sed error: “\1 not defined in the RE” under OS X

Question

I am using OS X 10.10.5 and when I tried the following commands:

$ ls foo*
foo123bar.pdf
$ for f in foo*; do echo "$f" | sed -E 's:foo:bar:';done
bar123bar.pdf
$ for f in foo*; do echo "$f" | sed -E 's:foo:(&):';done
(foo)123bar.pdf
$ for f in foo*; do echo "$f" | sed -E 's:\(foo\):(\1):';done
sed: 1: "s:foo:(\1):": \1 not defined in the RE

I got an error.

My question is: how to remedy the error yielded by the last command above?

Answer 1

Under -E, you need to use unquoted parentheses to create a capture group.

sed -E 's:(foo):(\1):'