RE: :alnum: broken?

[Drew Adams <drew.adams@oracle.com>]

> > 1. Warning during regexp input (e.g. regexp Isearch)
> >    would likely, itself, be quite confusing or
> >    annoying.
> 
> It's not unlike the error you get during input of
> an incomplete or otherwise malformed regexp.

Yes, I guess that could work.  (I guess you, like I,
are thinking about regexp Isearch, at least as one
such use case.)

The message should, in that case, be quite different
from the incomplete/malformed message.  It should
essentially point out that what you've typed so far
is a character class, not a character alternative.
E.g. "Did you perhaps mean [[:alnum:]]?"

Maybe even better might be to query the user to
just continue (y or SPC) or to go to the manual
node that talks about this.

(And the manual should probably explicitly point
out the gotcha of using a char-class outside of
a char alternative.  Actually, I maybe think it
did, at one time, and that was removed (?).)

Since different users will likely see such
interactive help as more or less helpful/annoying,
it should be configurable.  E.g. (1) no warning,
(2) on-the-fly brief reminder, (3) query to let
you visit the manual or get more explanation in
some other way (e.g. *Help*).