changing ownership for diferent users from the command line

Hi: I have a webmail service in a server and today after a backup & restore all the ownerships got messed up so now everything is owned by user root and group root, how can I do, perhaps with a little shell script, to change for example every file that starts with user_name (than_random_letters_generated_by_the_webmail_software) to have owenership user_name.users ??? Thanks!! Raul