Find there is an emoji in a string in python3

I want to check that a string contains only one emoji, using Python 3. For example, there is a is_emoji function that checks that the string has only one emoji.

def is_emoji(s):
    pass

is_emoji("") #True
is_emoji("◼️") #False

I try to use regular expressions but emojis didn't have fixed length. For example:

print(len("◼️".encode("utf-8"))) # 6 
print(len("".encode("utf-8"))) # 4