function toUpperCaseFormula(expression) {
  var PATTERN = /(\\"|"(?:\\"|[^"])*"|(\+))|(\\'|'(?:\\'|[^'])*'|(\+))/g;
  var strings = expression.match(PATTERN) || [];
  var index = -1;
  return expression.toUpperCase().replace(PATTERN, function () {
    index += 1;
    return strings[index];
  });
}
Upper case formula expression.

@param {String} expression Formula expression.
@returns {String}