Can someone please explain to me why this works and other ways to do this using re?

Question

Regular expressions

We need to create a number formatting system for a contacts database.
Create a program that will take the phone number as input, and if the number starts with "00", replace them with "+".
The number should be printed after formatting.

Sample Input
0014860098

Sample Output
[phone]
Notice that here you shouldn't convert the input string to integer, because you will work with symbols.

Code:

import re
num = input()
print(re.sub(r"^00","+",num))

Answer 1

It substitutes two 0's with a '+' IF the 0's are at the start of the number. The important part is the carat. It tests for items only at the beggining.