Dear ANTLR users! (developers?)
I have a very basic problem: supposed I have two rules, one which
basically includes any token, and another one that contains only a
subset of them, say:
any : A | B | C | D ;
some
: A B
| B
;
Now, I want to combine those two rules:
rule : ( options{ greedy=false;} : any )+ some ;
.. such that the inner loop should consume any's, as long as LA(1)!=B &&
(LA(1)!=A||LA(2)!=B). This should be easily recognizable by the parser
(in my opinion). However, ANTLR 3 (3.1.2) says:
warning: Decision can match input such as "A B" using multiple
alternatives: 1, 2
Note that when I change the rule "some" to:
some : A | B;
such that only a lookahead of one character is enough, everything works
as expected: "any" is consumed until a character is found from the
difference "any-some".
Now, I thought "ok, use backtracking". Enabling it globally seems to
work. But, if I try to enable it only locally, it does not seem to have
any effect. Actually, I tried to enable it for any rule in my grammar -
without any effect! (I include the grammar in question as attachement)
What can I do? Thanks for any hints!
Andreas Meyer
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grammar GreedyTest;
//options
//{
// backtrack = true;
//}
start
// options { backtrack = true; }
: rule EOF;
rule
// options { backtrack = true; }
: ( options{ greedy=false;} : any )+ some
;
any
// options { backtrack = true; }
: A
| B
| C
| D
;
some
// options { backtrack = true; }
// : A // this works fine.
: A B // this does not. why? the lookaheadset is the same: {A,B}.
| B
;
A: 'A';
B: 'B';
C: 'C';
D: 'D';
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