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):

One upper case letter
Two Lower case letters
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

Much easier/cleaner with regexp /([A-Z])([a-z]{3})([0-9]{3})/ (something like that, it's been a while) — JCOC611
– JCOC611, Commented Feb 28, 2016 at 18:36
Given that you don't have a specific problem, this is perhaps not a great question for stackoverflow. There are probably better sites on the stack exchange network (maybe programmers?) for this question. — Rook
– Rook, Commented Feb 28, 2016 at 18:38
Learn about regular expressions. They are really useful for this kind of tasks. — Yaroslav Admin
– Yaroslav Admin, Commented Feb 28, 2016 at 18:41
print('Correct Format' if re.match(r'[A-Z][a-z]{2}[0-9]{3}', u_id) else 'Wrong Format') should do it. — AChampion
– AChampion, Commented Feb 28, 2016 at 18:41
@JCOC611 Thanks, I'll look into it. Haven't learnt regexp yet. — Hassan.S
– Hassan.S, Commented Feb 28, 2016 at 18:41

JCOC611 · Accepted Answer · 2016-02-28 18:55:09Z

13

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.

edited Feb 28, 2016 at 18:55

answered Feb 28, 2016 at 18:43

JCOC611

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2 Comments

AChampion Over a year ago

Only 2 lower case characters, plus you don't really need need to group them (). Good idea on the $, so you probably should have the start ^ too.

JCOC611 Over a year ago

Lol I think it should be fine now. @AChampion thanks for the recommendations!

2 revs · Accepted Answer · 2016-02-28 18:57:10Z

2

If you want to solve it without regular expressions (mind you, in this case they are the right tool!), you could do something like this:

id_format = [
    "ABCDEFGHIJKLMNOPQRSTUVWXYZ", # or string.ascii_uppercase etc.
    "abcdefghijklmnopqrstuvwxyz",
    "abcdefghijklmnopqrstuvwxyz",
    "0123456789",
    "0123456789",
    "0123456789",
]

def check(input):
    # check for same length
    if len(input) != len(id_format):
        return False

    for test, valid in zip(input, id_format): # itertools.zip_longest can make
        if test not in valid:                 # the length check unnecessary
            return False       

    return True

check("Abc123") # True
check("abc123") # False

edited Feb 28, 2016 at 18:57

community wiki

2 revs
Georg Schölly

5 Comments

AChampion Over a year ago

return all(test in valid for test, valid in zip(input, id_format) would also work.

AlokThakur Over a year ago

Good one for interview

JCOC611 Over a year ago

You could also provide functions instead of strings (i.e str.isupper, str.islower, str.isdigit...) which could be more efficient.

Georg Schölly Over a year ago

@AChampion: That's beautiful!

Georg Schölly Over a year ago

@JCO611: I agree. I tried to create the clearest code possible, your regex solution is of course the "correct" one.

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