bash-2.03$ ./date_regex.pl 
1968-04-02 matches
-0045-01-01 matches
11968-04-02 matches
1968-04-02+05:00 matches
1968-04-02Z matches
invalids to follow does not match
68-04-02 does not match
1968-4-2 does not match
1968/04/02 does not match
04-02-1968 does not match
1968-04-31 matches