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
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