my ($month_name) =
  map  qr{ (?<! [[:alpha:]]) (?: $_) (?! [[:alpha:]]) }xms,
  join q{|},
  map  { $_, ucfirst($_), uc($_) }
  map  { $_, substr $_, 0, 3 }
  qw(january february march april may june
     july august september october november december)
  ;
;;
my $month_number = qr{ (?<! \d) (?: 0? [1-9] | 1 [012]) (?! \d) }xms;
;;
my $month = qr{ $month_name | $month_number }xms;
;;
my $day = qr{ (?<! \d) (?: 0? [[1-9] | [12] \d | 3 [01]) (?! \d) }xms;
;;
my $yr = qr{ (?<! \d) 20\d\d (?! \d) }xms;
;;
my $sep = qr{ [-/] | \s+ }xms;
;;
my $date = qr{
  $yr    $sep $month $sep $day
  |
  $month $sep $day   $sep $yr
  |
  $day   $sep $month $sep $yr
  }xms;