xxxx 2001-01-01 yyyy 200-09-09 zzz 2000-009-08 xxxx 2001-01-01 yyyy null zzz null
# nawk '$2!~"[0-9][0-9][0-9][0-9]-[0-9][0-9]-[0-9][0-9]"{$2="null"}{print}' infile xxxx 2001-01-01 yyyy null zzz null