Case-insensitive search and replace with sed

Question

I'm trying to use SED to extract text from a log file. I can do a search-and-replace without too much trouble:

sed 's/foo/bar/' mylog.txt

However, I want to make the search case-insensitive. From what I've googled, it looks like appending i to the end of the command should work:

sed 's/foo/bar/i' mylog.txt

However, this gives me an error message:

sed: 1: "s/foo/bar/i": bad flag in substitute command: 'i'

What's going wrong here, and how do I fix it?

Can you try updating your copy of sed? I is a GNU extension which might be not available with your copy of sed. — Lazer
EDIT: I struck through the OS X qualification, as the OP accepted an answer that doesn’t work on OS X. (As another answer indicated, sed on OS X does not support case-insensitive matching, contrary to Apple documentation.) — danorton
@danorton: Thanks for that; in case you derived the sense that the Apple documentation promises something the implementation doesn't deliver from my answer below: man sed IS consistent with the implementation - no mention of (and no support in practice) for case-insensitive matching; if you found a piece of documentation claiming otherwise, please let us know. — mklement0
@mklement0, yes, sorry, I stand corrected. The Apple documentation does not make any claim of case-insensitive matching for sed. — danorton
FWIW, the GNU versions of the tools whose BSD version comes with OS X are available from various package managers. I have the full suite of text utilities installed via Homebrew with a g prefix, so I can use gsed or gdate when I need a feature not found in the stock version. — Mark Reed

mklement0 mklement0 · Accepted Answer · 2012-10-14T22:48:01

Update: Starting with macOS Big Sur (11.0), sed now does support the I flag for case-insensitive matching, so the command in the question should now work (BSD sed doesn't reporting its version, but you can go by the date at the bottom of the man page, which should be March 27, 2017 or more recent); a simple example:

# BSD sed on macOS Big Sur and above (and GNU sed, the default on Linux)
$ sed 's/ö/@/I' <<<'FÖO'
F@O   # `I` matched the uppercase Ö correctly against its lowercase counterpart

^{Note: I (uppercase) is the documented form of the flag, but i works as well.}

Similarly, starting with macOS Big Sur (11.0) awk now is locale-aware (awk --version should report 20200816 or more recent):

# BSD awk on macOS Big Sur and above (and GNU awk, the default on Linux)
$ awk 'tolower($0)' <<<'FÖO'
föo  # non-ASCII character Ö was properly lowercased

The following applies to macOS up to Catalina (10.15):

To be clear: On macOS, sed - which is the BSD implementation - does NOT support case-insensitive matching - hard to believe, but true. The formerly accepted answer, which itself shows a GNU sed command, gained that status because of the perl-based solution mentioned in the comments.

To make that Perl solution work with foreign characters as well, via UTF-8, use something like:

perl -C -Mutf8 -pe 's/öœ/oo/i' <<< "FÖŒ" # -> "Foo"

-C turns on UTF-8 support for streams and files, assuming the current locale is UTF-8-based.
-Mutf8 tells Perl to interpret the source code as UTF-8 (in this case, the string passed to -pe) - this is the shorter equivalent of the more verbose -e 'use utf8;'.^{Thanks, Mark Reed}

(Note that using awk is not an option either, as awk on macOS (i.e., BWK awk and BSD awk) appears to be completely unaware of locales altogether - its tolower() and toupper() functions ignore foreign characters (and sub() / gsub() don't have case-insensitivity flags to begin with).)

A note on the relationship of sed and awk to the POSIX standard:

BSD sed and awk limit their functionality mostly to what the POSIX sed and POSIX awk specs mandate, whereas their GNU counterparts implement many more extensions.

Case-insensitive search and replace with sed

8 Answers