如何检查是否为InputStream Base64编码？(不转换为字节)

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.util.regex.Pattern;

public class Base64Checker {
    public static void main(String[] args) {
        InputStream inputStream = getInputStream(); // 获取输入流
        
        try {
            String data = convertInputStreamToString(inputStream); // 将输入流转换为字符串
            boolean isBase64 = isBase64Encoded(data); // 检查字符串是否为Base64编码
            
            if (isBase64) {
                System.out.println("输入流是Base64编码");
            } else {
                System.out.println("输入流不是Base64编码");
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
    
    private static InputStream getInputStream() {
        // 返回输入流
    }
    
    private static String convertInputStreamToString(InputStream inputStream) throws IOException {
        BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream));
        StringBuilder stringBuilder = new StringBuilder();
        String line;
        
        while ((line = reader.readLine()) != null) {
            stringBuilder.append(line);
        }
        
        return stringBuilder.toString();
    }
    
    private static boolean isBase64Encoded(String data) {
        String base64Pattern = "^[A-Za-z0-9+/=]+$";
        return Pattern.matches(base64Pattern, data);
    }
}