URL preg_match validation

hello everyone * im no expert in php and stuff but i have problem that i want the solution * i purchased i custom theme recently for my video website which has a user submission form to send videos as posts it’s good i like it but one problem some some viditors send some unwanted videos from unwanted websites i want to know how to define one or two websites that i want videos from , like youtube and vimeo only ; but when a visitor tries to send a video from another website error msg shows up * pleaz provide me with some working trick tnx in advance