[nrds]

> How do you define the language to be parsed? It's with a grammar.

False. This is how you define a language _to a parser generator_, but it is not how humans (and/or developers) define languages to each other.

> you won't realise you've made it

This is literally impossible in a recursive descent parser. I'm not saying getting it wrong is impossible, of course not. But what you literally cannot do (without concerted intentional effort) is make it ambiguous. Your parser will parse one first, or the other first, or either one left-to-right; and you will know which of these it does by reading the code.