Regex allows a search pattern such as "^$" to match a blank string. It appears that RegexpReplace() throws an error when the input argument is "". For example:
RegexpReplace("", "^$", "n/a")
I've tried various Error configs for MatchFunction="RegexpReplace" in my SeoTools.config.xml, but I can't seem to prevent the error popup.
As an aside, the error popup shows a blank title whenever RegexpReplace throws an error.