bug#29097: 25.1; to support '_' as a word constituent, subword-mode regexes need updating

[Eli Zaretskii]

> From: Chris Hecker <checker@d6.com>
> Date: Wed, 01 Nov 2017 05:08:28 +0000
> Cc: 29097@debbugs.gnu.org, yamato@redhat.com
> 
> That may be, I don't know the tradeoffs for that, but I do think the updated 
> regexes I sent are strictly superior in
> my testing so I think they should become the new defaults, unless I missed 
> some case.

I think you underestimate the number of possible use cases.  See the
responses by YAMATO-san.

I stand by my suggestion to make this a defcustom, then each user can
do whatever they want with this regexp.