ruby -v
# ==> ruby 1.8.4 (2005-12-24) [i486-linux]

class String; def =~(o); raise "S"; end; end
class Regexp; def =~(o); raise "R"; end; end

r = /x/
r =~ 'a'
# ==> RuntimeError: R
        from (irb):2:in `=~'
        from (irb):4
'a' =~ r
# ==> RuntimeError: S
        from (irb):1:in `=~'
        from (irb):5

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