I dunno about the live-regex/Incremental search for “In Files”
A tool called BareGrep offers live-regex/Incremental search for files, so it’s feasible, particularly given the powerful nature of computers these days. Admittedly, an option to disable it should be present though, if not required.
But XNews.exe has a very useful tool built-in, “Test Regex”
EmEditor already has this functionality for Find, except it will match against the current document, rather than a Test String. Given this, I don’t see a need for a separate “Text RegEx” tool.