search 

To search the Photo Stories in your Galleries, tap the the Search button (magnifying glass symbol) in the upper right corner and search for tags, captions or any text in your Gallery. You'll also notice the convenient built-in searches "On This Day", "During This Week" and "New Unread". The built-in searches make it easy to find old and new Photo Stories.

If you’re doing a search in a Gallery, the search only searches in that Gallery.