bug#23798: 25.0.90; Underscore for emphasis in Info?

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

FWIW -

In my own code I'm using this regexp now (by default - a user option):

"_\\(\\(\\sw\\(\\s-\\|\\sw\\|\\s.\\)*\\)\\|\\(\\(\\s-\\|\\sw\\|\\s.\\)\\sw*\\)\\)_"

It matches word, punctuation, and whitespace chars, plus hyphens,
with at least one word character.  Hyphen is included explicitly
because it generally has symbol syntax in Info.

This seems to work pretty will.  But there are of course some
places where it does not DTRT.

In my own code I also remove hiding of the enclosing `_' chars
when emphasis highlighting is turned off:

(while (re-search-forward Info-emphasis-regexp nil t)
  (let ((fn  (if Info-fontify-emphasis-flag
                 #'add-text-properties
               #'remove-text-properties)))
     (funcall fn (match-beginning 0) (1+ (match-beginning 0))
              '(invisible t front-sticky nil rear-nonsticky t))
     (funcall fn (1- (match-end 0)) (match-end 0)
              '(invisible t front-sticky nil rear-nonsticky t))
     (funcall fn (match-beginning 1) (match-end 1)
              '(font-lock-face info-emphasis))))