Form validation for zip codes
Changes I would make: (but not necessary)


Make sure inputtext has a value, and isn't undefined, so it wouldn't throw an error in your if statement.
Replace the if statement with return instead. (This is mostly to minimize the code a bit, something your minifier won't do for you).


Except for that, it seems ok (I'm not looking into the logic of the regex, since I'm not sure how a zipcode should be parsed).

function zipcode (inputtxt)    
{  

    if (!inputtxt || !inputtxt.value) return false;
    var zipcode = /^\+?([0-9]{2})\)?[-. ]?([0-9]{4})[-. ]?([0-9]{4})$/;  

    return ((inputtxt.value.match(zipcode));
}