there are many ways to slove this without regex

You are very correct!
The current implmentation did not account for when number is equal 0

doesnt metter

that is also work

yes, when there's only one statement to execute

No, there's a newline after the third digit: "123\n".

That's why the closing quote is below the digits.

Your code should be able to handle edge cases like the stated above.

thanks for the explanation!

does that mean the input have space like this "123 " ??

Why are people so afraid of regex.. It's an awesome tool.

it can easily be done without regex

See where the closing quote is.