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 result

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.Replace(@"/", @"//"),
RegexOptions.IgnoreCase);
Console.WriteLine(a);
Console.Read();

//value of a is True,but i don't know why