How do I correctly set up this validation in my model?

When I validate the format of a string, I'll do:

validates :link, :uniqueness => true,
               :format => { :with => (regular expression) }

I want the link to be either a youtube video by this regex:

/^http:\/\/www\.youtube\.com\/watch\?v=([a-zA-Z0-9_-]*)/开发者_高级运维

or a vimeo video:

/^http:\/\/www\.vimeo\.com\/(\d+)/

How do I set up this validation in my model?

You could combine those two regular expressions into one, though I presume you mean you have two different expressions instead of two identical ones:

validates :link,
  :uniqueness => true,
  :format => {
    :with => %r[http://(?:www\.youtube\.com/watch\?v=(\w+)|www\.vimeo\.com...)]
  }

Using %r[...] instead of /.../ saves you from having to escape all the slashes.

Also note that YouTube may use a youtu.be domain, so you may need yet another part to your regular expression. If this thing gets totally out of hand you may want to take a different approach and have a list of regexps you compare it against:

VALID_URLS = [
  %r[http://www\.youtube\.com/watch\?v=(\w+)],
  %r[http://www\.youtu\.be/(\w+)],
  %r[ ... ]
]

Then validate something like this:

validates :link,
  :uniqueness => true,
  :format => {
    :with => Regexp.new(VALID_URLS.join('|'))
  }