How to make sed remove lines not matched by a substitution

Question

I basically want to do this:

cat file | grep '<expression>' | sed 's/<expression>/<replacement>/g'

without having to write the expression twice:

cat file | sed 's/<expression>/<replacement>/g'

Is there a way to tell sed not to print lines that does not match the regular expression in the substitute command?

Answer 1

Say you have a file which contains text you want to substitute.

$ cat new.text 
A
B

If you want to change A to a then ideally we do the following -

$ sed 's/A/a/' new.text 
a
B

But if you don't wish to get lines that are not affected with the substitution then you can use the combination of n and p like follows -

$ sed -n 's/A/a/p' new.text 
a

Answer 2

This might work for you:

sed '/<expression>/!d;s//<replacement>/g' file

Or

sed 's/<expression>/<replacement>/gp;d' file

Answer 3

cat file | sed -n '/<expression>/{s//<replacement>/g;p;}'

Answer 4

How about:

cat file | sed 'd/<expression>/'

Will delete matching patterns from the input. Of course, this is opposite of what you want, but maybe you can make an opposite regular expression?

Please not that I'm not completely sure of the syntax, only used it a couple of times some time ago.