reply_regex bug on Linux?

[Gary Johnson <garyjohn@xxxxxxxxxxxxxxx>]

Until recently I had been using mutt-1.5.17 on Solaris.  Now I'm 
using the same version on Linux.  I just noticed that reply_regex is 
not working correctly on the Linux version.  If I open $MAIL on 
Solaris, execute ":reset reply_regex", and reply to a message with 
the Subject

   RE: Progress

my reply is created with the Subject

   Re: Progress

as I would expect.  However, if I do the same on Linux, my reply is 
created with the Subject

   Re: RE: Progress

In both cases, the reset value of 'reply_regex' is

   "^(re([\\[0-9\\]+])*|aw):[ \t]*"

If on the Linux version I execute

   :set reply_regexp="^(re([\[0-9\]+])*|aw):[ \t]*"

(i.e., changing the double backslashes to single backslashes), then 
reply to the message again, I get the expected Subject

   Re: Progress

Is this a known problem or should I investigate further?

Regards,
Gary