Insert line before match

Insert a comment line before every server_name directive

sed '/^server_name/i # Added by deploy script' nginx.conf

More sed commands

Append line after match

sed '/^\[defaults\]/a ansible_python_interpreter=/usr/bin/python3' ansible.cfg

Append a setting line after the [defaults] header
Case-insensitive substitution

sed 's/error/ERROR/gI' file.txt

GNU sed /I flag for case-insensitive match
Multiple expressions

sed -e 's/foo/bar/g' -e 's/baz/qux/g' file.txt

Apply multiple substitutions in one command
Strip leading/trailing whitespace

sed 's/^[[:space:]]*//;s/[[:space:]]*$//' file.txt

Trim leading and trailing whitespace from each line
Address range substitution

sed '5,15s/DEBUG/INFO/g' app.log

Apply substitution only to lines 5 through 15