How to replace 2 consecutive lines after Match with Sed

You can repeat the commands:

gsed '/^#ABC/{n;s/Size=bar/Size=foo/;n;s/Size=bar/Size=foo/}' file

See the online demo.

The n command "prints the pattern space, then, regardless, replace the pattern space with the next line of input. If there is no more input then sed exits without processing any more commands."

So, the first time it is used, you replace on the first line after line starting with #ABC, then you replace on the second line below that one.

gnu and some other sed versions allow you to grab a range using relative number so you can simply use:

sed -E '/^#ABC$/,+2 s/(Size=)bar/\1foo/' file
#ABC
oneSize=foo
twoSize=foo
threeSize=foo

Command details:

• /^#ABC$/,+2 Match range from pattern #ABC to next 2 lines
• s/(Size=)bar/\1foo/: Match Size=bar and replace with Size=foo, using a capture group to avoid repeat of same String in search and substitution

You may also consider awk to avoid repeating pattern and replacements N times if you have to replace N lines after matching pattern:

awk 'n-- {sub(/Size=bar/, "Size=foo")} /^#ABC$/ {n=2} 1' file

#ABC
oneSize=foo
twoSize=foo
threeSize=foo

Using sed, the loop will break when Size=bar is no longer found therefore replacing the first 2 lines after the match.

$ sed '/^#ABC/{:l;n;s/Size=bar/Size=foo/;tl}' input_file
#ABC
oneSize=foo
twoSize=foo
threeSize=foo

Using sed -z

sed -z 's/#ABC\noneSize=bar\ntwoSize=bar/#ABC\noneSize=foo\ntwoSize=foo/' file.txt
#ABC
oneSize=foo
twoSize=foo
threeSize=foo

Or

sed -E -z 's/#ABC\n(.*)bar\n(.*)bar/#ABC\n\1foo\n\2foo/' file.txt