using System;

using System.Collections.Generic;

using System.Text.RegularExpressions;

​

public class Program

{

    public static void Main()

    {

        var testError = "The element 'blah' has invalid child element 'blah 2'. List of possible elements expected: 'blah 3'. Error count: (1). First error found at Position 148801:...";

        var alternateError = "The element 'hydrogen' is missing a neutron. Please replace with deuterium. Error count: (1). First error found at position 1628395613581...";

        var errorFeed = new List<string>(){testError,testError,testError,alternateError,testError,testError};

        var matchString = @"(?<=Error count: \().(?=\))";

        var errorCountMatcher = new Regex(matchString);

        var errors = new List<string>();

        Console.WriteLine($"Number of actual errors: {errorFeed.Count}. Example error: {errorFeed[0]}");

​

        foreach (var newError in errorFeed) {

            if (errors.Count > 0) {

                int i = errors.FindIndex(error => error.StartsWith(newError.Substring(0, newError.IndexOf("Error count: ("))));

                if (i > -1) {

                    errors[i] = Regex.Replace(errors[i], matchString, (int.Parse(errorCountMatcher.Match(errors[i]).Value)+1).ToString());

                } else {

                    errors.Add(newError);

                }

            } else {

                errors.Add(newError);

            }

        }

        Console.WriteLine($"\n\nFinal list has {errors.Count} errors:");

        foreach (var error in errors) {

            Console.WriteLine(error);   

        }

    }

}