Gets the number of the column from the letter reference.

getColNum.gs

/**
 * Gets the number of the column from the letter reference
 * (eg. A -> 1, B -> 2, G -> 7, AC -> 29, etc.).
 * @param {string} letters
 *   The letters referring to column which will be converted to a number.
 * @return {number}
 *   The column number represented by the given letters.
 */
function getColNum(letters) {
  var num = 0;
  letters.replace(/[A-Z]/gi, function(m, i, s) {
    num += (parseInt(m, 36) - 9) * Math.pow(26, s.length - i - 1);
  });
  return num;
}