str1="2017-10-15 this is happy day..." >>> re.sub("(d{4})-(d{2})-(d{2})",r"2/3/1",str1) ##'10/15/2017 this is happy day...' >>> re.sub("(?P<year>d{4})-(?P<month>d{2})-(?P<day>d{2})",r"g<month>/g<day>/g<year>",str1) #'10/15/2017 this is happy day...'