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 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

As far as I know sed has not neither look-ahead nor look-behind. Switch to a more powerful language with similar syntax, like perl.

Answer 2

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  

Answer 3

You use perl compatible regular expression (PCRE) syntax which is not supported by GNU sed. You should rewrite your regex according to SED Regular-Expressions or use perl instead.