[antlr-interest] The unary not (~) vs. W3C EBNF dash operator

[Loring Craymer]

Wincent--

Thanks for the pointer.  Of course there are ways to
avoid re-lexing; I was giving the short answer. 
Andreas might be better off generating a Ragel lexer
and integrating that with ANTLR parsers than building
an ANTLR lexer.

--Loring

--- Wincent Colaiuta <win at wincent.com> wrote:

> El 8/10/2007, a las 22:36, Loring Craymer escribió:
> 
> > The W3C spec uses a bastardized extension of BNF
> that
> > supports pattern matching and comparison of
> matched
> > patterns (the '-' operator that you are concerned
> > with).  To process that requires re-lexing of
> input;
> > ANTLR does not support that, nor does any other
> sane
> > lexer generator.
> 
> I may have misinterpreted you but I think you're
> wrong about that.  
> The operation that Andreas talks about is
> implemented in Ragel, for  
> example, as a "difference" operator. Ragel
> (<http://www.cs.queensu.ca/ 
> ~thurston/ragel/>) is a (very sane) finite state
> machine compiler  
> which implements the operator as follows:
> 
> > The difference operation produces a machine that
> matches strings  
> > that are in machine one but are not in machine
> two. To achieve  
> > subtraction, a union is performed on the two
> machines. After the  
> > result has been made deterministic, any final
> state that came from  
> > machine two or is a combination of states
> involving a final state  
> > from machine two has its final state status
> revoked. As with  
> > intersection, the operation is completed by
> pruning any path that  
> > does not lead to a final state. The following
> example demonstrates  
> > the use of subtraction to exclude specific cases
> from a set.
> 
> So this is a straightforward combination of two
> finite state machines  
> and no re-lexing is required in order to implement
> it. If you look at  
> the Ragel user manual
> (<http://www.cs.queensu.ca/~thurston/ragel/ 
> ragel-guide-5.24.pdf>) you can see some nice state
> machine diagrams  
> that illustrate exactly what's being done.
> 
> Best wishes,
> Wincent
> 
> 



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