PHP isDate, checks for valid date, redux 3

I just can’t leave well enough alone. This third iteration of the PHP isDate function is a little more concise, and makes changing acceptable formats a little easier. The function signature stays the same, accepting a date, and the expected format of the date, using PHP date expressions.

1.  accepts a date, and a date format (not a regular expression)

function isDate($d, $f='m-H:%M'){

       $dateFmtRE = array(
            '/\//' => '\/',
            '/%d|%e/' => '(0?[1-9]|[12][0-9]|3[01])',
            '/%m/' => '(0?[1-9]|1[012])',
            '/%g|%G|%y|%Y/' => '(19\d\d|20\d\d)',
            '/%H|%I|%l/' => '([0-1]?\d|2[0-3])',
            '/%M/' => '([0-5]\d)');
        if (empty($d))  return true;

        #convert Unix timestamp to a std format (must not include regular expressions)
        if (preg_match('!\d{5,}!', $d))
            $d = strftime($f, $d);

        return (
            #does %d match the regular expression version of $f? if it does m/d/y are in $x
            preg_match('!^' .preg_replace(array_keys($dateFmtRE), array_values($dateFmtRE),$f) .'$!', $d, $x)
            && (checkdate($x[2], $x[1], $x[3]) || checkdate($x[1], $x[2], $x[3]) || checkdate($x[3], $x[1], $x[2]))
            ?true :false);

}