Karol S already provided a solution, but the reason might not be clear:
"\u1F600" is actually "\u1F60" followed by "0":
"\u1F60" # => "ὠ"
"\u1F600" # => "ὠ0"
You have to use curly braces for code points above FFFF:
"\u{1F600}" #=> "😀"
Therefore the character class [\u1F600-\u1F6FF] is interpreted as [\u1F60 0-\u1F6F F], i.e. it
matches "\u1F60", the range "0".."\u1F6F" and "F".
Using curly braces solves the issue:
/[\u{1F600}-\u{1F6FF}]/
This matches (emoji) characters in these unicode blocks:
- U+1F600..U+1F64F Emoticons
- U+1F650..U+1F67F Ornamental Dingbats
- U+1F680..U+1F6FF Transport and Map Symbols
You can also use unpack, pack, and between? to achieve a similar result. This also works for Ruby 1.8.7 which doesn’t support Unicode in regular expressions.
s="Hi!😀"
#=> "Hi!\360\237\230\200"
s.unpack('U*').reject{ |e| e.between?(0x1F600, 0x1F6FF) }.pack('U*')
#=> "Hi!"
Regarding your Rubular example – Emoji are single characters:
"😀".length #=> 1
"😀".chars #=> ["😀"]
Whereas kaomoji are a combination of multiple characters:
"^_^".length #=> 3
"^_^".chars #=> ["^", "_", "^"]
Matching these is a very different task (and you should ask that in a separate question).