True, the @XREPLACE function could solve this problem, but it is still requires a somewhat roundabout and non-straightforward syntax, because it would have to packaged within some sort of loop, rather than being run directly as a REN command.
I noticed that a couple of years ago vefatica suggested adding a /X option to REN which would do just this - it would treat the source and target names as regular expressions such that the target pattern would be able to include backreferences to the source name (see here: http://jpsoft.com/forums/threads/batch-renames.1521/
That would provide a much more elegant and straightforward solution to JohnQSmith's query.
Indeed, I would use it often, too (right now I use the free command-line utility BulkRenameUtility to do bulk regex-based renames, but the latter utility has serious bugs and shortcomings when it comes to applying regex expressions to unicode filenames - it will cause serious damage to non-English filenames - and hence I'd much rather have a regex rename in TCC where unicode is fully supported).
Given the number of threads that have raised this issue (in addition to this one and the aformentioned one, see also: http://jpsoft.com/forums/threads/regex-rename.2416/
), I think that there is sufficient interest. Unless there is any objection, I'll add this on the feature request site later today. If all of us regex-addicts vote on the feature-request site, perhaps we'll see it soon in TCC...
@XREPLACE, in Vincent's 4UTILS, might be what you're looking for.