module Regexp::Expression::ReferencedExpressions

def initialize_copy(orig)
  exp_id = [self.class, self.starts_at]
  # prevent infinite recursion for recursive subexp calls
  copied = self.class.instance_eval { @copied_ref_exps ||= {} }
  self.referenced_expressions =
    if copied[exp_id]
      orig.referenced_expressions
    else
      copied[exp_id] = true
      orig.referenced_expressions && orig.referenced_expressions.map(&:dup)
    end
  copied.clear
  super
end

def referenced_expression
  referenced_expressions && referenced_expressions.first
end