Bash – egrep matching group not printing

I'm not sure why you have an aversion to doing this correctly - either on the server a la

PrintMotd no
PrintLastLog no

and

#/etc/pam.d/ssh
# Print the message of the day upon successful login.
# session    optional     pam_motd.so

Or adding ~/.hushlogin for each user.

Hint, for ~/.hushlogin, add it to /etc/skel so new user home directories are created with the file.

Update:

Without more information about your backup cron job, my only other suggestion is to redirect the output of the command to a file (or let cron capture it in email) and the output of the ssh session to /dev/null. Something like:

0 0 * * * ssh backuphost "backup_script_that_writes_to_a_log" >/dev/null

Or

0 0 * * * ssh backuphost "backup_command 2>&1" >/dev/null

I'd have to play around with the commands a bit, but that should get you started.

I think the following Perl regexp matches what you want:

(?!.*msg-[-0-9]{4,16}\.html?$).*\.html?$

However AFAIK there isn't any place where bash supports Perl regexps. The =~ operator only supports extended regexps¹, which don't include zero-width lookahead assertions such as (?=…) and (?!…).

It is theoretically possible to convert a regexp with lookahaed assertions to one without, but the resulting regexp would be huge. It is much simpler to use two regexps:

[[ $string =~ \.html?$ && ! $string =~ msg-[-0-9]{4,16}\.html?$ ]]

_{¹ First there were basic regexps (BRE) (with several syntax variants), then came extended regexps (ERE) with more features (and again several syntax variants). Perl added yet more features, and many languages provide perl-compatible regexps (pcre). But bash sticks to ERE.}