String Format Checking in Python

Question

While preparing for my AS-Level Computer Science exam I came across a question in the pre-release material:

Prompt the user to input a User ID and check if the format of the ID corresponds with pre-defined formatting rules and output accordingly.

The Format (In order):

1. One upper case letter
2. Two Lower case letters
3. Three numeric characters (digits)

Example: "Abc123"

I came up with a solution using my language of choice(Python), however, I was wondering if there is a more elegant or better way to solve this. Especially the third check.

Here is my code:

#Task 2.2

u_id = input("Input User ID: ") #DECLARE u_id : string
numbers = [str(num) for num in range(10)]

#Checking if final 3 characters of User ID (u_id) are digits
for i in list(u_id[3::]):
    if i not in numbers:
        digit_check = False #DECLARE digit_check : bool
        break
    else:
        digit_check = True

#User ID format check
if (u_id[0:1].isupper() == True) and (u_id[1:3] == u_id[1:3].lower()) and (digit_check == True):
    print ("Correct Format")
else:
    print ("Wrong Format")

Ignore the DECLARATION comments. They are an exam requirement.

Thanks

Answer 1

If you are allowed to import re:

import re

u_id = input("Input User ID: ") #DECLARE u_id : string

rex = re.compile("^[A-Z][a-z]{2}[0-9]{3}$")
if rex.match(u_id):
    print("Correct format")
else:
    print("Incorrect")

Explanation of expression:

• ^ represents the beginning of a string.
• [A-Z] is a range, containing all uppercase letters (in the English alphabet).
• [a-z] is a range, containing all lowercase letters.
• [0-9] is a range, containing all numbers.
• {n} specifies that n items (items are whatever is before the curly brackets) will be matched.
• $ represents the end of the string.

Also, you can see more detailed explanations and test arbitrary strings against this regular expression here.