Q. How do I perform a case-insensitive search using sed under UNIX / Linux? I’d like to match all combination of word – foo, FOO, FoO and so on while replacing or performing other operations.

A. GNU sed and other version does support a case-insensitive search using I flag after /regex/.

UNIX / Linux: sed Case Insensitive Search

To perform a case-insensitive search, enter:

cat file.txt | sed -e ‘s/find-word/replace-word/gI’
cat file.txt | sed -e ‘s/find-word/replace-word/gI’ > output.txt
sed ‘s/find-word/replace-word/gI’ input.txt > output.txt

If you are using older sed version try,

sed ‘s/[wW][oO][rR][dD]/replace-word/g’ input.txt > output.txt

It is easy to match first few characters, for example match both Linux and linux word:

sed ‘s/[Ll]inux/Unix/g’ input.txt > output.txt