. -> any character (.) -> any character, grouped to reference it, since it's the first group it's referenced as \1 (.)* -> that, 0 or more times (.)* DO -> a

. -> any character (.) -> any character, grouped to reference it, since it's the first group it's referenced as \1 (.)* -> that, 0 or more times (.)* DO -> any character 0 or more times, followed by DO (.)* DO\1 -> any character 0 or more times, followed by DO, and the same character as in the beginning