Repetition count

[jmillan]

Hi there,

I have not found a way to specify a 'repetition count' for a parsing
expression. Is it possible?

Thanks a lot.

[David Majda]

Hi,

2011/1/21 jmillan <doct...@gmail.com>:

> I have not found a way to specify a 'repetition count' for a parsing
> expression. Is it possible?

currently it is not supported directly, you need to repeat expressions
in the grammar manually. I didn't add a special syntax for expressing
repetition since in most grammars it is used very rarely. Do you have
a specific use case in mind?

--
David Majda
Entropy fighter
http://majda.cz/

[jose luis millan]

Hello,

First of all, thanks for building this tool.

On Sun, Jan 23, 2011 at 11:37 AM, David Majda <da...@majda.cz> wrote:
> Hi,
>
> 2011/1/21 jmillan <doct...@gmail.com>:
>> I have not found a way to specify a 'repetition count' for a parsing
>> expression. Is it possible?
>
> currently it is not supported directly, you need to repeat expressions
> in the grammar manually. I didn't add a special syntax for expressing
> repetition since in most grammars it is used very rarely. Do you have
> a specific use case in mind?
>

This could be a representative example. A case where the grammar
accepts a number of a maximum of 5 digits for a specific rule:
_________
number = number: (DIGIT DIGIT? DIGIT? DIGIT? DIGIT?) { return
number.join("") }

DIGIT = DIGIT: [0-9]
_________

The specific use cases are network port ranges and IPv4 and and IPv6
network addresses.

Anyway, for this cases, expression repetition does the work.

[David Majda]

Hi,

2011/1/23 jose luis millan <doct...@gmail.com>:

> This could be a representative example. A case where the grammar
> accepts a number of a maximum of 5 digits for a specific rule:
> _________
> number  = number: (DIGIT DIGIT? DIGIT? DIGIT? DIGIT?)  { return
> number.join("") }
>
> DIGIT     = DIGIT: [0-9]
> _________
>
> The specific use cases are network port ranges and IPv4 and and IPv6
> network addresses.

Thanks for the use-case.

I still think adding repetition to the grammar syntax is not really
necessary, but if some nice syntax gets invented, I'd probably add it
for convenience.

[Jiangbin Zhao]

Hi, I started learning pegjs yesterday. Great tool. Please see my comment about repetition below.

I ran into the same situation wanting repetition count, too. I think having it will help simplify the grammar. What if an expression could repeat 20 times, or even more?

As for the syntax to specify the count, how about: "expression(min, max)"? The count would be in the range of min and max, inclusively.

Instead of manually repeating the "expression?" constructs many times, I ended up checking the array length in parser action, for example:

    id = digits:[0-9]+ {return (digits.length <= 20 && digits.length >= 10) ? digits.join('') : null;}

But that certainly wouldn't look as good as this:

    id = digits:[0-9](10,20) {return digits.join('');}

Regards,

Jiangbin

PS: Forgive me if this post appears twice. My first attempt seemed have not gone through.

[David Majda]

Hi,

sorry for a late reply, I am crawling through a big e-mail backlog.

I reconsidered my negative position towards specifying arbitrary
repetition count. I am now trying to decide the syntax. See the
following issue on GitHub where I tried to summarize my thinking:

https://github.com/dmajda/pegjs/issues/30

David

2011/9/25 Jiangbin Zhao <zhaoji...@gmail.com>:

--