如何检查字符串中的数字后面没有跟字母？

Question

The CS50 Problen Set 2 - Plates assignment is to write a program that will validate vanity plate input. The requirements are: “All vanity plates must start with at least two letters.” “… vanity plates may contain a maximum of 6 characters (letters or numbers) and a minimum of 2 characters.” “Numbers cannot be used in the middle of a plate; they must come at the end. For example, AAA222 would be an acceptable … vanity plate; AAA22A would not be acceptable. The first number used cannot be a '0'.” “No periods, spaces, or punctuation marks are allowed.” My code works, except for the third requirement: "Numbers cannot be used in the middle of a plate..." My code:

def main():
    plate = input("Plate: ")
    if is_valid(plate):
        print("Valid")
    else:
        print("Invalid")

def is_valid(plate):
    # Check that there is no punctuation
    if not plate.isalnum():
        return False

    # Check that the length is from 2 to 6 characters
    if len(plate) < 2 or len(plate) > 6:
        return False

    # Check to see if the first two letters are alphanumeric
    if not plate[0:2].isalpha():
        return False

    # Check to see that numbers do not start with "0"
    for i in range(len(plate)):
        # If the current character is a number and the previous character is a letter,
        # check if the number is "0".
        if plate[i].isnumeric() and plate[i-1].isalpha() and plate[i] == "0":
            return False

    # Check to see that numbers are not in the middle - this is where the problem lies !
    plate = plate[::-1]
    for i in range(len(plate)):
        if plate[i].isnumeric() and plate[i-1].isalpha():
            return False

    # If all checks pass, return True
    return True
main()

The last section in the is_valid function is supposed to check that a number is never followed by a letter. It does return those inputs as invalid, but also rejects inputs such as "CS50" which should be valid. In the version above, I have reversed the order of the variable (plate) in an attempt to see if a number follows a digit (which is not allowed in reverse order). I have tried all (I think) combinations of plate[i] and plate [i-1] etc. I have tried substituting isdigit() for isnumeric(), without success. I know there must be other ways of solving the problem, but I would really like to understand why this code is not working when the code immediately above it does. When running the code in pythontutor.com, the test seems to fail at the end of the range, where there are two letters, which makes no sense to me.

Answer 1

You have an error with your index.

if plate[i].isnumeric() and plate[i-1].isalpha():

Because it is python, at the start of the loop, the i-1 wraps around to the end of the string. Using i+1 and i raise the same problem the other way around.

You need to change your range() to start and end so that warping does not occur.

The same problem is also present in your previous check. You did not notice because there is an additional condition and the edge cases are filtered by the third check.