--- begin(n)

    n ܤʬʸƬΥեåȤ֤ޤ0 ϥޥå
    ̣ޤn ϰϳξ㳰 IndexError ȯ
    ޤn ܤʬʸ󤬥ޥåƤʤ nil 
    ֤ޤ

        /(foo)(bar)(BAZ)?/ =~ "foobarbaz"
        p $~.begin(0)   # => 0
        p $~.begin(1)   # => 0
        p $~.begin(2)   # => 3
        p $~.begin(3)   # => nil
        p $~.begin(-1)  # => `begin': index -1 out of matches (IndexError)

