str = "http://i.imgur.com/gallery/29P5XCh"
id = None
if 'gallery/' in str:
    id = str[(str.find("gallery/") + len("gallery/")):]
else:
    id = str[(str.index("imgur.com/") + len("imgur.com/")):]
if '.' in id:
    id = id[:id.index(".")]
print(id)