#!/usr/local/bin/perl -w
use strict;
 
# CCYY-MM-DD[tz]
my $pat = qr /^-?\d{4,}-(0\d|1[0-2])-\d\d(Z?|([+|-]{0,1}[0-2]?\d:[0-5]\d)?)$/;
while (<DATA>) {
    chomp;
    print "$_ ", /$pat/ ? "matches\n" : "does not match\n";
}
 
__DATA__
1968-04-02
-0045-01-01
11968-04-02
1968-04-02+05:00
1968-04-02Z
invalids to follow
68-04-02
1968-4-2
1968/04/02
04-02-1968
1968-04-31

##</code><code>##

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