[antlr-interest] Solving lexer ambiguities

[Jose Juan Tapia]

A lookahead I guess.


On Wed, Sep 12, 2012 at 2:20 PM, Jose Juan Tapia <jjtapia at gmail.com> wrote:

> Thank you for your suggestion. Unfortunately it still seems to be
> recognizing the .2 as a float. I was wondering if there was any way to tell
> the LEXER definition that any structure of the kind
>
> DOT DIGIT+
>
> should be recognized as a float, but if it has the form
>
> DOT DIGIT+ LETTER+, that is a DOT STRING where my STRING definition is
>
> STRING: (LETTER | DIGIT | '_')+
>
>
> it is recognized instead as a DOT STRING combination. instead of a FLOAT.
> Maybe I could be more strict with my STRING definition in some way?
>
>
> On Tue, Sep 11, 2012 at 10:41 PM, John B. Brodie <jbb at acm.org> wrote:
>
>> 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).
>>
>>
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>
>
>
> --
> José Juan Tapia Valenzuela
> Research Associate
> University of Pittsburgh
> 3076.1 Biological Sciences Tower 3
> Pittsburgh, Pa 15260
>



-- 
José Juan Tapia Valenzuela
Research Associate
University of Pittsburgh
3076.1 Biological Sciences Tower 3
Pittsburgh, Pa 15260