Javascript form validation (zip code). Something better?

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 (not looking into the logic of the regex, since im 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));
} 

Um. There is verry little code, so one can not say much about your code. It does, what it should, I assume.

One thing, I see is, that you are alerting a message, which has nothing to do with the validation. So if you are looking for SRP - the separation of concerns, you take the alert out and put it elsewhere.

Of course you could shrink the whole thing down to

function checkZipcode(zip) {
    if(!zip) throw new TypeError("zip is not defined"); 
    return /^\+?([0-9]{2})\)?[-. ]?([0-9]{4})[-. ]?([0-9]{4})$/.test(zip); 
}

done. But whether this is an improvement or not is open.