我如何获取字符串的单词？

import re
from collections import defaultdict

def get_words_from_string(input_string):
    # 使用正则表达式分割字符串，得到单词数组
    words = re.split(r'\W+', input_string)

    # 初始化一个字典，用于记录每个单词的出现次数
    word_count = defaultdict(int)

    for word in words:
        # 使用正则表达式去除非字母字符，只保留字母部分
        cleaned_word = re.sub(r'[^a-zA-Z]', '', word)
        if cleaned_word:
            # 统计单词出现的频率
            word_count[cleaned_word.lower()] += 1

    return word_count

# 测试代码
input_string = "Hello, world! This is a test string. Hello world!"
word_count = get_words_from_string(input_string)
for word, count in word_count.items():
    print(f"{word}: {count}")