[antlr-interest] Re: Problem with x: (A)(B)? ;

[Terence Parr]

On Feb 25, 2004, at 10:26 AM, dotlessbraille wrote:

> I really appreciate your trying to help but request you to please
> look at the generated Java. I understand that ANTLR will put in a
> test for an EOF if I don't put one in.  However, this test is at the
> highest level of alternatives.
>
> The problem I am encountering is that ANTLR interprets the (B)? as
> requiring either a B or an EOF directly after the A.  In other
> words, it won't handle the case where there are two A's.  It works
> fine if I use x:A(B)* which isn't what I want, however.
>
> startRule :
>       (x:A(B)?
>       |b:B
>       |eof:EOF
>       );

Let me take a look at this.  Hmm...i don't see how that will match two 
A's.  It matches A, AB, B, EOF.  You're use of x: confusing me.  Is 
this a rule or a label as you have here?  Assuming it's a label.  Ok, 
let me rephrase your question to see if I understand:

start : (A (B)* | B)? ;

works but

start : (A (B)? | B)? ;

does not.  This is as designed.  In the loop case, I avoid a lookahead 
test for the exit branch each time by simply saying "while lookahead 
consistent with stuff inside; B in this case".  For the (B)? optional 
case, i test the lookahead for what follows the (B)? subrule which is 
EOF.

You cannot escape that EOF follows your start symbol, I'm afraid.  Can 
u explain why you don't want EOF to match?  Anything non-B should fail 
to enter the (B)? subrule, right?

Ter
--
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