[antlr-interest] Handling optional spaces

[Austin Hastings]

You didn't include the full grammar, so it's hard to know what's going 
on. I'd bet it has something to do with how you defined ID, and whether 
you have a WHITESPACE or WS rule.

=Austin

Justin Crites wrote:
> Hi there,
>
> I am just getting started using ANTLR for a toy language I'm 
> developing.  I am trying to understand, why do these two grammars 
> behave differently?
>
> 1)
>
> expr     :    OptSpace ID OptSpace;
>
> OptSpace :    ' '*;
>
> 2)
>
> expr     :    ' '* ID ' '*;
>
> In grammar #1, the string "x" does not match, and I get mismatched 
> token exceptions.  It looks like ANTLR is trying to match OptSpace and 
> failing.  This confuses me because OptSpace's body is just ' '* which 
> should be able to match nothing, I thought.
>
> Why do these two grammars behave differently?  Does OptSpace always 
> have to match something, even if its body is potentially empty?
>
> More generally, how can I find out about and control ANTLR's handling 
> of whitespace?  I observed the behavior that strings with spaces in 
> them can be parsed even if space is not in the language definition 
> anywhere.  For example, if I have a rule like:
>
> expr : ID+;
>
> It will match input like "a b", even though technically " " should not 
> be matched by anything.  How can I make ANTLR _not_ accept strings 
> like this?  Would I have to write all my rules like this?
>
> expr : ID | (ID Space expr);
>
> Thanks for your help.
>
> -- 
> Justin Crites
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