Adrian Pepper writes:

(Mailman 2.1.12, some local mods, but not around topics...)

I had a utf-8 subject I was having difficulty matching with a topic regexp.

Eventually I concluded the subject still had newlines in it when it was matched against the regexp. (That is the continuation lines were not joined before matching). And "." would not match the newline character(s)).

Am I correct in my conclusion that .* won't match newline characters, but <space-chars><not-space-chars><linefeed><carriage-return> will ? (And also, that that is the character class I created).

Yes. Here are the docs for Python regular expressions as used in Mailman: https://docs.python.org/2.7/library/re.html.

In general this problem would be addressed with the DOTALL flag:

The special characters are:

'.'
(Dot.) In the default mode, this matches any character except a
newline. If the DOTALL flag has been specified, this matches any
character including a newline.

Note that the definition of "newline" here is exactly "\n".

However, in your case I think there's a simpler method.

For production I might need to put [\s\S\n\r]* between every pair of characters after a reasonable point in the expression. Unless I can enumerate the possibilities more precisely. (Which will probably result in an even longer looking character class).

Well, actually what you need is just "\s*" (or perhaps "\s+" or "(\s|_)+") wherever a space might occur in the topic regexp, I think. Line folding can only occur at whitespace (breaking this rule would be noticed by everybody, and so is not likely to go unfixed), and "\s" already includes "\n".

Empirically I see ?=\n =?utf-8?q?_ after "Weekly" and before "Ac". (And it seems the matching is done on the incoming subject, not the one formatted for resending, which, with my tag, and the utf-8 of an incoming tag pushes the expression entirely onto the second line where I think the ".*" variant (or even [_ ]) would match.

That would explain your observations, but I am not familiar with the topic code. I don't have time to address that until the weekend, and maybe not then as $DAYJOB is piling up work on me, and Mark is on vacation in Croatia, so you may have to wait a bit for a final answer on that. I'm sorry about that, but I think at least for now the "\s*" bandaid will get you most of the way to where you want to go.