RE: :alnum: broken?

[Drew Adams]

> > Maybe there is a way to do it that won't be confusing.
> > If it waits until you type RET to finish the regexp.
> > then flashes a warning on the screen for two seconds or until there is
> > more input, it might be clear and not annoying.
> 
> We could also have `read-regexp` show a warning *on the side* while you
> type the input, a bit like isearch does when the regexp is
> incomplete/invalid.
>
> It would also make sense to make `read-regexp` emit such warnings about
> incomplete/invalid regexps.

Both suggestions sound good.