The user has to enter 6 digit #PNR Number. The number of digits=6, In that digits at least have one upper case , one lower case & one number. Need Regex for this condition.

@Saicharan Pittala, You can use below formula

NOT(AND((LEN(PNR_Number__c) = 6, REGEX( '(?=.*[a-z])(?=.*[A-Z])(?=.*\d)')))