Re: [abnf-discuss] Mail regarding draft-seantek-constrained-abnf

[Paul Kyzivat <[e-mail]>]

On 7/8/16 12:23 PM, Sean Leonard wrote:

>> I have difficulty imagining how you would fit such constraints into
>> the syntax. The syntax of ABNF is pretty nice for what it does, but it
>> doesn't leave much room for extension because we are running out of
>> special characters. (Perhaps the possibilities could be expanded by
>> using unicode characters. Imagine what could be done with emojis as
>> operator characters.)
>
> Length constraints are expressible with current ABNF. Generically,
> suppose you have an identifier comprised of ALPHAs and DIGITs:
>
> identifier = 1*(ALPHA / DIGIT)
>
> Well, if you want to limit identifier to 20 chars, then you can say max
> twenty:
>
> identifier = 1*20(ALPHA / DIGIT)
>
> As Paul says, this is a repetition constraints, and is easy to analyze
> with the repeated symbols are fixed-width. But what about variable width?
>
> Well, suppose you have Domain from RFC 5321:
>
>    Domain         = sub-domain *("." sub-domain)
>
>    sub-domain     = Let-dig [Ldh-str]
>    Let-dig        = ALPHA / DIGIT
>    Ldh-str        = *( ALPHA / DIGIT / "-" ) Let-dig
>
>
>
> Now suppose you want to constrain Domain to be 255 chars. Such a
> constrained domain might be called DNSDomain. Then you can say:
>
>    DNSDomain ^ Domain = 1*255ASCII

Ah. Interesting!


> Regarding Paul's Unicode point: it would be nice for ABNF to have an
> extension that defines the domain of the input symbols, with specific
> options for: ASCII-only, octets, fixed-bit-width units, UTF-8,
> UTF-16LE/BE, and Unicode scalar values (0-0xD7FF, 0xE000-0x10FFFF), and
> Unicode characters (non-characters such as 0xFFFF are excluded). The
> most important of these are ASCII-only, octets, and Unicode scalar values.

Note that if you are using quoted strings in ABNF then you are 
restricted to their mapping to ASCII, which then extends obviously to 
Unicode.

I am not well informed about Unicode, but ISTM that if you are using 
ABNF with it then it makes sense to only do it over Unicode scalar 
values, while leaving the conversion of those to/from a particular 
encoding like UTF-8 can be via a pre/post-processor. You need to be a 
glutton for punishment to define your Unicode-based grammar over octets. 
And is there any operational difference between defining ABNF over 
unsigned integers and defining it over Unicode scalar values?

	Thanks,
	Paul