RE: grammar

[by way of Hans Aberg]

Hi,

thanks for your answer, but I need some explanation on the third rule you
gave to me ( I am just starting learning).

> a* := x, where x:   | x a      -- Zero or more
>
Does that mean that if we cannot apply any rule, then consider an empty
string and read a "a"?

For example, if I have those rules:
x= a s t d
s = b ¦ s b
t = ¦ t c

and I want to parse a b b c c c d

I'll get to a s . c c c d

Does the 3rd rule mean : I cannot do anythind, so consider an empty string
and push t? (in which case it would allow me to parse correctly the
message).
Message would then be a s t . c c c d

Thanks

Bertrand


> ----------
> From:         Hans Aberg[SMTP:address@hidden
> Sent:         Saturday, April 14, 2001 11:50
> To:   address@hidden
> Cc:   address@hidden
> Subject:      Re: grammar
>
> At 14:03 +0200 2001/04/13, address@hidden wrote:
> >Could someone tell me how I can write the grammar to parse such message:
> >
> >abbcccd
> >
> >in which
> >a is mandatory and non repetitive,
> >b is mandatory and repetitive,
> >c is optional and repetitive,
> >d is mandatory and non repetitive.
>
> If you are only interested in regular words, and do not intend to attach
> actions to the rules, you can use Flex
>   ftp://ftp.digital.com/pub/GNU/non-gnu/flex/flex-2.5.4a.tar.gz
> If you need greater generality, under C++, you could create a istream that
> outputs characters representing grammar symbols.
>
> Alternatively, using Bison, you could write your grammar as EBNF, and then
> apply the definitions reducing to BNF (using Bison notation)
>   a? := | a                      -- Optional.
>   a+ := x, where x: a | x a      -- One or more.
>   a* := x, where x:   | x a      -- Zero or more.
>
> One the more advanced level, one could try tweak Bison so that it
> understands EBNF directly. -- This is the way that mid-actions are
> implemented (i.e., by introducing a new empty rule with an action). If you
> do this, please don't report it to Bison bugs.
>
>   Hans Aberg
>
>


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