One of the nice things about php is the online manual. Give that a check, it will tell you what can and can't be done via .htaccess.
The above link will go directly to the page you need, and tell you how to do it. There ARE strings that can't be used, and they're described on that page, and the pages linking to it
WHMCS Guru - WHMCS addons, management, support and more. WHMCS Notifications Extended - Add slack, hipchat, SMS, pushover to WHMCS !!
Linux Problems? WHMCS Issues? +1-866-546-8914 (linux-14) or @whmcsguru on twitter!