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));
  

  }

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));
}