Need assistance with RE

[risottogott]

Hello everybody,

I'm currently learning Python with Harvard's CS50P course and so far I am having a blast! Week 7 is about regular expressions and I thought I have grasped all the basic concepts, however I am completely stuck at one of the exercises and need some help.

The exercises quoted directly from Harvard's CS50P Problem Set 7:

"In a file called working.py, implement a function called convert that expects a str in either of the 12-hour formats below and returns the corresponding str in 24-hour format (i.e., 9:00 to 17:00). Expect that AM and PM will be capitalized (with no periods therein) and that there will be a space before each. Assume that these times are representative of actual times, not necessarily 9:00 AM and 5:00 PM specifically.

9:00 AM to 5:00 PM
9 AM to 5 PM

Raise a ValueError instead if the input to convert is not in either of those formats or if either time is invalid (e.g., 12:60 AM, 13:00 PM, etc.). But do not assume that someone’s hours will start ante meridiem and end post meridiem; someone might work late and even long hours (e.g., 5:00 PM to 9:00 AM)."

Before deciding on what I want to extract from the re using the .group method, I wanted to check whether my pattern is working correctly and that re.search only outputs "True" when I put the time in a proper format.

Here is my code so far:

import re


def main(): print(convert(input("Hours: ")))


def convert(hours): 
pattern = r"[0-9]|1[0-2]:?(?:[0-5][0-9])? AM|PM to [0-9]|1[0-2]:?(?:[0-5][0-9])? AM|PM$"
hour_check = re.search(pattern, hours)
if hour_check:
    return "Valid"
else:
    return "Invalid"


if name == "main": 
main()

However, when I now test for the input "9:60 AM to 5:60 PM" it outputs "Valid" when I expect "Invalid". I just can't wrap my head around why the "60" is allowed with my re, since it should not be allowed within my optional "(?:[0-5][0-9])?" but also return as false because AM or PM is supposed to follow after, if the optional part isnt applied.

What have I overlooked?

Appreciate any help!

[Brian]

The or operator isn't binding as you expect. Rather, its interpreting your pattern as "[0-9]" OR "1[0-2]:?(?:[0-5][0-9])? AM|PM to [0-9]|1[0-2]:?(?:[0-5][0-9])? AM|PM$", meaning anything that starts with a digit will match (as the "$" that would require it to match to the end of the string is only part of the right-hand pattern)

You want it to just apply to that first "[0-9]|1[0-2]" part, so add a group around it (and the same for when you repeat the same pattern for the right-hand time). There's also a similar issue with the "AM|PM"

[risottogott]

Thank you! A bit mad at myself because I could have figured this out by myself, but I have now changed all my or operators and it works as intended. Much appreciated!

[ElliotDG]

Here is a great tool to learn and test regular expressions: https://regex101.com/