python-hard-way/caeser-cipher.py

import re # Import Regex tools
import string # Import string tools
import collections

ciphertext = input("Please type in your cipher text:") # Set variable cipertext to the input from user.
shiftNum = input("Please enter the shift used:") # Get input
ciphertext = re.sub (r'([^a-zA-Z ]+?)', '', ciphertext) # Remove all non-letters.
ciphertext = ciphertext.lower() # Make it all lowercase. https://www.geeksforgeeks.org/isupper-islower-lower-upper-python-applications/
letters = list(string.ascii_lowercase) # Use a list of lowercase letters. https://stackoverflow.com/questions/43918437/how-to-iterate-through-the-alphabet
letterCount = 0 # Set variable for keeping track of which letter in the ciphertext we're on.
letterPos = 0 # Set variable for keeping track of the letter's position in the alphabet.
answer = "" # The solution.

for c in ciphertext: # For every letter in the ciphertext
    #Upletters = list(string.ascii_uppercase)
    while letterPos < 26: # While the letter's position in the alphabet is less than 26 (and thus valid), continue.
        if ciphertext[letterCount] == letters[letterPos]: # Match the letter in the ciphertext to a letter in the alphabet and once they match, continue.
            letter = int(letterPos) + int(shiftNum) # Take the position of the letter and the shift number and add them for the enciphered letter.
            if letter > 25: # If the enciphered letter's position is not valid because it is too high, fix it by rotating around the alphabet.
                letter = letter - 26
            if letter < 0:
                letter = letter + 26 # If the enciphered letter's position is not valid because it is too low, fix it by rotating around the alphabet.
            answer = answer + letters[letter] # Add letters together to get the enciphered text.

        letterPos = letterPos + 1 # Iterate through letter positions in the alphabet. (neccessary to find one that matches.)

    if letterPos > 25: # If the letter position
        letterPos = letterPos - 26

    letterCount = letterCount + 1


def freq():
    print(collections.Counter(answer).most_common()[0][0]) # Find most common letter https://stackoverflow.com/questions/47251934/how-to-count-the-most-frequent-letter-in-a-string

print("\nYour enciphered text is: " + answer) #  NOTE TO SELF: catch spaces for nice formatting in answer.
freq()

'''
On line #4 Regex
This feeds a regex to re.sub literally "If it doesn't match letters or spaces, convert it to nothing, process this variable."
Scrub extranous info or typos.
r marks it as a string literal (no escaping needed)
[] indicates a set of characters, special characters become normal.
^ = start of the string. In a set like this, it matches anything that is not in the string.
a-z matches lowercase letters.
Z-Z matches caps letters
 matches a space
+? essential means "try to match this till you can't"
More info: https://docs.python.org/3/library/re.html
'''

'''
### Stackoverflow thanks to:
First char of string: https://stackoverflow.com/questions/48973202/how-to-get-first-char-of-string-in-python
Length of string: https://stackoverflow.com/questions/4967580/how-to-get-the-size-of-a-string-in-python
Regex to scrub var of extranous info/symbols: https://stackoverflow.com/questions/44315941/regex-to-strip-all-numbers-and-special-characters-but-space-and-letters
Proper syntax for if blocks: https://stackoverflow.com/questions/37376516/python-check-if-multiple-variables-have-the-same-value
https://www.tutorialspoint.com/python/python_if_else.htm
Find substring in a string: https://stackoverflow.com/questions/3437059/does-python-have-a-string-contains-substring-method
'''