How to insert text between lines using sed?

Question

Command line

It is necessary to insert a part of the text between lines containing certain substrings (or falling under a regular expression).

Answer 1

Well, now the question makes more sense. The clarification problem is solved as follows:

echo -e "aaa\nbbb\naaa\nddd" | sed '/aaa/{n;/bbb/i new_line
}'

Answer 2

It is not entirely clear what it means between the lines. But anyway:
a. if you need to insert after the line with the template:

echo -e "1\n2\n3\n4" | sed '/2/a test'

if you need to insert after each line between the start and end according to the pattern:

echo -e "1\n2\n3\n4" | sed '/2/,/4/a test'