Total Commander

I think 'contains(regex) and '!contains(regex)' is much more logical than just 'regex' and '!regex' since TC searches not exact match but text that contains (or not contains) specified regular expression. String doesn't reguire additional translation, since 1577 line has translated 'contains' and 'regex' (also translated) in braces may be appended to it.

Unfortunately this would break all existing saved searches, so I prefer to leave it as it is now. I will move this thread to "suggestions".

I think you can leave INI line format as is but in Plugins tab of Search dialog display 'contains(regex)' instead of 'regex', so all will work fine.
Translated operator names already can have another names than specified in INI.