Unable to understand a line in SED

Question

I run the following code

sed 's/\([^ ]+\) your \([^ ]+\)/ \2\1er/' < fail

The file fail is

fail your test

The above command gives me

fail your test

although it should give "testfailer".

The second and first globs \2\1 should be at the start of the word "er". This suggests me that the problem may be in the regexes in the search part. However, they seem to be correct for me.

Do you see any mistake in the code?

Answer 1

Common or garden variety sed regex doesn't understand +. Yeah, I know, how stupid is that. So this is an equivalent, working version of your command line:

sed 's/\([^ ][^ ]*\) your \([^ ][^ ]*\)/ \2\1er/' < fail

Also works to request extended regex, in which case you ditch the backslashes on the parens:

sed -r 's/([^ ]+) your ([^ ]+)/ \2\1er/' < fail

Answer 2

Your code does work when you escape the plus signs:

sed 's/\([^ ]\+\) your \([^ ]\+\)/\2\1er/' < fail