for the found string, or nil if none found."
  (let (start end)
    (while (and (not start)
                (re-search-forward "\\([\"']\\|/\\*\\|//\\)" nil t))
      (cond ((= (preceding-char) ?*)
             ;; Disregard comments.
             (search-forward "*/"))
            ((= (preceding-char) ?/)
             ;; Disregard C++ comments.
             (end-of-line)
             (forward-char 1))
            ((= (preceding-char) ?\')
             ;; Disregard character constants.
             (forward-char (if (= (following-char) ?\\) 3 2)))
            ((save-excursion
               (beginning-of-line)
               (looking-at "^# *\\(include\\|line\\)"))
             ;; Disregard lines being #include or #line directives.
             (end-of-line))
            ;; Else, find the end of the (possibly concatenated) string.
            (t (setq start (1- (point))
                     end nil)
               (while (not end)
                 (cond ((= (following-char) ?\")
                        (if (looking-at "\"[ \t\n\\\\]*\"")
                            (goto-char (match-end 0))
                          (forward-char 1)
                          (setq end (point))))
                       ((= (following-char) ?\\) (forward-char 2))
                       (t (skip-chars-forward "^\"\\\\"))))
               ;; Check before string for keyword and opening parenthesis.
               (goto-char start)
               (skip-chars-backward " \n\t")
               (if (= (preceding-char) ?\()
                   (progn
                     (backward-char 1)
                     (skip-chars-backward " \n\t")
                     (let ((end-keyword (point)))
                       (skip-chars-backward "_A-Za-z0-9")
                       (if (member (list (buffer-substring-no-properties (point)
                                                                         end-keyword))
                                   keywords)
                           ;; Disregard already marked strings.
                           (progn
                             (goto-char end)
                             (setq start nil
                                   end nil))
                         ;; String found.  Prepare to resume search.
                         (goto-char end))))
                 ;; String found.  Prepare to resume search.
                 (goto-char end)))))
    ;; Return the found string, if any.
    (and start end
         (list (po-extract-unquoted (current-buffer) start end) start end))))

(defun po-mark-c-string (start end keyword)
  "Mark the C string, from START to END, with KEYWORD.
Leave point after marked string."
  (goto-char end)
  (insert ")")
  (save-excursion
    (goto-char start)
    (insert keyword)
    (or (string-equal keyword "_") (insert " "))
    (insert "(")))