weechat-log-mode.el

(setq weechat-timestamp-regex
      "\\([0-9]\\{4\\}-[0-9]\\{2\\}-[0-9]\\{2\\}\\)?[[:blank:]]?\\([0-9]\\{2\\}:[0-9]\\{2\\}:[0-9]\\{2\\}\\)")
(setq weechat-timestamp-prefix-regex
      (format "^[[:blank:]]*%s"
              weechat-timestamp-regex))
(setq weechat-info-line-regex
      (format "%s[[:blank:]]+\\(<?-->?\\)[[:blank:]]+\\(.+\\)"
              weechat-timestamp-prefix-regex))
(setq weechat-msg-line-regex
      (format "%s[[:blank:]]+\\([^[:blank:]]+\\)[[:blank:]]+\\(.+\\)"
              weechat-timestamp-prefix-regex))

(setq weechat-log-mode-keywords
      `((,weechat-timestamp-regex 1 font-lock-function-name-face t t)
        (,weechat-timestamp-regex 2 font-lock-function-name-face t t)
        (,weechat-info-line-regex 3 font-lock-warning-face t t)
        (,weechat-info-line-regex 4 font-lock-comment-face t t)
        (,weechat-msg-line-regex 3 font-lock-keyword-face nil t)
        (,weechat-msg-line-regex 4 bold nil t)))

(define-derived-mode weechat-log-mode fundamental-mode "weechat-log"
  "major mode for viewing weechat logfiles"
  (setq
   font-lock-defaults '(weechat-log-mode-keywords t)))

(provide 'weechat-log-mode)

I've made some progress in the current revision but some lines do not match.

http://ergoemacs.org/emacs/elisp_font_lock_mode.html helped a lot here.

Looking better now.