#!/usr/bin/perl -w
#============validdate.pl===================================
# Checks date dd/mm/yyyy
# 4/3/2002 danny@vrijdag.xs4all.nl

    print "Check dates of the form dd/mm/yyyy\nq quits.\n-:";
    &test;

# just a wrapper:
sub test {
    while (<>) {
        last if m/(?:^q|bye|end|exit|quit|stop)/i;
        chop;
        my $val= &validate($_);
        print " '$_': $val\n-:";
    }
    print $_."\n";
}

# The core:
sub validate {
    my ($in)=@_;
    return "No dd/mm/yyyy" unless m !
        ^(?:                # the start
           (?:              #   A: not leapsensitive or not leap:
             (?:0?[1-9]|1\d|2[0-8]) # A1: dd (0)1 - 28
             /              #          separator (or [\.- ])
             (?:0?[1-9]|1[0-2]) #          mm (0)1 - 12 (any month)
            |               #      or A2: 
             (?:29|30)      #          dd 29 or 30
             /              #          separator
             (?:0?[13-9]|1[0-2]) #     mm (0)1,3-12: any month but 2
            |               #      or A3: 
             31             #          31
             /                          
             (?:0?[13578]|1[02]) #     a long mm: 1,3,5,7,8,10,12
           )                #   end dd/mm non-leap
             /                         
             \d{4}          #   4 digits: any yyyy goes in A.
          |                 #   B: leapday in a leapyear:
           29/0?2/          #      that is 29/(0)2 february
             (?:            #      B1:  
               \d\d         #         any century and
               (?:          #         divisable
                 0[48]      #         by 4 but not by 100
                |           #         (so 04, 08 but not 00)
                 [2468][048] #        even tens:20,24,28,40..
                |
                 [13579][26] #        odd tens: 12, 16, 32, 36, 52..
              )
             |              #      B2:
              (?:           #         yyyy divisible by 400
                [02468][048] #        even mill: 00xx, 04xx, 08xx..
               |
                [13579][26] #         odd mill: 12xx, 16xx, 32xx..
              )00           #         so divisible by 400
           )                #   end leapday
        )                   # end day
        $!x;                # nothing else, nothing more

    return "Ok";
}