regex - return all before the second occurrence

Question

Given this string:

DNS000001320_309.0/121.0_t0

How would I return everything before the second occurrence of \"_\"?

DNS0000         


        

Answer 1

I think this might do the task (regex to match everything befor the last occurence of _):

_([^_]*)$

E.g.:

> sub('_([^_]*)$', '', "DNS000001320_309.0/121.0_t0")
[1] "DNS000001320_309.0/121.0"