js-credit-card-spacing.js

function spaceCard(str) {
  var ret = [];
  var i;
  var len;

  str = str.replace(/\s|\D+/gi, '');

  if( str.match(/^3[47]/) ) {
      // american express only
      var sec = 4;
      for(i = 0, len = str.length; i < len; i+= sec) {
          switch(i) {
              case 4:
                  sec = 6;
                  break;
              case 10:
                  sec = 5;
                  break;
          }
          ret.push(str.substr(i, sec))
      }
  } else {
      // group by four
      for(i = 0, len = str.length; i < len; i += 4) {
          ret.push(str.substr(i, 4))
      }
  }

  return ret.join('  ');
}

$('#card-number').on('keyup', function() {
    var $el = $(this);
    $el.val( spaceCard($el.val()) );
});