you can use datetime for this (in python 2.7)
from datetime import datetime
date_string = '20170808'
if len(date_string) == 8:
datetime.strptime(date_string, '%Y%m%d')
else:
raise ValueError('date need to be in format YYYYMMDD ')
if you date is not valid than you get:
a ValueError Exception
ValueError: unconverted data remains: 0
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solved validate date format in python [with regex or any built in method]