It returns None because it looks only at the position 0 (first char) only.
You need search instead:
import re
x = re.search('[A-Z]{1}', 'ABCDEF').group()
print(x)
# This doesn't work as expected
x = re.search('[A-Z]{1}', 'aBCDEF').group()
print(x)
Output:
A
B
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