def METHOD_NAME(s: str, replacement_char: str = "_") -> str:
    """
    Replace all non-word characters in string with the replacement character.
    Reference SO: https://stackoverflow.com/questions/1276764/stripping-everything-but-alphanumeric-chars-from-a-string-in-python/70310018#70310018
    >>> word_characters_only("Hello, world!")
    'Hello__world_'
    >>> word_characters_only("Hello, world! 123", '')
    'Helloworld123'
    """
    return re.sub(r"\W", replacement_char, s)