[antlr-interest] Solving lexer ambiguities

[John B. Brodie]

Greetings!

You might try something like the following --- obviously untested since 
you did not provide complete example of your issue:

FLOAT:
   (DIGIT)+ '.' (DIGIT)* EXPONENT?
| (DIGIT)+ EXPONENT;

  DOT: '.' ( (DIGIT)+ EXPONENT? {$type=FLOAT;} )? ;

hopefully in your language the 2structure strings  can never match a 
FLOAT.....
(e.g. something like 1structure.2E5.35 isnt permitted....)

Hope this helps...
    -jbb

On 09/11/2012 08:45 PM, Jose Juan Tapia wrote:
> So I was gaving a problem with my lexer recognition where my double token
> is defined as follows.
>
> FLOAT:
>    (DIGIT)+ '.' (DIGIT)* EXPONENT?
> | '.' (DIGIT)+ EXPONENT?
> | (DIGIT)+ EXPONENT;
>
>
> However additional to that I have certain structures where the following
> syntax:
>
> 1structure.2structure .35
>
> Should be recognized by the following grammar
>
> STRING (DOT STRING)? FLOAT
>
> The problem being of course, that my lexer is recognizing the .2 token as a
> FLOAT and I'm not sure how can I make it so that it choses the alternative
> solution. (I've tried using  backtracking to no avail. Maybe I'm doing it
> wrong but my current assumption is that since the ambiguity is at the lexer
> rather than at the parser level the parser can't do much to solve the
> conflict).