I just discovered that one of my mail filters is behaving in what seems like an unpredictable manner. After realizing I was missing a few mails, I went digging and found a seemingly random jumble of them them in a subfolder that was populated by this filter:

Subject -> Matches wildcard condition -> [GS/SS Beta]*
then File into folder -> MySubFolder

MySubFolder ended up containing mails with subjects like:

September Expense Reports
Thanks a million
advice for tomorrow?

If I switch the match type to "Contains", this problem does not occur. However, even with possible syntactic problems, I can't see how the wildcard match above should result in matching those subjects.

Unless... Does the filtering treat the brackets like a regexp? i.e. "G or S or / or B or e", etc.? Are my non-technical users really supposed to understand regexp escapes when they create their own filters? Though the user guide makes a relatively clear case for escaping asterisks, it does not mention handling square brackets or any other characters.

Can anyone explain this behavior?