Use grep to report back only line numbers

Question

I have a file that possibly contains bad formatting (in this case, the occurrence of the pattern \\\\backslash). I would like to use grep to return

Answer 1

All of these answers require grep to generate the entire matching lines, then pipe it to another program. If your lines are very long, it might be more efficient to use just sed to output the line numbers:

sed -n '/pattern/=' filename