Parse part of content (ignore/skip ALL until known tokens are found)

Good morning!

I'm just a novice in javacc struggling to get some simple stuff running:

I have a file going like that:

...upper random content...
known-content1
...random content...
known-content2
known-content3


(the known-content1,2,3 are optional)

When I define the syntax for "known-content1", it says that "upper random content" is not identified. Also

What I ultimately want to do is to get javacc only parse the _known content_, the one it recognizes. I want it to ignore the rest of content whatever it is.

Could you please tell how that thing is done?

Best Regards!