Unable to generate valid slug for Category ".."

• I have read the Filing Issues and subsequent “How to Get Help” sections of the documentation.
• I can reproduce this problem with stock/default settings file, theme, and sample content (as described in above “How to Get Help” sections of the documentation).
• I have searched the issues (including closed ones) and believe that this is not a duplicate.

• OS version and name: Windows 10
• Python version: Python 3.14.4
• Pelican version: 4.12.0
• Link to theme: Seafoam
• Links to plugins: (many)
• Link to your site: https://blog.minchin.ca/
• Link to your source: https://github.com/minchinweb/blog.minchin.ca
• Link to a Gist with the contents of your settings file: https://github.com/minchinweb/blog.minchin.ca/blob/pelican/pelicanconf.py

Issue

Upgrading from Pelican 4.11 to 4.12 throws a new warning:

[16:46:10] WARNING  Unable to generate valid slug for Category "..".                                   urlwrappers.py:46

No other changes seem applicable.

Previously mentioned here __> https://github.com/getpelican/pelican/issues/3596#issuecomment-4315960125 (but not related to that issue).

This can be suppressed, add this to your settings:

CATEGORY_REGEX_SUBSTITUTIONS = [
    (r"\.\.", "-dotdot-"),  # needed to keep a "phantom" category from throwing an build error

    # defaults
    (r"[^\w\s-]", ""),   # remove non-alphabetical/whitespace/"-" chars
    (r"(?u)\A\s*", ""),  # strip leading whitespace
    (r"(?u)\s*\Z", ""),  # strip trailing whitespace
    (r"[-\s]+", "-"),    # reduce multiple whitespace or "-" to single "-"
]

Sites immediately before the above bypass:

• https://github.com/minchinweb/blog.minchin.ca/commit/db04ad005f1df997ace5c705ed231d9245555964
• https://github.com/minchinweb/strathcona-power-website/commit/573ac74a890af36a844c589b25a02245f57da485