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

Question

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

gsed -i -E \'s/^(?!foo)(.*)$/bar\\1/\' file.txt

I\'m trying to put bar in front

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