[ragel-users] ragel scanner construct question
thurston at complang.org
Tue Apr 21 02:10:52 UTC 2009
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?
> 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.
>> 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
>>> 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.
>>> ragel-users mailing list
>>> ragel-users at complang.org
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