Update: Oops, I misread Enlil's (??{}) as (?{}) which is very different. His original code works, it just isn't a great way to write that. Better to write it directly as \xe7.

No, no this is wrong. (?{}) is zero width and is always true. That's no help to the person who just wants to get the character #231 into their regex. Some other nice monks noticed that you could write it in hex form \xe7 or BrowserUK's verbose (and slightly slower) @{[ chr 231 ]}.