I am attempting to create a validation rule on a field that ensures a time is formatted correctly. This is only if the user enters a time, whereas if left blank, one is automatically generated and that is fine, thus the validation rule should not fire.
Currently I have this, which isn't working (Rule fires no matter what):
( NOT(REGEX( Presentation_Start_Time__c , "[0-1]?[0-9]:[0-5][0-9] [A|P]M")) || NOT(ISBLANK(Presentation_End_Time__c)) )
&&
( NOT(REGEX( Presentation_End_Time__c , "[0-1]?[0-9]:[0-5][0-9] [A|P]M")) || NOT(ISBLANK(Presentation_End_Time__c)) )