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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def process_pub_date(year, mon, day):
 """Create pub_date from what Pubmed provides in Journal PubDate entry """


    pub_date = None
    if year and re.match("[a-zA-Z]+", mon):
        pub_date = datetime.datetime.strptime(
            f"{year}-{mon}-{day}", "%Y-%b-%d"
        ).strftime("%Y-%m-%d")
    elif year:
        pub_date = f"{year}-{mon}-{day}"

    return pub_date