How to install gnu sed on Mac OS X and set it as the default

install-gnu-sed-on-mac-osx.sh

# Check which version of sed is used when you run the `sed` command
# The version that ships with Mac OS X is 
#   /usr/bin/sed
which sed

# Install gnu-sed using Homebrew
# The `--with-default-names` option configures `sed` to use gnu-sed
# Without that option, you'll need to type `gsed` to use gnu-sed
brew install --default-names gnu-sed 

# Verify which version of sed is used when you run the `sed` command
#  OK:  /usr/local/bin/sed
#  BAD: /usr/local/bin/gsed 
# If BAD, run `brew uninstall gnu-sed`, then reinstall with `--with-default-names`
which sed

For zsh, after installing gnu-sed using brew install gnu-sed, I just added this at the end of the ~/.zshrc:

export PATH="/opt/homebrew/opt/gnu-sed/libexec/gnubin:$PATH"

You path may vary and you can acquire it using brew info gnu-sed (as mentioned above). The key is to add it late in the ~/.zshrc. Also note, that if you are using a different shell, you will want to change the path variable in the corresponding configuration file.