Preventing Illegal character entry in a form text control

I have a single Line Text Box form control called "Opportunity Name" which is used as a string element in a workflow generated library folder name. Obviously I want to prevent Users from entering illegal characters in the test box as these cause the subsequent create folder workflow to fail, I would like to use a "Contains({self},.... validation rule to achive this, giving users the opportunity to create a meaningful title, rather than forcing the naming convention using a linked "Replace" calculation, any thoughts??