use Regexp::Assemble;
use Date::Calc;
my $re = Regexp::Assemble->new;
for my $y ( 1900 .. 2100 ) {
  for my $m ( 1 .. 12 ) {
    for my $d ( 1 .. 31 ) {
      if ( Date::Calc::check_date( $y, $m, $d ) ) {
        $re->add( sprintf( "^%4d.%02d\.%02d\$", $y, $m, $d ) );
      }
    }
  }
}

print $re;