bug#6227: Color isearch regexp submatches differently

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

> > What bothers me is that faces isearch-group-6 to isearch-group-9 are
> > not defined.  Should we define them?
> 
> I'd rather go the other way, to be honest -- just leave group-1 to -3,
> perhaps.  I think it's rather unusual to have a that many sub-groups
> interactively...

It is unusual, as in not so common.  But when you do
have a complex regexp with many groups, that's exactly
when such highlighting can be most useful.

I settled on 8 levels, for Isearch+, but that was
without any special research into the question.

It doesn't hurt, in any way that I'm aware of, to
have more rather than less.  In the case where many
would actually be used (which, as you say, is not
usual), a little more time would be needed, but it's
likely that in such a complex case there would not
be many matches to highlight.  I think any time cost
of such highlighting is negligible.