C# 截取字符串，区分中英文情况

public static string SubstringByCN(string strInput, int begin, int length)
        {
            if (!Regex.IsMatch(strInput, "[u4E00-u9FA5uF900-uFA2D]", RegexOptions.IgnoreCase))
            {
                if (strInput.Length < length)
                    return strInput;
                else
                    return strInput.Substring(0, length);
            }
            else
            {
                string checkstr = Regex.Replace(strInput, "[u4E00-u9FA5uF900-uFA2D]", "**", RegexOptions.IgnoreCase);
                if (checkstr.Length < length)
                    return strInput;
                else
                {
                    string strOut = "";
                    int strLength = 0;
                    for (int i = 0; i < strInput.Length; i++)
                    {
                        if (strLength >= length)
                            break;
                        strOut += strInput.Substring(i, 1);
                        if (!Regex.IsMatch(strInput.Substring(i, 1), "[u4E00-u9FA5uF900-uFA2D]", RegexOptions.IgnoreCase))
                            strLength += 1;
                        else
                            strLength += 2;
                    }
                    return strOut;
                }
            }
        }