Regular Expression help

I've been beating my head against the wall for a while trying to come up with the RE I need to use. I have a camel case word - let's use JimJoeBobBriggs. I need to come up with a RE that will fine all the upper case characters and insert an underscore prior to those characters with the exception of a possible first character. So using the aforementioned word, the RE would change it to Jim_Joe_Bob_Briggs. Any help would be greatly appreciated! thnx, Chris