class Expression:
    """ Useful regexes."""
    DIGIT = r'\d'
    WORD = r'\w'
    SPACE = r'\s'
    TAB = r'\t'
    NEW_LINE = r'\n'
    CARRIAGE_RETURN = r'\r'
    FORM_FEED = r'\f'
    WORD_BOUNDARY = r'\b'
    ESCAPE = r'\e'
    VERTICAL_TAB = r'\v'
    NOT_DIGIT = r'\D'
    NOT_WORD = r'\W'
    NOT_SPACE = r'\S'
    NOT_WORD_BOUNDARY = r'\B'
    MATCH_START = r'^'
    MATCH_END = r'$'
    RANGE_LOWERCASE_ALPHABET = r'a-z'
    RANGE_UPPERCASE_ALPHABET = r'A-Z'
    RANGE_NUMBERS = r'0-9'
    ZERO_OR_ONE = r'?'
    ZERO_OR_MORE = r'*'
    ONE_OR_MORE = r'+'
    OR_ = r'|'
