nroff-mode .\" comment filling

[Kevin Ryde <user42@zip.com.au>]

I've had a bit of trouble trying to get .\" recognised as a comment
prefix when filling in nroff mode.  In simplified form,

emacs -Q
(progn
  (switch-to-buffer "foo")
  (nroff-mode)
  (setq comment-start-skip "\\(^[.]\\)?[\\]\"[ \t]*")
  (setq fill-column 20)
  (insert ".\\\" this is a line of text more then twenty chars\n")
  (goto-char (point-min)))

and press M-q gives

.\" this is a line
 \" of text more
 \" then twenty
 \" chars

where I hoped it would be .\" instead of \" on the new lines.

A .\" like this is quite common, in fact I think it's usual in man
pages.  I wondered if comment-start had to be set, but adding

  (setq comment-start ".\\\"")

into the above doesn't change the result.  As I mentioned on the sources
list I nosed around a bit and was suspicious that comment-search-forward
seems to choose \" even when point is right on a .\"  Both of course
match, but I think I would have expected the first match of
comment-start-skip on a line to be what's used.



In GNU Emacs 22.1.1 (i486-pc-linux-gnu, GTK+ Version 2.10.13)
 of 2007-07-07 on raven, modified by Debian
configured using `configure  '--build=i486-linux-gnu' '--host=i486-linux-gnu' '--prefix=/usr' '--sharedstatedir=/var/lib' '--libexecdir=/usr/lib' '--localstatedir=/var/lib' '--infodir=/usr/share/info' '--mandir=/usr/share/man' '--with-pop=yes' '--enable-locallisppath=/etc/emacs22:/etc/emacs:/usr/local/share/emacs/22.1/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/22.1/site-lisp:/usr/share/emacs/site-lisp:/usr/share/emacs/22.1/leim' '--with-x=yes' '--with-x-toolkit=gtk' '--with-toolkit-scroll-bars' 'build_alias=i486-linux-gnu' 'host_alias=i486-linux-gnu' 'CFLAGS=-DDEBIAN -g -O2''

Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: en_AU
  locale-coding-system: iso-8859-1
  default-enable-multibyte-characters: t