* meta character \X matches Unicode 9.0.0 characters with some workarounds
  for UTR #51 Unicode Emoji, Version 4.0 emoji zwj sequences.
  [Feature #12831] [ruby-core:77586]

The term "character" can have many meanings bytes, codepoints, combined
characters, and so on. "grapheme cluster" is highest one of such words,
which means user-perceived characters.
Unicode Standard Annex #29 UNICODE TEXT SEGMENTATION specifies how to
handle grapheme clusters (extended grapheme cluster).
But some specs aren't updated to current situation because Unicode Emoji
is rapidly extended without well definition.
It breaks the precondition of UTR#29 "Grapheme cluster boundaries can be
easily tested by looking at immediately adjacent characters". (the
sentence will be removed in the next version)
Though some of its detail are described in Unicode Technical Report #51
UNICODE EMOJI but it is not merged into UTR#29 yet.

http://unicode.org/reports/tr29/
http://unicode.org/reports/tr51/
http://unicode.org/Public/emoji/4.0/

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56949 b2dd03c8-39d4-4d8f-98ff-823fe69b080e