[antlr-interest] Fwd: Common Prefix question

[Douglas Godfrey]

the lexer rule RESULT must come before the rule R.
the way you specify them, RESULT will never be matched and Result in the
input text will give an error.

Lexer rules must be ordered so that all rules with a common prefix are
ordered longest first.

On 11/10/11 11:05 PM, "Eric" <researcher0x00 at gmail.com> wrote:

>---------- Forwarded message ----------
>From: Eric <researcher0x00 at gmail.com>
>Date: Thu, Nov 10, 2011 at 9:55 PM
>Subject: Re: [antlr-interest] Common Prefix question
>To: Alexander Kaiser <alexander.g.kaiser at gmail.com>
>
>
>
>
>On Mon, Nov 7, 2011 at 3:56 PM, Alexander Kaiser <
>alexander.g.kaiser at gmail.com> wrote:
>
>> Hi,
>>
>> I am new to antlr (antlr3) and struggling with lexing common prefixes.
>> My simple grammar:
>>
>> grammar simple;
>>
>>
>> R : 'R';
>> E : 'e';
>> D : '1';
>> RESULT : 'Result';
>>
>>
>> expr : ( RESULT | R E D )  EOF;
>>
>>
>> On input 'Result' I expect token RESULT, and on input Re1 I expect
>>tokens R
>> E D but input 'Re1' only throws NoViableAltException.
>> I searched for info 'antlr common prefixes' on the web and tried
>>increasing
>> k, lookahead predicates ..., things like
>>
>> RESULT options{filter=true;} : 'Result';
>>
>>
>>
>> , but to no avail. I would be very much obliged if anyone could give me
>>a
>> hint.
>>
>
>Hi Alexander,
>
>Usually you want to convert yoru input into an AST and make the lexer and
>parser as simple as possible. A common mistake of beginners is to try and
>modify the lexer or parser into doing work that should be done with the
>AST. In this case, the goal is to end of with one of two tokens, either
>RED
>or RESULT. While it may be possible to contrive the lexer and parser into
>doing this, it is much easier to do with AST rewrites.
>
>Here is an example.
>
>grammar mail_004;
>options
>{
> output=AST;
>}
>tokens
>{
> RESULT;
> RED;
>}
>s : prefix suffix
> EOF;
>prefix  : R E
> ;
>
>suffix : D  -> RED
> | SULT  -> RESULT
> ;
>R : 'R';
>E : 'e';
>D : 'd';
>SULT : 'sult';
>
>If AST and rewrites are new concepts to you, then you should get a copy of
>"The Definitive ANTLR Reference" and read it. You can find some info on
>the
>website, but there is no substitute for the book.
>
>
>Eric
>
>
>
>
>
>>
>>
>>
>> --
>> Alexander
>>
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