How to insert a newline in front of a pattern?

Question

Not how to insert a newline before a line. This is asking how to insert a newline before a pattern within a line.

For example,

sed 's/regexp/&\n/g'

will insert a newline behind the regexp pattern.

How can I do the same but in front of the pattern?

Here is an example input file

somevariable [phone]

Should become

somevariable
[phone]

Answer 1

To insert a newline to output stream on Linux, I used:

sed -i "s/def/abc\\\ndef/" file1

Where file1 was:

def

Before the sed in-place replacement, and:

abc
def

After the sed in-place replacement. Please note the use of \\\n. If the patterns have a " inside it, escape using \".

Answer 2

sed -e 's/regexp/\0\n/g'

\0 is the null, so your expression is replaced with null (nothing) and then...
\n is the new line

On some flavors of Unix doesn't work, but I think it's the solution at your problem.

echo "Hello" | sed -e 's/Hello/\0\ntmow/g'
Hello
tmow

Answer 3

in sed you can reference groups in your pattern with "\1", "\2", .... so if the pattern you're looking for is "PATTERN", and you want to insert "BEFORE" in front of it, you can use, sans escaping

sed 's/(PATTERN)/BEFORE\1/g'

i.e.

  sed 's/\(PATTERN\)/BEFORE\1/g'

Answer 4

After reading all the answers to this question, it still took me many attempts to get the correct syntax to the following example script:

#!/bin/bash
# script: add_domain
# using fixed values instead of command line parameters $1, $2
# to show typical variable values in this example
ipaddr="127.0.0.1"
domain="example.com"
# no need to escape $ipaddr and $domain values if we use separate quotes.
sudo sed -i '$a \\n'"$ipaddr www.$domain $domain" /etc/hosts

The script appends a newline \n followed by another line of text to the end of a file using a single sed command.