sed Case Insensitive Search Matching and Replacement

UNIX / Linux: sed Case Insensitive Search and Replace

Replacing or substituting string is simple. The below example replaces the word wiwek with vivek in the file:
sed 's/wiwek/vivek/' names.txt
# The g flag applies the replacement to all matches to the regexp, not just the first. #
sed 's/wiwek/vivek/g' names.txt
In the above example the “s” specifies the substitution operation. The “/” are delimiters. The mispelling “wiwek” is the search pattern (more like word in this case) and the “vivek” is the replacement string. By default, sed is case-sensitive. You can add an “i” OR “I” flag at the end of the substitution to change this. 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
sed -i 's/Unix/Linux/gi' input

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

A note about GNU sed version

The syntax is pretty simple
sed -i 's/SEARCH-WORD/REPLACMENT-WORD/gi' input
The -i option edit and update file in place.

The BSD implementation of sed does NOT support case-insensitive matching

Please note that macOS comes with BSD version of sed which does not support case-insensitive matching. Hence, if you are on macOS install GNU sed using the following brew command. First, install Homebrew on macOS and then type the following brew command:
$ brew install gnu-sed
Sample outputs:

Updating Homebrew...
==> Auto-updated Homebrew!
Updated 1 tap (homebrew/core).
No changes to formulae.
 
==> Downloading https://homebrew.bintray.com/bottles/gnu-sed-4.4.sierra.bottle.tar.gz
Already downloaded: /Users/veryv/Library/Caches/Homebrew/gnu-sed-4.4.sierra.bottle.tar.gz
==> Pouring gnu-sed-4.4.sierra.bottle.tar.gz
==> Using the sandbox
==> Caveats
The command has been installed with the prefix "g".
If you do not want the prefix, install using the "with-default-names" option.
 
If you need to use these commands with their normal names, you
can add a "gnubin" directory to your PATH from your bashrc like:
  PATH="/usr/local/opt/gnu-sed/libexec/gnubin:$PATH"
 
Additionally, you can access their man pages with normal names if you add
the "gnuman" directory to your MANPATH from your bashrc as well:
  MANPATH="/usr/local/opt/gnu-sed/libexec/gnuman:$MANPATH"
==> Summary
?  /usr/local/Cellar/gnu-sed/4.4: 12 files, 491KB

Now use gsed command as follows:

cat file.txt | gsed -e 's/find-word/replace-word/gI' 
cat file.txt | gsed -e 's/find-word/replace-word/gI' > output.txt
gsed 's/find-word/replace-word/gI' input.txt > output.txt

Onec gsed installed, create the alias as follows:
alias sed='gsed'
Now we can use the sed command as follows:
sed -i 's/Unix/Linux/gI' file

A note about Perl for case-insensitive search and replace instead of sed

Another option is to use the perl tool for case-insensitive search & replace as follows:

## find foo and replace with bar case-insensitive ##
perl -pi -e 's/old_word/new_word/gi' file
perl -pi -e 's/SEARCH/REPLACE/gi' filename.txt
perl -pi -e 's/foo/bar/gi' input.txt

Use the cat command to verify changes:
cat input.txt
We can replace a single word in a large number of files in Unix or Linux using the Perl as follows:
perl -pi -e 's/cyberciti/nixcraft/gi' *.txt
Perl would change every instance of “cyberciti” it found into “nixcraft.

Conclusion

You learned about case-insensitive search and replace with sed under Linux, macOS and Unix-like systems. See sed command man page here for more info or type the following command at the shell promot:
man sed