sed to insert new line in file code example

Example 1: insert a line at a line number sed

sed -i '8i8 This is Line 8' FILE

8 This is Line 8

Example 2: sed add newline

For substituting with newline use sed command to replace a match with a
not used char and tr command to replace that char with a newline '\n'
#For example:
echo "123." | sed -E 's/([[:digit:]]*)\./\1|next line/' | tr '|' '\n'