Overriding =~ method on a String subclass generates inconsistency
In the source for the String#=~ method, Ruby handles the special case where the argument is of a built-in Regexp type used by the parser, which is the case when we write s =~ /abc/.
The rb_reg_match method can be found as being the Regexp#=~ method.
Thus, if you really want this behavior you can monkey patch the Regexp class to accept objects of MyString for the =~ operator, but that might go wrong very, very easily.
Another approach would be composition. You can either include the Forwardable module or go with plain old method_missing. Here's an example:
class MyString attr_reader :string def initialize(str) @string = str end def method_missing(*args) string.public_send(*args) end def =~(obj) "Overriden" endends = MyString.new "abc"s =~ /abc/ # => "Overriden"s =~ Regexp.new("abc") # => "Overriden"