5-2.py

#!/usr/bin/env python

from string import ascii_uppercase
import re

input = []
with open('5_input.txt') as f:
    for line in f:
        input.append(line)

#input = [ 'dabAcCaCBAcCcaDA' ]

def makeReaction(input):
    alphabet = ascii_uppercase
    finished = False
    while not finished:
        for letter_upper in alphabet:
            letter_lower = letter_upper.lower()
            if letter_lower + letter_upper in input:
                input = re.sub(letter_lower + letter_upper, '', input)
                break
            elif letter_upper + letter_lower in input:
                input = re.sub(letter_upper + letter_lower, '', input)
                break
            elif letter_upper == 'Z':
                finished = True
    return len(input)

def polyReact(a, b):
    if a.islower():
        if a.upper() == b:
            return True
    elif a.isupper():
        if a.lower() == b:
            return True
    return False

def findShortestPolymer(input):
    alphabet = ascii_uppercase
    alpha_dict = {}
    for letter_upper in alphabet:
        letter_lower = letter_upper.lower()
        input_test = re.sub("[{0}{1}]".format(letter_lower, letter_upper), '', input)
        alpha_dict[letter_lower] = makeReaction(input_test)
    lowest_result = min(alpha_dict, key=lambda i: alpha_dict[i])
    result = alpha_dict[lowest_result]
    return result

if __name__ == '__main__':
    input = input[0].splitlines()[0]
    result = findShortestPolymer(input)
    print(result)