org-mode html publish fails with invalid regex error

Question

I am using org-mode for generating a static website for my notes. For this, I followed the tutoriala at:

https://orgmode.org/worg/org-tutorials/org-publish-html-tutorial.html

And this is the relevant part on my .emacs file:

;; Org-mode html publish
(require 'ox-publish)
(setq org-publish-project-alist
  '(
    ("org-notes"
        :base-directory "~/org/"
        :base-extension "org"
        :publishing-directory "~/public_html/"
        :recursive t
        :publishing-function org-html-publish-to-html
        :headline-levels 4             ; Just the default for this project.
        :auto-preamble t
    )
    ("org-static"
        :base-directory "~/org/"
        :base-extension "css\\|js\\|php\\|jpg\\|png\\"
        :publishing-directory "~/public_html/"
        :recursive t
        :publishing-function org-publish-attachment
    )
    ("org" :components ("org-notes" "org-static"))
))

And I used this one for quite some time now. But lately, when I do M-x org-publish-project and pick org-static, it fails with the error message Invalid regexp: "Unmatched ( or \\(". I wonder where is the problem in my config and how this started failing all of a sudden. The project org-notes still works fine.

Answer 1

Check the comment from @NickD, regex:

:base-extension "css\\|js\\|php\\|jpg\\|png\\"

is indeed wrong if we follow the tutorial exactly. I removed the last \\. Still, as I said, this was working before. The actual error was that a package named cl is not supported by Emacs anymore. For that, I had to see which packages are depending on it and I evaluated following on *scratch*:

(require 'loadhist)
(file-dependents (feature-file 'cl))

and saw that the package htmlize, which I use for org publishing was depending on it. The newer versions of this package are updated to work without the cl package. So, I had to reinstall htmlize basically and things started working again.