import re
from collections import defaultdict, Counter


def count_words_v1(sentence):
    count = defaultdict(int)
    for word in sentence.split():
        count[word.lower()] += 1
    return count


def count_words_v2(sentence):
    regex = r"[\w|']+"
    count = defaultdict(int)
    for word in re.findall(regex, sentence):
        count[word.lower()] += 1
    return count


def count_words_v3(sentence):
    """Augmented by trey's regex."""
    regex = r"\b[\w'-]+\b"
    count = defaultdict(int)
    for word in re.findall(regex, sentence):
        count[word.lower()] += 1
    return count


def count_words_v4(sentence):
    """And the most compact yet pythonic solution."""
    return Counter(re.findall(r"\b[\w'-]+\b", sentence.lower()))


count_words = count_words_v4