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
