John Hanauer

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negate regexp (does not start w/'$', 'a'): ^(?!\$).+ ^(?!a).+
at least 2 occurrences of same letter: \b\w*(\w)\w*\1\w*\b
exactly 2 of the same letter: (\w*([a-zA-Z]).*\2{N-1,}\w*)
cidr