[antlr-interest] assigning preference to one of two intersectingrules

[cgorski at cgorski.org]

Thanks.  I suppose my real question is, "is it considered 
acceptable practice to have ambiguities similar to those 
that I specified?"

I think I read the following in some documentation and got 
it in my head that every ambiguity warning has some 
elegant resolution drastically different from what what I 
was doing:

"What happens when the same character predicts more than a 
single lexical rule? ANTLR generates an nondeterminism 
warning between the offending rules, indicating you need 
to make sure your rules do not have common left-prefixes. 
ANTLR does not follow the common lexer rule of "first 
definition wins" (the alternatives within a rule, however, 
still follow this rule). Instead, sufficient power is 
given to handle the two most common cases of ambiguity, 
namely "keywords vs. identifiers", and "common prefixes"; 
and for especially nasty cases you can use syntactic or 
semantic predicates."

-Chris


On Fri, 17 Aug 2007 10:26:01 -0700
  "Diehl, Matthew J" <matthew.j.diehl at intel.com> wrote:
>> 
>> In the grammar that follows, I am attempting to match 
>>all 
>> strings such as:
>> 
>> "54"
>> "a54"
>> "#75"
>> "n.ob89"
>> 
>> I'd like to be able to extract the Prefix and Number 
>> values for later use in a parser.  What's the best way 
>>to 
>> go about this?  How do I tell
>> ANTLR to attempt to match first the first rule in 
>> BoxNumberComplete?
> 
> I don't think you can tell it to match the first...but 
>since your lexer
> already works, don't change it and in the parser just 
>analyze the string
> and get the location of the split manually:
> 
> identifier :
>  BoxNumberComplete
> {
>  String tmp = BoxNumberComplete.text;
>  int i;
>  for(i = tmp.length(); i >= 0; i--){
>    if (!Character.isAlphaNumeric(tmp.charAt(i))) //or 
>something like
> that
>      break;
>  }
>  //i is the location of the end of your first rule in
> BoxNumberComplete.
> }
>  ;
> 
> I like modifying the parser a lot better than the lexer.
> Good luck,
> Matt