检测字节流是否是UTF8编码

几天前偶尔看到有人发帖子问“如何自动识别判断url中的中文参数是GB2312还是Utf-8编码”

也拜读了wcwtitxu使用巨牛的正则表达式检测UTF8编码的算法。

使用无数或条件的正则表达式用起来却是性能不高。

刚好曾经在项目中有类似的需求，这里把处理思路和整理后的源代码贴出来供大家参考

先聊聊原理：

UTF8的编码规则如下表

看起来很复杂，总结起来如下：

ASCII码（U+0000 - U+007F），不编码

其余编码规则为

•第一个Byte二进制以形式为n个1紧跟个0 (n >= 2), 0后面的位数用来存储真正的字符编码，n的个数说明了这个多Byte字节组字节数（包括第一个Byte） 
•结下来会有n个以10开头的Byte，后6个bit存储真正的字符编码。 
因此对整个编码byte流进行分析可以得出是否是UTF8编码的判断。

根据这个规则，我给出的C#代码如下：

/// <summary>

///   Determines whether the given <paramref name="inputStream"/>is UTF8 encoding bytes.

/// </summary>

/// <param name="inputStream">

///    The input stream.

///  </param>

/// <returns>

///   <see langword="true"/> if given bystes stream is in UTF8 encoding; otherwise, <see langword="false"/>.

/// </returns>

/// <remarks>

///   All ASCII chars will regards not UTF8 encoding.

/// </remarks>

public static bool IsTextUTF8(ref byte[] inputStream)

{

    int encodingBytesCount = 0;

    bool allTextsAreASCIIChars = true;

 

    for (int i = 0; i < inputStream.Length; i++)

    {

        byte current = inputStream[i];

 

        if ((current & 0x80) == 0x80)

        {                   

            allTextsAreASCIIChars = false;

        }

        // First byte

        if (encodingBytesCount == 0)

        {

            if ((current & 0x80) == 0)

            {

                // ASCII chars, from 0x00-0x7F

                continue;

            }

 

            if ((current & 0xC0) == 0xC0)

            {

                encodingBytesCount = 1;

                current <<= 2;

 

                // More than two bytes used to encoding a unicode char.

                // Calculate the real length.

                while ((current & 0x80) == 0x80)

                {

                    current <<= 1;

                    encodingBytesCount++;

                }

            }                   

            else

            {

                // Invalid bits structure for UTF8 encoding rule.

                return false;

            }

        }               

        else

        {

            // Following bytes, must start with 10.

            if ((current & 0xC0) == 0x80)

            {                       

                encodingBytesCount--;

            }

            else

            {

                // Invalid bits structure for UTF8 encoding rule.

                return false;

            }

        }

    }

 

    if (encodingBytesCount != 0)

    {

        // Invalid bits structure for UTF8 encoding rule.

        // Wrong following bytes count.

        return false;

    }

 

    // Although UTF8 supports encoding for ASCII chars, we regard as a input stream, whose contents are all ASCII as default encoding.

    return !allTextsAreASCIIChars;

}

再附上单元测试代码：

/// <summary>

///This is a test class for EncodingHelperTest and is intended

///to contain all EncodingHelperTest Unit Tests

///</summary>

[TestClass()]

public class EncodingHelperTest

{

    /// <summary>

    ///  Normal test for this method.

    ///</summary>

    [TestMethod()]

    public void IsTextUTF8Test()

    {

        for (int i = 0; i < 1000; i++)

        {

            List<Char> chars = new List<char>();

            chars.Add('中');

 

            List<UnicodeCategory> temp = new List<UnicodeCategory>();

            Random rd = new Random((int)(DateTime.Now.Ticks & 0x7FFFFFFF));

 

            for (int j = 0; j < 255; j++)

            {

                char ch = (char)rd.Next(0xFFFF);

                UnicodeCategory uc = System.Globalization.CharUnicodeInfo.GetUnicodeCategory(ch);

                if (uc == UnicodeCategory.Surrogate || // Single surrogate could not be encoding correctly.

                    uc == UnicodeCategory.PrivateUse || // Private use blocks should be excluded.

                    uc == UnicodeCategory.OtherNotAssigned

                    )

                {

                    j--;

                }

                else

                {

                    chars.Add(ch);

                    temp.Add(uc);

                }

            }

 

            string str = new string(chars.ToArray());

 

            byte[] inputStream = Encoding.UTF8.GetBytes(str);

            bool expected = true;

            bool actual;

            actual = EncodingHelper.IsTextUTF8(ref inputStream);

            Assert.AreEqual(expected, actual, string.Format("UTF8_Assert Fails at:{0}", str));

 

            inputStream = Encoding.GetEncoding(932).GetBytes(str);

            expected = false;

 

            actual = EncodingHelper.IsTextUTF8(ref inputStream);

            Assert.AreEqual(expected, actual, string.Format("ShiftJIS_Assert Fails at:{0}", str));

        }

    }

 

    /// <summary>

    ///   Check with All ASCII chars

    /// </summary>

    [TestMethod]

    public void IsTextUTF8Test_AllASCII()

    {

        string str = "ABCDEFGHKLHSJKLDFHJKLHAJKLSHJKLHAJKLSHDJKLAHSDJKLHAJKLSDHJKLASHDJKLHASJKLDHJKLASD";

 

        byte[] inputStream = Encoding.UTF8.GetBytes(str);

        bool expected = false;

        bool actual;

        actual = EncodingHelper.IsTextUTF8(ref inputStream);

        Assert.AreEqual(expected, actual, string.Format("UTF8_Assert Fails at:{0}", str));

 

 

    }

}

另：

如果是判断一个文件是否使用了UTF8编码，不一定非用这种方法，因为通常以UTF8格式保存的文件最初两个字符是BOM头，标示该文件使用了UTF8编码。

参考：

维基百科：http://en.wikipedia.org/wiki/UTF-8

http://www.cnblogs.com/powertoolsteam/archive/2010/09/20/1831638.html