How can I make regular expression in python match only alphanumeric and not numerical or alphabet characters?

How do I make regular expressions in python match this13 or 13this but not this or 13?

>>> re.search(r'[\w\d]+', 'this13') # returns this13 which is ok
>>> re.search(r'[\w\d]+', '13this') # returns 13this which is ok
>>> re.search(r'[\w\d]+', 'this')   # returns this but I want empty string as result
>>> re.search(r'[\w\d]+', '13')     # returns 13 but I want empty string as result

I would use an alternation here:

inp = ["this13", "13this", "this", "13"]
output = [bool(re.search(r'^(?:[a-z]+[0-9]+|[0-9]+[a-z]+)$', x, flags=re.I)) for x in inp]
print(output)  # [True, True, False, False]

The regex pattern used above says to match:

^                 from the start of the string
(?:
    [a-z]+[0-9]+  letters followed by numbers
    |             OR
    [0-9]+[a-z]+  numbers followed by letters
)
$                 end of the string