Find a string using a given regular expression. 
    If the string cannot be found, returns None.
    The regex should contain one matching group, 
    as only the result of the first group is returned.

    s - The string to search.
    regex - A string containing the regular expression.
    
    Returns a unicode string or None.

def find_string(regex, s):
    """Find a string using a given regular expression. 
    If the string cannot be found, returns None.
    The regex should contain one matching group, 
    as only the result of the first group is returned.

    s - The string to search.
    regex - A string containing the regular expression.
    
    Returns a unicode string or None.
    """
    m = re.search(regex, s)
    if m is None:
        return None
    return m.groups()[0]