#[regex(r"#[0-9]+=", |lex| lex.slice()[1..lex.slice().len() - 1].to_string())]
    DatumLabeler(String),
    #[regex(r"#[0-9]+#", |lex| lex.slice()[1..lex.slice().len() - 1].to_string())]
    DatumLabel(String),

    #[test]
    fn lex_datum_labels() {
        let source = r"#0=(#0# 2 3)";
        let data = [
            (Token::DatumLabeler("0".to_string()), (0..3), "#0="),
            (Token::LParen, (3..4), "("),
            (Token::DatumLabel("0".to_string()), (4..7), "#0#"),
            (Token::Integer((2, 0)), (8..9), "2"),
            (Token::Integer((3, 0)), (10..11), "3"),
            (Token::RParen, (11..12), ")"),
        ];
        let lexer = Token::lexer(source);
        for ((tok, span), (lexed, tspan, tslice)) in lexer
            .spanned()
            .filter(|(tok, _)| tok.is_err() || tok.as_ref().is_ok_and(|v| v != &Token::Whitespace))
            .zip(data.into_iter())
        {
            let slice = &source[span.clone()];
            assert!(tok == Ok(lexed));
            assert!(span == tspan);
            assert!(slice == tslice);
        }
    }