C# 删除字符串中除字母和数字之外的其它字符，C#将字符串中的大写转小写，小写转大写

private static string RemoveNotLetter(string title)
{
var listSign = new List<string> { "|", "'", ",", "&", ".", "!" };
var notLetter = Regex.Split(title, @"[a-zA-Z]/[0-9]", RegexOptions.IgnoreCase).Where(r => r.Trim() != string.Empty).ToList();
var newLetter = new List<string>();

for (int i = notLetter.Count - 1; i >= 0; i--)
{
if (notLetter[i].Trim().Length == 0)
{
notLetter.RemoveAt(i);
continue;
}

if (notLetter[i].Trim().Length > 1)
{
for (int j = 0; j < notLetter[i].Trim().Length; j++)
{
newLetter.Add(notLetter[i].Trim().Substring(j, 1));
}
notLetter.RemoveAt(i);
}
}

notLetter.AddRange(newLetter);
foreach (string sign in notLetter)
{
if (sign.Trim().Length == 0) continue;

if (!listSign.Contains(sign.Trim()))
{
title = title.Replace(sign.Trim(), "");
}
}
return title;
}

private string  str(string strs){
string str=strs;
char[] ch = str.ToCharArray();
System.Text.RegularExpressions.Regex R = new System.Text.RegularExpressions.Regex("[A-Z]");
str = "";
foreach (char s in ch)
{
if (R.IsMatch(s.ToString()))
{
str += s.ToString().ToLower();
}
else
{
str += s.ToString().ToUpper();
}
}
return str;
}