Regex Help

Greetings list, I have been given a list of products, and I need some help building a regular expression to split the category from the sub category. Example: CamerasDigital_CannonXLRshot <-Original entry in list Cameras Digital Cannon XLRshot <-Desired result. I can use str_replace() for the underscore, but I also want to be able to split on lowercase char followed by an uppercase char. I have not worked with regular expressions much before, and I just need a push in the right direction. Thanks in advance, Keith