[antlr-interest] The eternal question: Why is this ANTLR grammar ambiguous??

[Harald M. Müller]

[I have posted this to the pccts newsgroup; and on the antlr forum 
on jguru - I'm sorry if this is "against the rules", but thos two 
forums seem to have quite low answer rates, and I have found this 
mailing list only now]

I have already written a few ANTLR lexers and parsers (for 
productive systems, with sometimes weird grammars) - but the 
following puzzles me: 
Why does the following simple grammar create an ambiguous warning?: 


class MyLexer extends Lexer;

options {
	charVocabulary = '\3'..'\377';
	k=3;
	caseSensitive=false;
}

COMMENT_COMMAND
  : '-' '-' '$'
  ;
  
SQL_STATEMENT
  : ( SYMBOL )+ ';'
  ;

protected SYMBOL
  : ('-' (~ '-')) => '-'   //1
  | 'a'
  | '"' (~ '"')* '"'       //2
  ;

When I remove either of //1 or //2, the ambiguity goes away. 
When I replace //2 with 

  | '"' (~ ('"'|'$'))* '"'

- i.e., I also exclude $ inside the "string" -, the ambiguity also 
goes away. 
But why would a $ after(!!) a " lead to an ambiguity????? 

Thanks for any help! 

Harald M.











 
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