Dictionary<string,string> myCollection = new Dictionary<string,string>();

myCollection.Add("(.*)orange(.*)","Oranges are a fruit.");
myCollection.Add("(.*)apple(.*)","Apples have pips.");
myCollection.Add("(.*)dog(.*)","Dogs are mammals.");
// ...

string input = "tell me about apples and oranges";

var results = from result in myCollection
              where Regex.Match(input,result.Key,RegexOptions.Singleline).Success
              select result;

foreach (var result in results)
{
    Console.WriteLine(result.Value);
}

// OUTPUT:
//
// Oranges are a fruit.
// Apples have pips.