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if ("2020-01-31"=~ /^2020-(?!12).+-31/) { print "OK!\n"; } # => OK!
if ("2020-12-01"=~ /^2020-(?!12).+-31/) { print "OK!\n"; } # =>
if ("2020-05-31"=~ /^2020-(?!12|01).+-31/) { print "OK!\n"; } # => OK!