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