ANTLR ambiguity

I need to match in ANTLR a message containing 2 fields separated by a / First field can have 1..3 digits, the second field can have 1..2 digits,

this does not work

msg: f1 '/' f2;

f1: DIGIT(DIGIT(DIGIT)?)? ;
f2: DIGIT(DIGIT)?

1. How can I avoid ambiguity in such a case?
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2. Is there a more elegant way to express the number of repetitions with ANTLR?

Thanks a lot Chris

AFAIR (it's some time ago I last used ANTLR), you can use "fragment" to avoid having f1 and f2 as a top level token:

msg: f1 '/' f2;

fragment f1: DIGIT(DIGIT(DIGIT)?)? ;
fragment f2: DIGIT(DIGIT)?

This allows you to have an ambiguity between f1 and f2, because they don't have to match on their own.