Because you used "_" the result you get is going to be a string object, not a character vector.
strfind() returns positions inside the scalar string object
Positions inside a scalar string object are not the same thing as indices into the string object. A scalar string object has index 1 only -- the index for a string object is which complete string you are accessing. Like ["hello", "bye"] has indices 1 for "hello" and 2 for "bye".