def insert_after(content, regex, content_to_insert):
    """ Inserts content_to_insert into content immediately after regex. 

    Args:
        content: Existing string.
        regex: String identifier indicating where string needs to be inserted.
        content_to_insert: String to be inserted into content.

    Returns:
        New string with content_to_insert inserted into content directly after regex,
        or None if regex was not found.
    """
    match = regex.search(content)
    if match is not None:
        insert_index = match.end()
        content = content[:insert_index] + content_to_insert + content[insert_index:]
    return None if match is None else content