Regular Expression Match Always Returns Null

I found this regular expression on the internet and it works fine when I test it in various validators on the web.

Code:

^(((0?[1-9]|1[012])/(0?[1-9]|1d|2[0-8])|(0?[13456789]|1[012])/(29|30)|(0?[13578]|1[02])/31)/(19|[2-9]d)d{2}|0?2/29/((19|[2-9]d)(0[48]|[2468][048]|[13579][26])|(([2468][048]|[3579][26])00)))$

It's purpose is to check for valid date and allows formats mm/dd/yyyy, m/dd/yyyy, mm/d/yyyy or m/d/yyyy.

When I try it with the code below it always returns null.

Code:

function isValidDate(/* String */ p1_date) {
var x = "^(((0?[1-9]|1[012])/(0?[1-9]|1d|2[0-8])|(0?[13456789]|1[012])/(29|30)|(0?[13578]|1[02])/31)/(19|[2-9]d)d{2}|0?2/29/((19|[2-9]d)(0[48]|[2468][048]|[13579][26])|(([2468][048]|[3579][26])00)))$";

[Code].....