[antlr-interest] how to parse a fraction

[Gavin Lambert]

At 18:46 26/09/2008, Sven Prevrhal wrote:
>One like 1/2 ?
>INT ‘/’ INT does not work as it requires spaces 
>between the numbers and the slash


If you've defined your grammar in the standard 
way (and if the above is in a parser rule), it 
won't actually require spaces (but it won't 
object if they're there).

INT : ('0'..'9')+;
SLASH : '/';
WS : (' ' | '\t' | '\r' | '\n')+ { $channel = HIDDEN; };

fraction : INT SLASH INT;

The above should match "1/2", "1   /2", "1/   2", 
and "1     /    2" (among other combinations).

Now, if you explicitly want to *forbid* 
whitespace to appear in there, you'll have to 
match it entirely as a lexer rule:

fragment INT : ('0'..'9')+;
SLASH : '/';
FRACTION
   :  INT
      ( (SLASH) => SLASH INT
      | { $type = INT; }
      )
   ;
WS : (' ' | '\t' | '\r' | '\n')+ { $channel = HIDDEN; };

value : INT | FRACTION;

Of course, if you have a division operator then 
all of the above is probably redundant, since the 
fraction one-half is the result of treating "1/2" 
as a division expression (unless you're doing 
integer division, of course).