Use sed with ignore case while adding text before some pattern

You can use the following:

sed 's/[Ff][Ii][Rr][Ss][Tt]/last/g' file

Otherwise, you have the /I and n/i flags:

sed 's/first/last/Ig' file

From man sed:

I

i

The I modifier to regular-expression matching is a GNU extension which makes sed match regexp in a case-insensitive manner.

Test

$ cat file
first
FiRst
FIRST
fir3st
$ sed 's/[Ff][Ii][Rr][Ss][Tt]/last/g' file
last
last
last
fir3st
$ sed 's/first/last/Ig' file
last
last
last
fir3st

if you want to save some typing, try awk. I don't think sed has that option

 awk -v IGNORECASE="1" '/first/{your logic}' file

You can try

sed 's/first/somethingelse/gI'

GNU sed

sed '/first/Ii This line to be added' file