from collections import defaultdict
from itertools import repeat
import re

input_data = '''
Beautiful is better than ugly.
Explicit is better than implicit.
Simple is better than complex.
Complex is better than complicated.'''

word_freq = defaultdict(repeat(0).__next__)
for word in re.findall('\\w+', input_data):
    word_freq[word.lower()] += 1

freq_words = defaultdict(list)
for word, freq in word_freq.items():
    freq_words[freq].append(word)
print(freq_words)