sometimes you you ask for a hostname and you want to offer your user the option of entering the 'http://' or not

just the host and scheme from a uri

def scheme_host(url)
  begin
    p_uri = URI.parse(url)
    p_uri.scheme = 'http'
    return nil if p_uri.host == 'http'
    p_uri.host = url if !p_uri.host
    p_uri.select(:scheme, :host).join('://')
  rescue URI::InvalidURIError
    nil
  rescue URI::InvalidComponentError
    nil      
  end  
end
 
#shoulda tests
scheme_host('google.com').should == 'http://google.com'
scheme_host('http://google.com').should == 'http://google.com'
scheme_host('http://goog__le.com').should == nil
scheme_host('http://http://goog__le.com').should == nil        
scheme_host('http://').should be_nil
scheme_host('/google.com').should be_nil
scheme_host('http:/google.com').should be_nil