Any way to keep the whitespaces or tab stops when I use /usr/bin/ex? 

[foo@jumpspeeder ~]$ echo  'passwd:     files nis sss' >~/nsswitch.conf
[foo@jumpspeeder ~]$ cat ~/nsswitch.conf
passwd:     files nis sss
         ^
         |
         |
     Look here...
    

[foo@jumpspeeder ~]$ ex -s -c '%s/^\(passwd:\|shadow:\) \+\(files\) \+nis \+\(sss\)/\1 \2 \3/' -c w -c q ~/nsswitch.conf
[foo@jumpspeeder ~]$ cat ~/nsswitch.conf
passwd: files sss
       ^
       | 
       |
  ...here too.

Thanks in advance.