linux - Match a string that contains a newline using sed

Question

I have a string like this one:

    #
    pap

which basically translates to a # pap and I want to replace it with:

    #
    pap
    python

which translates to # pap python.

Tried this with sed in a lot of ways but it's not working maybe because sed uses new lines in a different way. I tried with:

sed -i "s/#
pap/#python
pap/" /etc/freeradius/sites-available/default

...and many different other ways with no result. Any idea how can I do my replace in this situation?

Answer 1

This might work for you (GNU sed):

sed '/^#$/{n;/^pap$/{p;s//python/}}' file

If a line contains only # print it, then if the next line contains only pap print it too, then replace that line with python and print that.