Walking Through Mail Headers

Hi, I need a way to walk through mail headers and search for a specific string in the subject. If the message contains the string, it will be forwarded to a different account, otherwise it will trigger an autoresponse with a certain message to the sender. Does anyone have any infomation on doing this, or can point me where to start. I think the thing I'm caught up on is how I walk through the mail headers in the mail file (We are on FreeBSD). Thanks. Jason