How can I find and replace with a variable using a list of filenames piped to sed?

Question

Currently I find and replace using the following:

ack -l oldstring | xargs sed -i '' -e s/oldstring/newstring/g

Is there a way to do this without having to type out oldstring more than once? This is what I'm trying that does not work:

ack -l oldstring | xargs sed -i '' -e s/{}/newstring/g

Answer 1

I finally decided to create two bash functions to save typing:

# Function to find and replace.
function ags {
  #!/bin/bash
  ag -l "$1" | xargs sed -i '' -e "s/$1/$2/g";
}
# Function to delete any lines that contain a string.
function agd {
  #!/bin/bash
  ag -l "$1" | xargs sed -i '' -e "/$1/d";
}