Same pattern of Regular Expression but output is different between PHP and C# .NET

Question

I'm a new in PHP and .NET. Here is my question. I write the same Regular Expression in PHP and .NET. But I don't know why their countRow are different answers. I want their outputs are the same(countRow = 1). Please help me solve this problem. Thank a lot.

This is my PHP code.

<?php

$p = "S:";
$pattern = "/^([A-Z]:+)$/";
$countRow = 0;
if(preg_match($pattern,$p)){
   $countRow = $countRow + 1;           
}
echo $countRow;

//countRow is 1;
?> 

This is my C#.NET code.

using System;
using System.Text.RegularExpressions;

public class Example
{
   public static void Main()
   {
      int countRow = 0;
      string p= "S:";
      string pattern = @"/^([A-Z]:+)$/";
      if(Regex.IsMatch(p,pattern)){
          countRow = countRow + 1;                
      } 
      Console.WriteLine("{0}",countRow);
     //countRow is 0
   }
}