Trouble with grep -o regex

Question

I'm trying to use a grep command with the --only-matching flag, but it's not behaving as I would expect it to.

This command:

echo "1/2/3/4/5" | grep -oE "^([^/]+/){0,2}"

Gives this output:

1/2/
3/4/

I was expecting just 1/2/

What's going on..? 3/4/ shouldn't match "^([^/]+/){0,2}" because it's not at the beginning of the line..

(running GNU grep 2.5.1)

This gives just 1/2/ for me on Debian Squeeze with GNU grep 2.6.3. Maybe you should add some more specific details about your distro and grep version. — Arrowmaster, Mar 9, 2011 at 4:05
Tested on GNU/Linux with GNU grep 2.5.1 and showing result as @Acorn . — chanchal1987, Mar 9, 2011 at 6:14
@ Zarkdav,its not working for me on GNU grep 2.5.4 echo "1/2/3/4/5" | grep -oE "^([^/]+/){0,2}" 1/2/ — sush, Mar 9, 2011 at 7:11

zarkdav · Accepted Answer · 2011-03-09 07:43:00Z

8

It was a bug in versions of GNU Grep earlier than this commit (i.e. earlier than GNU version 2.5.3).

Quoting the relevant part of the changelog:

Previously failing tests relative to left anchors (^ and \<) and -w should now pass.

The initial commit that described the bug also added a test for it:

# End of a previous match should not match a "start of ..." expression.
grep_test "word_word/" "word_/" "^word_*" -o

answered Mar 9, 2011 at 7:20

zarkdav

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This bug, where grep -o/grep --only-matching allows a caret to match after the end of a previous match instead of only at beginning of line, is still present in Mac OS X 10.9.4.
– Robert Tupelo-Schneck
Aug 22, 2014 at 13:22
This bug is still present today at the end of 2020, in Mac OS X Catalina 10.15.7. That's 9 years, 8 months after this question was posted. This was driving me crazy: surely there can't be a BUG IN GREP??? But alas, there is.
– David M. Perlman
Nov 26, 2020 at 6:48

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