Yes, excellent, this appears to be the most canonical response, e.g. replace "en(\d+)" with "\1\e0". That works, and follows the same pattern as "\U" and other special characters in replace-strings.
(For the probably-less-than-3 people who read this and are wondering what "\U" does, it allows you to do case changes on the replace string, e.g. if you had a list of names in a column, and wanted to change "Harris, Bryan" to "HARRIS, Bryan", you could do by with finding "(.+)(,.+)" and replacing it with "\U\1\E\2". "\U" and "\L" are uppercase and lowercase respectively, and they operate until the next "\E". You can also do a case-change on just the first letter with "\l" and "\u", terminating with "\e". I cannot find any mention of these in the documentation, but it may be that I just don't know where to look.)
Thanks @Craige_Hales and @jthi !