with this format "20130411". Not "2013.kjh" or "NN.201304", or "2013mmkk04".
You might need to refine your definition of 'this format'. Must it be a valid date ? Can it be in the future ? Can it be anywhere in the past ? Could it be less than 8 digits ? ... Etc. If you'd want _precisely_ 8 digits and wouldn't care about validness of the date,