Possible bug in formal grammar for extended grapheme cluster

Mark Davis ☕️ via Unicode unicode at unicode.org
Sun Dec 17 11:17:57 CST 2017

Thanks for the feedback. You're correct about this; that is a holdover from
an earlier version of the document when there was a more basic treatment of
RI sequences.

There is already an action to modify these. There is a placeholder review
note about that just above


(scroll up just a bit).


Mark <https://twitter.com/mark_e_davis>

On Sun, Dec 17, 2017 at 4:16 PM, David P. Kendal via Unicode <
unicode at unicode.org> wrote:

> Hi,
> It’s possible I’m missing something, but the formal grammar/regular
> expression given for extended grapheme clusters appears to have a bug
> in it.
> <https://unicode.org/reports/tr29/#Table_Combining_Char_
> Sequences_and_Grapheme_Clusters>
> The bug is here:
>     RI-Sequence := Regional_Indicator+
> If the formal grammar is intended to exactly match the rules given the
> the “Grapheme Cluster Boundary Rules” section below it as-is, then
> this should be
>     RI-Sequence := Regional_Indicator Regional_Indicator
> since as given it would cause any number of RI characters to coalesce
> into a single grapheme cluster, instead of pairs of characters. That
> is, the text U+1F1EC U+1F1E7 U+1F1EA U+1F1FA would represent one
> grapheme cluster instead of the correct two.
> --
> dpk (David P. Kendal) · Nassauische Str. 36, 10717 DE · http://dpk.io/
>    we do these things not because they are easy,      +49 159 03847809
>       but because we thought they were going to be easy
>           — ‘The Programmers’ Credo’, Maciej Cegłowski
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