import re


def extract_url(input_string):
    """Extracts a URL from a blob of text.

    Params:
    - input_string: (type: string) string to parse.

    Returns:
    - result: (type: string) extracted URL.
    """
    if input_string:
        url_search = re.search(
            r'http[s]?://(?:[a-zA-Z]|[0-9]|[$-_@.&+]|[!*\(\),]|(?:%[0-9a-fA-F][0-9a-fA-F]))+',
            input_string)

        if bool(url_search):
            return url_search.group()

    return None