peek

        pub fn peek(self: *Self) ?[]const T {
            // move to beginning of token
            while (self.index < self.buffer.len and self.isDelimiter(self.index)) : (self.index += switch (delimiter_type) {
                .sequence => self.delimiter.len,
                .any, .scalar => 1,
            }) {}
            const start = self.index;
            if (start == self.buffer.len) {
                return null;
            }

            // move to end of token
            var end = start;
            while (end < self.buffer.len and !self.isDelimiter(end)) : (end += 1) {}

            return self.buffer[start..end];
        }