Creator here - can you elaborate? What is your family name? The example in this thread ("Grüneis") matches and displays correctly in all the browsers I've tested.
Are you perhaps trying to use a RegEx feature that is not supported by JS? Currently, RegExr only supports the JS flavour of RegEx.
Creator here - we are currently relying on the JS RegExp API, and thus only support features of that engine, which are somewhat limited. In the future, we may support other flavours. We may also add specific errors for more common features that are not supported, as I've already done for lookbehinds.
Creator here - this is because \w* matches 0 characters, and thus matches infinitely. You can roll over the "infinite" error for details, or look in the help.
Are you perhaps trying to use a RegEx feature that is not supported by JS? Currently, RegExr only supports the JS flavour of RegEx.