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

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Answer 1

The sed FAQ addresses the closely related case-insensitive search. It points out that a) many versions of sed support a flag for it and b) it's awkward to do in sed, you should rather use awk or Perl.

But to do it in POSIX sed, they suggest three options (adapted for substitution here):

1. Convert to uppercase and store original line in hold space; this won't work for substitutions, though, as the original content will be restored before printing, so it's only good for insert or adding lines based on a case-insensitive match.

2. Maybe the possibilities are limited to FOO, Foo and foo. These can be covered by

   s/FOO/bar/;s/[Ff]oo/bar/
   

3. To search for all possible matches, one can use bracket expressions for each character:

   s/[Ff][Oo][Oo]/bar/