Searching line by line text between first two matches
$ sed -r 's/[^_]+_([^_]+)_.*/\1/' file
ASSET-D
STRING-H
FILE-FD
Explanation
-
-rUse ERE -
s/old/new/replaceoldwithnew -
[^_]+some characters that are not underscore -
(some chars)savesome charsfor later -
.*any number of any characters -
\1the saved pattern
You can use awk with the following parameters:
-
-F "_"- which uses_as separator -
'{print $2}'- which prints the 2nd element$ awk -F "_" '{print $2}' input_file ASSET-D STRING-H FILE-FD