Regex Crossword character class solver helper

regex_crossword_helper.py

"""Helps solving https://regexcrossword.com/ that use character classes."""
def letters(string):
    """Converts a string representing a character class into the set of letters that can match it.
    
    Args:
        string (str): String representing a Character class. Can be '^A-CX-Z' or '[^A-CX-Z]'
    
    Returns:
        set: A set of (upper case) letters matching the character class
    """
    string = string.upper()
    def pop_left():
        nonlocal string
        first, *string = string
        return first
    
    def letters_set(a, b):
        return set(''.join([chr(x) for x in range(ord(a), ord(b)+1)]))
    
    if string[0] == '[':
        pop_left()
    if string[0] == '^':
        reverse = True
        pop_left()
    else:
        reverse = False
    
    result = set()
    
    while string:
        start = pop_left()
        if start == ']':
            continue
        if string[0] == '-':
            pop_left()
        end = pop_left()
        result = result | letters_set(start, end)
    
    if reverse:
        result = letters_set('A', 'Z') - result
    return result

# https://regexcrossword.com/challenges/doublecross/puzzles/1
# v = vertical
# h = horizontal
# l = left
# r = right
v1t = letters('[D-HJ-M]')
v1b = letters('[^F-KM-Z]')
v2t = letters('[^A-RU-Z]')
v2b = letters('[A-KS-V]')

h1l = letters('[A-GN-Z]')
h1r = letters('[^A-DI-S]')

print(v1t & v1b & h1l & h1r)
print(v2t & v2b & h1r & h1r)