[ragel-users] ragel scanner construct question

[Adrian Thurston]

Oh, my bad. In that case yes! You can do something like this.

word = [a-z];
number = [0-9];

token = word | number;
op = '+' | '-' | '*' | '\\';

OWS = [ \t\r\n]*;

main := OWS token OWS ( op OWS token OWS )*;

Pramukta Kumar wrote:
> I understand that an algebraic expression can be described by a  
> context free grammar and not by a regular grammar (I thought due to  
> the recursive requirement).  However, i'm only referring to the  
> validation of the next symbol.  All I want to express within ragel is  
> that within the buffer, "2 3" shouldn't match since it doesn't make  
> sense for a number to be directly followed by another number whereas  
> "2 +" or "2 )" would make sense.  I can see doing this via the  
> lookahead strategy you talk about in the manual, but that seems to be  
> a little excessive.  It just seems like all I am making use of to  
> implement this portion is a next-state table so it can't be so off the  
> wall.  Am I still totally off the mark?
> 
> ~pramukta
> 
> also, is the dragon book still the best place to look for this stuff?
> 
> 
> On Apr 20, 2009, at 7:16 PM, Adrian Thurston wrote:
> 
>> You can't do that in Ragel. To understand why you'll have to do some
>> reading on context-free VS regular languages. Pick up a compiler book.
>> It's a big topic.
>>
>> -Adrian
>>
>> Pramukta Kumar wrote:
>>> I am experimenting with ragel to validate and tokenize an algebraic
>>> expression in one step.  Basically convert something like "3 + A *  
>>> 2 /
>>> ( 1 - 5 )" to "[3.0, 'A' , 2.0, :*, 1.0, 5.0, :-, :/, :+]" if it's
>>> valid, and throw an error otherwise. I'm using the ruby target, and
>>> the standard "shunting yard" algorithm or whatever.
>>>
>>> I have the thing working (I think, still testing) by using the  
>>> scanner
>>> construct and making an "allowed transition" table by hand.  Each of
>>> the tokenizing actions validates against this table to see whether it
>>> should continue.  It seems to me that there is probably a nice way to
>>> do this within ragel itself but I can't seem to figure it out.  Can
>>> somebody point me in the right direction?
>>>
>>> the files are up on github: http://github.com/prakatmac/expression-parser/tree/master
>>>
>>> Sorry if this is a dumb question.  I'm pretty new at this stuff.
>>> Thanks.
>>>
>>> ~pramukta
>>>
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> 
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