Re: Converting BNF grammar to EBNF

On 9 Aug 2006 00:00:35 -0400, "Scorpio" <sandy.pec@gmail.com> wrote:


>Hi
>
>I am a newbie with the parsing stuff, and I am trying to convert a BNF
>grammar to an EBNF one. I have a lexer rule which states Set ::= Set1 -
>Set2; I am using ANTLR v3.0 Beta 3 and unable to get how to convert
>this into the ANTLR grammar. ANTLR just rejects the "-" operation on
>sets. Please help!
>


It's been a while since I've done a parser with ANTLR so don't depend
on the syntax below.




Depending on what Set2 actually is, you may be able to use a syntactic
predicate like the following that proceeds only if the next token is
not from Set2.


      Set: ( ~Set2 )=> Set1;




There is also a way (but I forget how - check the manual) to
programmatically examine the token so you could write a semantic
predicate to check for and reject a token from Set2. Semantic
predicates throw an exception if they fail which you will have to
catch. Use an alternation like the following:


      Set: { /* code that returns false if token in Set2 */}?
                exception
                    catch [SemanticException ex]
                        { /* do nothing */}


            | Set1
            ;




Hope this helps.
George

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