bug#51733: 27.1; Detect impossible email addresses better

[Lars Ingebrigtsen]

I'm now looking at 5.3 Mixed-Number Detection:

d U+09EA ( ৪ ) BENGALI DIGIT FOUR can be confused with U+0038 ( 8 )
DIGIT EIGHT.

Right, but they recommend implementing this by looking at the digit
version of the character first...  but...  Does Emacs have a function to
get the number value of ৪?  (Which should be 8.  😀)  They then
recommend comparing the value with the zero value of that system, and
I'm pretty sure we don't have that.

I don't quite understand why it's not sufficient to see that we have
numbers from two different numbering systems (which is trivial by
looking at the Nd category and then comparing the scripts).

Does anybody understand why they're doing this in a much more convoluted
manner here?  I must be missing something:

https://www.unicode.org/reports/tr39/#Mixed_Number_Detection

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   bloggy blog: http://lars.ingebrigtsen.no