How do I search for "\n" without a preceding "\r" in vim?

Question

Vim won't convert fileformat if it sees inconsistent line endings. How can I find those ?

Answer 1

Zero-width look-behind assertion

"How do I search for “\n” without a preceding “\r” in vim ?"

/\r\@<!\n

How do you search and replace these occurances with "\r\n"?

:%s/\r\@<!\n/\r\n/gc

Get rid of the final "c" if you want to accept every match without confirmation.

Edit: However, if you want an answer that you can remember how to do without having used look ahead/behind assertions a billion times, see Jackson Miller's answer. Sometimes it is better to simplify your problem and use tools that don't require you to read the manual constantly :)