Convert HSBC CSV files to Xero Format

xero.el

(defun convert-hsbc-to-xero ()
  (interactive)
  (let ((filename (buffer-file-name))
        (new-name (replace-regexp-in-string "\.csv$" "-XERO.csv" (buffer-file-name)))
        (hsbc-header-start-regexp "^Date")
        (invalid-rows-regexps '("^$" "Opening Balance" "Closing Balance"))
        (header-row "*Date,*Amount,Payee,Description,Reference,Check Number\n")
        (record-regexp "^\\([0-9/]+\\),\\([[:alpha:][:blank:][:digit:]-/]+\\),\\([0-9.]*\\),\\([0-9.]*\\),\\([0-9.]*\\).*$"))
    (copy-file filename new-name)
    (find-file new-name)
    (goto-char 1)
    (re-search-forward hsbc-header-start-regexp)
    (end-of-line)
    (forward-char 1)
    (delete-region (point-min) (point))
    (insert header-row)
    (while (re-search-forward record-regexp nil t)
      (let ((date (match-string 1))
            (desc (match-string 2))
            (withdrawal (- 0 (string-to-number (match-string 3))))
            (deposit (string-to-number (match-string 4)))
            (balance (string-to-number (match-string 5))))
        (replace-match (format "%s,%s,,%s,,"
                               date
                               (if (< withdrawal 0) withdrawal deposit)
                               desc))))
    (mapc
     (lambda (r) (flush-lines r (point-min) (point-max)))
     invalid-rows-regexps)))