Linux Shell Scripting Cookbook - Third Edition

Quoting

The sed expression is commonly quoted with single quotes. Double quotes can be used. The shell will expand double quotes before invoking sed. Using double quotes is useful when we want to use a variable string in a sed expression.

Consider this example:

    $ text=hello
    $ echo hello world | sed "s/$text/HELLO/" 
    HELLO world 

$text is evaluated as hello.