Question

How can I do a sed regex swap on all text that preceed a line matching a regex.

e.g. How can I do a swap like this

s/foo/bar/g

for all text that precedes the first point this regex matches:

m/baz/

I don’t want to use positive/negative look ahead/behind in my sed regex, because those are really expensive operations on big files.

Answer 1

If you mean that you want to do the substitution on every line preceding the given match, this is your answer:

The substitution takes an optional address range; you can use both numbers and patterns. In this case, start from line 1, go until your pattern:

sed '1,/baz/s/foo/bar/g'

In awk:

awk '
/baz/ { done = 1 }
{
    if (!done) {
        gsub(/foo/, "bar")
    }
    print
}'

(It’s really short enough to leave out the line breaks, but they make it readable)