query-replace casing not ok when string contains _1

[Philippe Waroquiers]

In GNU Emacs 20.7.1 (hppa1.1-hp-hpux11.00, X toolkit)
 of Mon Aug 14 2000 on rhino
configured using `configure  hppa1.1-hp-hpux11.00 --with-gcc=no --with-x11r6 
--x-includes=/usr/include/X11R6:/usr/contrib/X11R6/include:/usr/include 
--x-libraries=/usr/lib/X11R6:/usr/contrib/X11R6/lib --with-x-toolkit=lucid 
--prefix=/opt/contrib/emacs --exec-prefix=/opt/contrib/emacs/20.7 
--mandir=/opt/contrib/man'

Please describe exactly what actions triggered the bug
and the precise symptoms of the bug:

start emacs with :
emacs --no-init-file --no-site-file
then type the following 3 lines in a buffer :
Foo_1 Fupper will result in bar_1 (not ok!!!!)
foo_1 flower will result in bar_1 (ok)
FOO_1 F_O_O_allupper will result in BAR_1 (ok)

then, at the beginning of the buffer, execute :
M-x query-replace
foo_1
bar_1

It looks to me that in the first line, Foo_1 is wrongly replaced
by bar_1  (I was expecting Bar_1).
foo_1 and FOO_1 are replaced according to what I expected.

Note that the same bug (if it is a bug :) happens also with emacs 21.0.100.