I'm trying to find sentences having only one digit number along with.

sentence="I'm 30 years old."
print(re.match("[0-9]", sentence)

then it returns

<re.Match object; span=(0, 1), match='3'>

but it's 30 which is two digit number actually, and I don't want it to be matched. seems like each consisting number 3 and 0 is recognized as one independent number. And these numbers are two-byte number which is usually used in my country.

How can I change my regular expression? Thanks!

You should add negative lookbehind and negative lookahead to avoid digits before and after a stand-alone digit:

re.findall("(?<!\d)\d(?!\d)", "200 20 1 20 200 20 2")
#['1', '2']
re.findall("(?<!\d)\d(?!\d)", "200 20 20 200 20")
#[]
if not re.findall("(?<!\d)\d(?!\d)", "200 20 20 200 20"):print("no single-digit numbers")
else:print("some single-digit numbers")