grep --ignore-case --only
RegexBashUnixGrepGnuRegex Problem Overview
grep fails when using both --ignore-case and --only-match options. Example:
$ echo "abc" | grep -io abc
abc
$ echo "ABC" | grep -io abc
$
But
$ echo "abc" | grep -i abc
abc
$ echo "ABC" | grep -i abc
ABC
According to man page: > -o, --only-matching > Show only the part of a matching line that matches PATTERN. > -i, --ignore-case > Ignore case distinctions in both the PATTERN and the input files.
Is it a bug of grep or I didn't get the map page?
I am using Mac OS X 10.6.8 and
$ grep --version
grep (GNU grep) 2.5.1
Found this link: http://lists.gnu.org/archive/html/bug-gnu-utils/2003-11/msg00040.html
Of course it is possible to use workaround like grep -o [aA][bB][cC]
, but this doesn't seem to be a good option.
Regex Solutions
Solution 1 - Regex
This is a known bug on the initial 2.5.1, and has been fixed in early 2007 (Redhat 2.5.1-5) according to the bug reports. Unfortunately Apple is still using 2.5.1 even on Mac OS X 10.7.2.
You could get a newer version via Homebrew (3.0) or MacPorts (2.26) or fink (3.0-1).
Edit: Apparently it has been fixed on OS X 10.11 (or maybe earlier), even though the grep version reported is still 2.5.1.
Solution 2 - Regex
It could be a problem in your version of grep.
Your test cases are working correctly here on my machine:
$ echo "abc" | grep -io abc
abc
$ echo "ABC" | grep -io abc
ABC
And my version is:
$ grep --version
grep (GNU grep) 2.10
Solution 3 - Regex
If your grep -i does not work then try using tr command to convert the the output of your file to lower case and then pipe it into standard grep with whatever you are looking for. (it sounds complicated but the actual command which I have provided for you is not !).
Notice the tr command does not change the content of your original file, it just converts it just before it feeds it into grep.
1.here is how you can do this on a file
tr '[:upper:]' '[:lower:]' <your_file.txt|grep what_ever_you_are_searching_in_lower_case
2.or in your case if you are just echoing something
echo "ABC"|tr '[:upper:]' '[:lower:]' | grep abc
Solution 4 - Regex
I'd suggest that the -i
means it does match "ABC", but the difference is in the output. -i
doesn't manipulate the input, so it won't change "ABC" to "abc" because you specified "abc" as the pattern. -o
says it only shows the part of the output that matches the pattern specified, it doesn't say about matching input.
The output of echo "ABC" | grep -i abc
is ABC
, the -o
shows output matching "abc" so nothing shows:
Naos:~ mattlacey$ echo "ABC" | grep -i abc | grep -o abc
Naos:~ mattlacey$ echo "ABC" | grep -i abc | grep -o ABC
ABC