using System;

using System.Text.RegularExpressions;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var input = "how to make money. I love you. I want do make something.";

        var regex = new Regex("\\s*[A-Za-z,;'\"\\s]+[.?!]");

        var matches = regex.Matches(input);

        var sentencesMatchingStr = matches

            .Cast<Match>()

            .Select(pr => pr.Value)

            .Where(pr => pr.Contains("make something"))

            .First();

        if(sentencesMatchingStr == null) {

            return;

        }

        Console.WriteLine(sentencesMatchingStr);

    }

}