Find the line number which contains the pattern with custom delimiter

As in example I'm trying to get line numbers which contains the pattern. My pattern contains slashes so I wanted to add custom delimiter.

This simple one works:

sed -n '/file/=' temp.txt

Using delimiter for string replace works too:

sed 's|file|gile|' temp.txt

but when I want to add delimiter to first example it doesn't:

sed -n 's|file /etc|=' temp.txt

I know I can escape slashes but I would prefer to add custom delimiter. Any idea how to fix my command?