Fundamentals Exam 03. Anonymous Vox - 05 Nov 2017 Part II

namespace _02._AnonymousVox
{
    using System;
    using System.Text.RegularExpressions;

    class AnonymousVox
    {
        static void Main(string[] args)
        {
            //Read the text
            string text = Console.ReadLine();
            //Read the placeholders
            string[] placeholders = Console.ReadLine().Split(new char[] {'{', '}'}, StringSplitOptions.RemoveEmptyEntries);

            //Why this pattern? Because we have a sequence of letters, then we have a sequence
            //of ANY ASCII character, then we have another sequence of letters WHICH MUST BE EQUAL TO THE FIRST
            //Well ... How do we do that with Regex? A simple backreference.
            //What's a backreference? Regex Presentation - "\number – matches the value of a numbered capture group"
            //THE VALUE, not the pattern, so its MATCHES THE SAME VALUE THAT THE FIRST GROUP MATCHES
            //Which is exactly what we need for our problem.
            string pattern = @"([a-zA-Z]+)(.+)\1";
            Regex regex = new Regex(pattern);

            //Match them all
            MatchCollection matches = regex.Matches(text);

            //We'll need to follow the placeholders with some index
            int placeholderIndex = 0;

            //Foreach the matches
            foreach (Match match in matches)
            {

                //If the placeholders are more than the values, then we break.
                if (placeholderIndex >= placeholders.Length) break;

                //If not, we REPLACE FIRST, wait ... Why first. Because we need to replace ONLY ONE PLACEHOLDER WITH ONE VALUE.
                text = ReplaceFirst(text, match.Groups[2].Value, placeholders[placeholderIndex++]);
            }

            Console.WriteLine(text);
        }

        //A method for replacing first, works like string.Replace.. But only replaces the first result.
        static string ReplaceFirst(string text, string oldValue, string newValue)
        {
            string substringWithOldValue = text.Substring(0, text.IndexOf(oldValue) + oldValue.Length);

            string substringWithNewValue = substringWithOldValue.Replace(oldValue, newValue);

            return substringWithNewValue + text.Substring(substringWithOldValue.Length);
        }
    }
}