--- to_s        ruby 1.7 feature

    ɽʸɽ֤ޤ֤ʸ¾ɽ
    Ǥ⤽ΰ̣ݻ褦ˤʤäƤޤ

        re = /foo|bar|baz/i
        p re.to_s       # => "(?i-mx:foo|bar|baz)"
        p /#{re}+/o     # => /(?i-mx:foo|bar|baz)+/

    Ȥޤɽϰտ̤ˤϤʤʤ礬ޤ
    ϸȤֹǤǤʤǤ

        re = /(foo|bar)\1/      # \1 ϡfoo  bar
        p /(baz)#{re}/          # \1 ϡbaz

        # => /(baz)(?-mix:(foo|bar)\1)/

