linux - What's wrong with my lookahead regex in GNU sed?

Question

This is what I'm doing (simplified example):

gsed -i -E 's/^(?!foo)(.*)$/bar1/' file.txt

I'm trying to put bar in front of every line that doesn't start with foo. This is the error:

gsed: -e expression #1, char 22: Invalid preceding regular expression

What's wrong?

Answer 1

sed -i '/^foo/! s/^/bar/' file.txt

• -i change the file in place
• /^foo/! only perform the next action on lines not ! starting with foo ^foo
• s/^/bar/ change the start of the line to bar