ruby正则表达式使用锚点

string = "abc\ndef\nghi"
puts string
puts /^abc/=~string
puts /abc$/=~string
puts /^def/=~string
puts /def$/=~string
puts /^ghi/=~string
puts /ghi$/=~string

string = "abc\ndef\nghi"
puts /\Aa/=~string
puts /\Adef/=~string
puts /ghi\Z/=~string

string  = "abc\ndef\nghi"
str2="abc\ndef\nghi\n"
puts /ghi\Z/ =~ str2    # 8
puts /ghi\Z/=~string    #8
puts /ghi\z/ =~string    # 8
puts /ghi\z/ =~str2      # nil