JavaCC: Matching with wildcard but not consuming for state switch

Question

In JavaCC, for example in state DEFAULT , I want to perform a state switch, if the next token is <A> , I want to switch to state STATE_A , otherwise, I want to switch to state STATE_B .

I tried to use something like the following code with "" as a wildcard:

TOKEN:
{
  <A: "aa"> : STATE_A
| <NOT_A: ""> : STATE_B
}

But it doesn't work, when a character that cannot be reduced to A is met, the function returns immediately, and doesn't get switched to STATE_B , therefore "" doesn't seem to be able to do the job.

Do you have any suggestions? Thanks.

Answer 1

Well I find that this actually works . The empty string will be matched when A cannot be matched, however we need to explicitly refer to NOT_A in the definitions of non-terminals. Therefore expressions like

[ <A> ]

should be rewritten as

( <A> | <NOT_A> )

to enforce a state switch.