Using lambda or Linq to split a string

Hi,

need some help trying to split strings that look something like the following:

aaa_bbb_F0001
ccc_ddd_eee_Y0005

Using Linq or a lambda expression, how I can turn it into somethign like the following:

aaa_bbb_F0001  becomes F0001
ccc_ddd_eee_Y0005 become Y0005

the number of underscores varies from string to string.

thanks