Function preprocess_text does not seem to strip punctuations

```
def preprocess_text(text):
    text = ' '.join(word.lower() for word in text.split(" "))
    text = re.sub(r"([.,!?])", r" \1 ", text)
    text = re.sub(r"[^a-zA-Z.,!?]+", r" ", text)
    return text
```

Calling `preprocess_text('Are you a, boy or a girl?')` returns:

> ''are you a , boy or a girl ? "
