import re

docs = {
    "P1": "Unused items can be returned within 30 days.",
    "P2": "Standard shipping usually takes 5 days."
}
query = "How many days before unused items can be returned?"

def words(text):
    return set(re.findall(r"[a-z]+", text.lower()))

scores = {key: len(words(query) & words(text)) for key, text in docs.items()}
best = max(scores, key=scores.get)
print("Source:", best)
print("Evidence:", docs[best])