currently out-of-the-box regex is a little bit dumb in vvvv. in regex you have some flags to control how it will interpret incoming text. for example you have to provide a specific flag for multiline text. Unfortunately you cannot set these flags manually in vvvv. it looks like in native vvvv regex the multiline flag was set and in the regex replace multiline flag was not set.
i have a regex implementation here which is much more versatile than what we have in vvvv currently. It doesn’t replace text but it’s pretty easy to modify to do so (regex in C# has built-in replace functionality) https://github.com/microdee/VVVV.Packs.mcro/tree/master/nodes/Plugins/StringRegex