How can I a put a line like “==========” quickly in vim

Question

I am editing restructuredtext files. I often need to put some charactors like \"=-`~\" in one line, and I want the length of the line match the previous line. How should I

Answer 1

When the cursor is placed on a long long line you could use something like

:s/\(.*\)/\=submatch(1) . nr2char(13) . repeat('=', strlen(submatch(1)))/

In order to make it more easy to do the substitution, I'd then use a map:

nmap __ :s/\(.*\)/\=submatch(1) . nr2char(13) . repeat('=', strlen(submatch(1)))/

So, you can underline the line where the cursor is on with typing __.