An implementation of a simple caesar cipher

caesar_cipher.py

# This is an attempt at creating a caesar cipher using the ord()
# function and taking input from the user.
import re

def caesar_cipher(mode, input_str, key):
    return_str = ''
    char_as_num = None
    num_as_char = None
    char_after_cipher = None

    if mode == 'ENCRYPT':
        for char in input_str:
            char_as_num = ord(char)
            char_after_cipher = char_as_num + key
            
            if re.match(r"[^A-Za-z]", char):
                return_str += char
                
            # Why not just "char == char.upper()"? --BF
            elif char == char.lower().upper():
                if char_after_cipher > 90:
                    num_as_char = chr(char_after_cipher - 26)
                    return_str += num_as_char
                else:
                    num_as_char = chr(char_after_cipher)
                    return_str += num_as_char
            else:
                if char_after_cipher > 122:
                    num_as_char = chr(char_after_cipher - 26)
                    return_str += num_as_char
                else:
                    num_as_char = chr(char_after_cipher)
                    return_str += num_as_char
    
    if mode == 'DECRYPT':
        for char in input_str:
            char_as_num = ord(char)
            char_after_cipher = char_as_num - key
            
            if re.match(r"[^A-Za-z]", char):
                return_str += char
                
            # Same as L19
            elif char == char.lower().upper():
                if char_after_cipher < 65:
                    num_as_char = chr(char_after_cipher + 26)
                    return_str += num_as_char
                else:
                    num_as_char = chr(char_after_cipher)
                    return_str += num_as_char
            else:
                if char_after_cipher < 97:
                    num_as_char = chr(char_after_cipher + 26)
                    return_str += num_as_char
                else:
                    num_as_char = chr(char_after_cipher)
                    return_str += num_as_char
    
    return return_str

def validate_entry(variable, input_str):
    return_input = input_str
    while True:
        # Instead of using regex, this would be less costly and more readable:
        # if variable == 'mode' and return_input.upper() in ('ENCRYPT', 'DECRPYPT'):
        if variable == 'mode' and re.match(r"(^ENCRYPT|^DECRYPT)$", \
        return_input.upper()) == None:
            print('"{0}" is not a valid input for mode...'.format(return_input))
            return_input = input('> ')
        
        # Same as L64
        elif variable == 'key' and re.match(r"(^[1-9]|^1[0-9]|^2[0-5])$", \
        return_input) == None:
            print('"{0}" is not a valid input for key...'.format(return_input))
            return_input = input('> ')
        else:
            break
            
    return return_input
    
def main():
    mode = None
    input_str = None
    result = None
    key = None

    print('Welcome to a simple caesar cipher!')
    mode = input('Choose mode (encrypt/decrypt): ')
    mode = validate_entry('mode', mode)
    print('')
    print('Enter the string that you want to {0}:'.format(mode.lower()))
    input_str = input()
    print('')
    key = input('Choose key (1-25): ')
    key = validate_entry('key', key)
    print('')
    
    result = caesar_cipher(mode.upper(), input_str, int(key))
    print('Result: {0}'.format(result))
    
if __name__ == '__main__':
    main()