four-operators-switch

The @as(i64, c - '0') pattern is starting to repeat. Add a tiny helper to cut the noise:

fn digit(c: u8) i64 {
    return @as(i64, c - '0');
}

Then digit(input[0]) instead of @as(i64, input[0] - '0').

Handle all four operators. Read input[1] and use a switch. In your main add this:

    check("3+5", 8);
    check("9-3", 6);
    check("4*7", 28);
    check("8/2", 4);

In Zig switch works like this:

const month: i64 = 2;
const month_string: []const u8 = switch (month) {
    0 => "Jan",
    1 => "Feb",
    else => "got bored typing this already",
};

For division, use @divTrunc(a, b) -- Zig insists you be explicit about truncation.

fn eval(input: []const u8) i64 {
    const a: i64 = digit(input[0]);
    const op: u8 = input[1];
    const b: i64 = digit(input[2]);
    return switch (op) {
        '+' => a + b,
        '-' => a - b,
        '*' => a * b,
        '/' => @divTrunc(a, b),
        else => 0,
    };
}

Four oks. We can evaluate any single-operator expression on two single digits. It's not much -- but think about what just happened. A string went in. A number came out. That's the whole game. Everything from here is just making the string more interesting.