How to validate a string?

Question

I was just wondering how one would write a code that validates a string? For example a user inputs a postal code (string). i have to make sure it follows a L#L#L# format L-> represents a letter only and #-> represents only a number not decimal ...if not ask user to enter again

Answer 1

String Methods more info

For your example you could slice the string with a step of 2 checking every other if it is a digit/letter:

.isdecimal checks for characters that make up base-10 numbers systems (0-9).

.isalpha checks for letters (A-Z)

test_good = 'L5L5L5'
test_bad = 'LLLLLL'

def check_string(test):
    if test[0::2].isalpha() and test[1::2].isdecimal():
        return True
    else:
        return False

Test it out:

check_string(test_good)
>>>True

Negative test:

check_string(test_bad)
>>>False

Regex more info regexr

Regex does pattern matching operations and really a lot more. In the example below I compiled the pattern ahead of time so that it looks clean and can be reused if needed.

I also use re.fullmatch() which requires the entire provided string match, not just one part of it. On its own it will return None or the match object, so I check to see if it exists (meaning it matched) and return True or if not (None) return False.

import re

def match(test):
    re_pattern = re.compile('[A-Z][0-9][A-Z][0-9][A-Z][0-9]')
    if re.fullmatch(re_pattern, test):
        return True
    else:
        return False