import re


def create_source_url(date_string: str) -> str:
    """Creates the URL to source file of date.

    Args:
        date_string (str): A date string.

    Returns:
        A URL string of the source file.
    """
    if not re.match(r"^(0[1-9]|1[012])-(0[1-9]|[12][0-9]|3[01])-20\d\d$", date_string):
        raise ValueError("Invalid date format, must be MM-DD-YYY! Try using normalise_datetime()!")
    gh_branch_url = "https://raw.githubusercontent.com/CSSEGISandData/COVID-19/master"
    source_path = "/csse_covid_19_data/csse_covid_19_daily_reports/"
    file_name = f"{date_string}.csv"
    return gh_branch_url + source_path + file_name