check string in c

what the fast way to check that string contain only characters within some set like "a-z0-9"?

>>22
This one time I developed a game in Sepples for a Uni project. I ended up running out of time so I tried to focus on finishing the actual game. Even though I did complete it, I ended up turning it in with 29 memory leaks still there　ａｎｄ　Ｉ　ｆｅｅｌ　ｋｉｎｄ　ｏｆ　ｂａｄ　ａｂｏｕｔ　ｉｔ　：（