Replace whole line containing a string using Sed

You need to use wildcards (.*) before and after to replace the whole line:

sed 's/.*TEXT_TO_BE_REPLACED.*/This line is removed by the admin./'

You can use the change command to replace the entire line, and the -i flag to make the changes in-place. For example, using GNU sed:

sed -i '/TEXT_TO_BE_REPLACED/c\This line is removed by the admin.' /tmp/foo