Why can be so about Regex.IsMatch with double backslash?

In C #, / is the escape character, character escape sequences as follows
/' Single quote
/" Double quote
// Backslash
/0 null
/a warning
/b backspace
/f new page
/n new line
/r enter
/t horizontal tab
/v vertical tab

string str= "The destination 'c://test.txt' already exists.";string strP = "The destination 'c://test.txt' already exists.";Console.WriteLine(str);Console.WriteLine(strP);Console.WriteLine("Done");bool a = Regex.IsMatch( str, strP, RegexOptions.IgnoreCase);Console.WriteLine(a);Console.Read();//value of a is False, so the double backslash will affect the resultstring str= "The destination 'c://test.txt' already exists.";string strP = "The destination 'c://test.txt' already exists.";Console.WriteLine(str);Console.WriteLine(strP);Console.WriteLine("Done");bool a = Regex.IsMatch( str, strP.Replace(@"/", @"//"), RegexOptions.IgnoreCase);Console.WriteLine(a);Console.Read();//value of a is True,but i don't know why