using System.Text.RegularExpressions;
...
string pattern1 = "This is \n\n a test.";
string pattern2 = "This is \n pattern 2.";
Regex test_pattern=new Regex("[^\n]\n[^\n]");
if(test_pattern.IsMatch(pattern1))
Console.WriteLine("Pattern 1 matches.");
if(test_pattern.IsMatch(pattern2))
Console.WriteLine("Pattern 2 matches.");