#!/usr/bin/perl -w

use strict;
use constant MAIL_ALIASES => "/etc/mail/aliases";

my $site = shift;

# all users of a site have home directories under 
# /home/sites/XXX/users/.
my @users = (map { m{/([^/]*)$} }   # `basename' with a RE
             (grep { -d $_ }        # only directories
              </home/sites/$site/users/*>  # glob
              ));

# build a big regular expression with each user name;
# use \b to anchor the pattern match, so that a user called
# `jane' won't show up entries for `janet'
my $pattern = '\b(' . join("|", @users) . ')\b';

open ALIASES, MAIL_ALIASES
    or die("Failed to open ".MAIL_ALIASES."; $!");
while (<ALIASES>) {
    print if m/$pattern/o;
}
close ALIASES;