[antlr-interest] distinguish "3 + 4" from "3 +4"

[Andreas Liebig]

Hello,
I am not very experienced with ANTLR, and I would like to ask for some ideas how to solve this task:

I have to distinguish input streams like
"3 + 4" (parsed as three tokens NUMBER PLUS NUMBER) from 
"3 +4" (parsed as NUMBER NUMBER, because the + is part of the number +4).

I would like to ignore whitespace in general using the "$channel=HIDDEN;" syntax. But only in this situation whitespace does matter.
Can you guide me to a good explanation of a possible solution?

Thanks
Andreas