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

Given the limitations of the = command, I think it's easier to divide the job in two (actually, three) parts. With GNU sed you can do:

$ sed -n '/###/=' test > lineno

and then something like

$ sed -e '/###/R lineno' test | sed '/###/{:r;N;s/###\([^\n]*\n\)\([^\n]*\)/\2\1/;tr;:c;s/\n\n/\n/;tc}'

I'm afraid there's no simple way with sed because, as well as the = command, the r and GNU extension R commands don't read files into the pattern space, but rather directly append the lines to the output, so the contents of the file cannot be modified in any way. Hence piping to another sed command.

If the contents of test are

fooo
bar ### aa
test
zz ### bar

the above will produce

fooo
bar 2 aa
test
zz 4 bar