I know this is a common question, but I haven't found a solution that works yet.

Basically, I have a file that has many instances of a block of text, like:

line1
line2
line3


And I want to replace this with a new single line of text:

newline4


It seems that text searches using sed or vi only match single lines of text...so anything that spans multiple lines is no good. E.g.:

sed s/line1\n/line2\n/line3\n/newline4/g infile > outfile

This produces no matches, and the text in the outfile is identical to infile.

I've been searching around for a solution for about an hour, and nothing sensible has turned up.

Does anyone have any ideas?

Ok...I figured out a reasonable workaround (at least, one that works in my case):

sed s/line1/newline4/g infile | grep -v line2 | grep -v line3 > outfile


This replaces the first line, and filters out line2 & line3 individually. The problem with this is that it's treating each line individually. As long as it doesn't matter whether lines 1 - 3 are in a group, or occur individually, then this is okay (which just happens to be the case for me), but I'd rather replace the text as a block if I can (I know there will be other situations where I'll need to do that).

How about using a range and the "c" command?

Let's say you want to find a pattern beginning with a line containing "fred" and ending with a line containing "alice". Then replace this (multi-line) pattern with "susan":

sed '/fred/,/alice/c susan' filename

Good SED tutorial here:
http://www.grymoire.com/Unix/Sed.html

You can add the beginning (^) and termination ($) special characters to the pixellany's solution to make the regular expressions match unique lines:
otherwise /line1/ and /line3/ will match every other line containing "line1" and "line3" (such as "hello line1" or "line3xyz").

For other solutions see the SED FAQ, here.

Indeed....
When using a range (or any other regex), you have to put in **enough** information to get the desired result

Very cool...thanks for the tips.

Er...just out of curiosity...is it possible to do the same in vi? I'm trying to get the hang of that as well...I found out how to do the equivalent of s/search/replace/g (hit colon, then enter %s/search/replace/). I'm sure there's something in the vi manual somewhere...

In vi use the syntax as in your first attempt with sed:
this works for me (using vim 7.1).

Awesome, thanks again.