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

freq_words = dict()
for word, freq in word_freq.items():
    freq_words.setdefault(freq, []).append(word)

print(freq_words)