Get a file extension from a URL in Golang

I am presenting a quick code snippet in case anyone else needs a safe way to get the file extension from a URL using Go (Golang). The issue with URLs is that they have many “parts” that using a regular expression might not be the best possible (and readable) way to solve this with. Instead, I am using the standard Go url library to isolate the path (without the host or the query-string), and extract the extension from it.

func GetFileExtensionFromUrl(rawUrl string) (string, error) {
   u, err := url.Parse(rawUrl)
   if err != nil {
      return "", err
   }
   pos := strings.LastIndex(u.Path, ".")
   if pos == -1 {
      return "", errors.New("couldn't find a period to indicate a file extension")
   }
   return u.Path[pos+1 : len(u.Path)], nil
}

Tests:

func TestGetFileExtensionFromUrl(t *testing.T) {
   tests := []struct{
      fullUrl, want string
      wantErr bool
   }{
      {
         fullUrl: "https://assets.somedomain.com/assets/files/mypicture.jpg?width=1000&height=600",
         want: "jpg",
         wantErr: false,
      }, {
         fullUrl: "https://google.com",
         wantErr: true,
      },
   }

   for _, tt := range tests {
      t.Run(tt.fullUrl, func(t *testing.T) {
         ext, err := utils.GetFileExtensionFromUrl(tt.fullUrl)
         if tt.wantErr {
            assert.NotNil(t, err)
         } else {
            assert.Nil(t, err)
            assert.Equal(t, ext, tt.want)
         }
      })
   }
}

Tip: if you need the period ( . ) along the returned extension, just change u.Path[pos+1 : len(u.Path)] to u.Path[pos : len(u.Path)]

This is inspired by https://stackoverflow.com/a/44570361