Mageia forum

The Kmail Help says "Matches if a part of the searched item matches the given regular expression (or does not match it). If the regular expression editor is installed then you can edit the regular expression by clicking on the Edit... button."

What software package is the regular expression editor? I answered my own question. for KDE4, there is none. See bottom of my post.

I think I do not have it installed, because I do not see an Edit button when I select a regular expression condition in the Filter Criteria window.

The KDE forum has addressed this problem. See https://forum.kde.org/viewtopic.php?f=2 ... expression

This has been an issue with KDE4 since 2010. Do not hold your breath.

This is an example of an "Upstream" issue in the Linux world. With much , and regret, this is the current answer.

Sad, but there are definitely some drawbacks. Not all is golden with KDE.
I've also some pretty nasty regressions or features that were present in KDE3 that have never been ported, or default behaviour of some applications got worse since then.

Anyway, mark the thread accordingly by editing the topic of the first post and prefix it by [DONE], thanks