using System;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        //options/configuration values which may work better as parameters

        string source = "The first world war occurred in the early 20th century, many countries around the world were originally agnostic to the early battles. An obvious reason why is because there are many countries in the world that do not have relations to nation states that were fighting in the early battles.";

        string template = "Roses {0} red, scarlets {1} blue, this poem doesn't have {2} ending, so embrace the unknown because it is {3}.";

        //When true both "around" and "An" will match "a". When false only "around" will

        bool ignoreCase = true;

        int numberOfBlanks = 4;

        //word to use in blanks if not enough matching words are found. use an empty string "" if you don't want anything filled

        string defaultWord = "banana";

        string matchingLetter = "a";

        string[] madgabWords = new string[numberOfBlanks];

        Array.Fill(madgabWords, defaultWord);

        string[] textSplit = source.Split(" ")

            .Where(x=> x.StartsWith(matchingLetter, ignoreCase, null))

            .ToArray();

        for(int i = 0; i < textSplit.Length && i < numberOfBlanks; i++)

        {

            madgabWords[i] = textSplit[i];

        }

        Console.WriteLine(template, madgabWords);

        //if you are building a string to return or assign to a variable instead of printing directly to console then use:

        var madgab = string.Format(template, madgabWords);

    }

}