Sed replace pattern with line number

Question

I need to replace the pattern ### with the current line number.

I managed to Print in the next line with both AWK and SED.

sed -n \"/###/{

Answer 1

This might work for you (GNU sed):

sed = file | sed 'N;:a;s/\(\(.*\)\n.*\)###/\1\2/;ta;s/.*\n//'