects the end of a line, not the end of the file

\z instead?

Yeah, that seems closer to what you want

Thanks! I’ll give it a try. And see if I can produce something to repro the infinite loop with that and without that change

I'd still be a little concerned at the potential for matching infinite EOF's

OK. I see what you mean. I’ll have a think about a different way of expressing this

It's possible you don't need to explicitly match on EOF, because instaparse will only consider a parse valid if it consumes the whole string