require 5.010;
my $sep         = qr{ [/.-] }x;               #allowed separators
my $any_century = qr/ 1[6-9] | [2-9][0-9] /x; #match the century
my $any_decade  = qr/ [0-9]{2} /x;            #match any decade or 2 digit year
my $any_year    = qr/ $any_century? $any_decade /x; #match a 2 or 4 digit year

#match the 1st through 28th for any month of any year
my $start_of_month = qr/
    (?:                         #match
        0?[1-9] |               #Jan - Sep or
        1[0-2]                  #Oct - Dec
    )
    ($sep)                      #the separator
    (?:
        0?[1-9] |               # 1st -  9th or
        1[0-9]  |               #10th - 19th or
        2[0-8]                  #20th - 28th
    )
    \g{-1}                      #and the separator again
/x;

#match 28th - 31st for any month but Feb for any year
my $end_of_month = qr/
    (?:
        (?: 0?[13578] | 1[02] ) #match Jan, Mar, May, Jul, Aug, Oct, Dec
        ($sep)                  #the separator
        31                      #the 31st
        \g{-1}                  #and the separator again
        |                       #or
        (?: 0?[13-9] | 1[0-2] ) #match all months but Feb
        ($sep)                  #the separator
        (?:29|30)               #the 29th or the 30th
        \g{-1}                  #and the separator again
    )
/x;

#match any non-leap year date and the first part of Feb in leap years
my $non_leap_year = qr/ (?: $start_of_month | $end_of_month ) $any_year/x;

#match 29th of Feb in leap years
#BUG: 00 is treated as a non leap year
#even though 2000, 2400, etc are leap years
my $feb_in_leap = qr/
    0?2                         #match Feb
    ($sep)                      #the separtor
    29                          #the 29th
    \g{-1}                      #the separator again
    (?:
        $any_century?           #any century
        (?:                     #and decades divisible by 4 but not 100
            0[48]       |
            [2468][048] |
            [13579][26]
        )
        |
        (?:                     #or match centuries that are divisible by 4
            16          |
            [2468][048] |
            [3579][26]
        )
        00
    )
/x;

my $any_date  = qr/$non_leap_year|$feb_in_leap/;
my $only_date = qr/^$any_date$/;