This request is for a new, default-off, regex mode for dolphin's filter bar. The mode could be toggled via a new toggle button on the filter bar, next to the existing keep-filter-when-changing-folders toggle button. It should remember its state.

The particular regex implementation used shouldn't matter much as long it's not annoyingly exotic and it supports groups/backrefs.

It would also be very useful to be able to quickly toggle the case-sensitivity of the regex filter, but I'm not sure how to best do that. I think adding another toggle button that is always available on the filter bar (whether in regex mode or not) makes sense because it can be useful outside of regex mode as well, and keeps things pretty simple. It should remember its state. But if the devs are willing to add only one toggle button at this time, then I'd propose the searches stay case-insensitive in regex mode.

Personally I find the bash-ish but not bash-exact behavior of the filterbar to be less than straightforward, and backrefs and regex features obviously offer capability boosts.

For case sensitivity, these icons may be useful: format-text-{{lower,upper}case,remove-formatting}

For regex, I don't know if any common theme icons are handy.