using System;

using System.Linq;

using System.Collections.Generic;

public static class GetUpperCaseWordsFromString

{

    public static void Main(string[] args)

    {

        Console.Write("Input the string : ");

        var input = Console.ReadLine();

​

        Console.Write("\nThe UPPER CASE words are :\n ");

        foreach (string word in input.SelectWords().WhereIsUppercase())

        {

            Console.WriteLine(word.RemoveTrailingPunctuation());

        }

        Console.ReadLine();

    }

​

    // Converts a string in to an enumeration of words

    // Word will still contain any trailing punctuation, such as comma or period.

    static IEnumerable<string> SelectWords(this string s, bool ignoreDoubleWhiteSpace = false)

    {   

        int start = 0;

​

        for (var current = 0; current < s.Length; current++)

        {

            // Return next word on each space.

            if (char.IsWhiteSpace(s[current]))

            {

                var word = s.Substring(start, current - start);

                if (!ignoreDoubleWhiteSpace) yield return word;

                else if ( word.Length > 1 || (word.Length == 1 && !char.IsWhiteSpace(word[0]))) yield return word;

                start = current+1;

            }               

        }               

        // Return the remainder of the string, after the last space.

        if (start < s.Length) yield return s.Substring(start, s.Length - start);            

    }

    // Returns only the upper-case words from an enumeration of words.

    static IEnumerable<string> WhereIsUppercase(this IEnumerable<string> seq)

    {

        foreach( var s in seq)

        {

            // Each character, in a word, must be either an upper-case letter or a non-letter.

            if (s.All(x=> char.IsLetter(x) && char.IsUpper(x) || !char.IsLetter(x))) yield return s;                      

        }   

    }

    // Removes the last character, if it is punctuation.

    static string RemoveTrailingPunctuation(this string word) 

    {

        return word.Length > 0 && char.IsPunctuation(word[word.Length - 1]) ? word.Substring(0, word.Length - 1) : word;

    }

}

​