Mailman Error: "bad escape \s at position 0"

Symptoms

In cPanel version 126 with Mailman 2.2.0.19-1, errors similar to the following are generated:

Apr 30 16:13:35 2025 (163123) Uncaught runner exception: bad escape \s at position 0
Apr 30 16:13:35 2025 (163123) Traceback (most recent call last):
  File "/usr/lib64/python3.9/sre_parse.py", line 1051, in parse_template
    this = chr(ESCAPES[this][1])
KeyError: '\\s'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/local/cpanel/3rdparty/mailman/Mailman/Queue/Runner.py", line 113, in _oneloop
    self._onefile(msg, msgdata)
  File "/usr/local/cpanel/3rdparty/mailman/Mailman/Queue/Runner.py", line 184, in _onefile
    keepqueued = self._dispose(mlist, msg, msgdata)
  File "/usr/local/cpanel/3rdparty/mailman/Mailman/Queue/IncomingRunner.py", line 130, in _dispose
    more = self._dopipeline(mlist, msg, msgdata, pipeline)
  File "/usr/local/cpanel/3rdparty/mailman/Mailman/Queue/IncomingRunner.py", line 153, in _dopipeline
    sys.modules[modname].process(mlist, msg, msgdata)
  File "/usr/local/cpanel/3rdparty/mailman/Mailman/Handlers/CookHeaders.py", line 101, in process
    prefix_subject(mlist, msg, msgdata)
  File "/usr/local/cpanel/3rdparty/mailman/Mailman/Handlers/CookHeaders.py", line 426, in prefix_subject
    prefix_pattern = p.sub(r'\s*\d+\s*', prefix_pattern)
  File "/usr/lib64/python3.9/re.py", line 327, in _subx
    template = _compile_repl(template, pattern)
  File "/usr/lib64/python3.9/re.py", line 318, in _compile_repl
    return sre_parse.parse_template(repl, pattern)
  File "/usr/lib64/python3.9/sre_parse.py", line 1054, in parse_template
    raise s.error('bad escape %s' % this, len(this))
re.error: bad escape \s at position 0

Apr 30 16:13:35 2025 (163123) SHUNTING: 1745838183.0014486+a18556ed9bb5c0aa96156d274d6c2e94bad50bb2

Description

A bad escape in a pattern-match is causing the above error on certain emails that Mailman processes.

We've opened an internal case for our development team to investigate this further. For reference, the case number is CPANEL-47080. Follow this article to receive an email notification when a solution is published in the product. 

Workaround

None at this time.