How can I write a filetype plugin that evaluates the first lines of the file?

Question

I'm trying to modify my filetype.vim such that it highlights a unified diff file with diff syntax, overriding whatever syntax is normally associated with the file's extension.

So far, I'm just trying to see if the first line in the file begins with ---. However, what I have will treat the file as a diff if any line begins with ---.

au BufRead,BufNewFile *
  \ if search('^---', "n") |
  \   set filetype=diff |
  \ endif
augroup END

I've tried various combinations of setting the cursor position, adding a stop parameter to search, but nothing I've tried works.

Answer 1

One simple tweak is to use \%^ which only matches at the start of the file.

You'll also want to pass search() a 'c' flag, to match at the current cursor position (which should be the start of the file.)

au BufRead,BufNewFile *
  \ if search('\%^---', "cn") |
  \   set filetype=diff |
  \ endif

Another option would be to use getline(1) to get the contents of the first line only and then use match() on it.

if match(getline(1), '^---') >= 0 | ...

You can also use the pattern matching operator =~# instead of match(). (The # at the end makes it case insensitive, which doesn't matter much in this example but it's a good practice to use it explicitly.)

if getline(1) =~# '^---' | ...

Either should be fine for your use case.