Rewriting file from bash

[Julius Hamilton]

Hey,

How could I print the nth region of text of a file that is between blank
newline regions, at the command line? So for example:

file.txt:

line1

line2
line3

line4



$ region[2]

would return

line2
line3

I presume this would be with grep, sed and/or awk.

How could I then write a replacement for that region, from the command line
(i.e. not using Vim or something)? Something like:

$ region [2] “this is the replacement line”

Thank you very much,
Julius