using System;
using System.Text.RegularExpressions;
using System.Collections.Generic;

public class Test
{
    public static void Main()
    {
        Dictionary<string, string> configuracoes = new Dictionary<string, string>();

        string re = "([^\\s].*?)=([^;]+?)(;|$)";

        string input = "Data Source=.\\SQLEXPRESS;Initial Catalog=Target_Database;User Id=User; Password=MyPassword";

        foreach (Match m in Regex.Matches(input, re)) {

            //Exibe os resultados encontrados, pode remover isto é só para testes
            //Console.WriteLine("Chave: {0} - Valor: {1}", m.Groups[1].Value, m.Groups[2].Value);

            //Salva o item
            configuracoes[m.Groups[1].Value] = m.Groups[2].Value;
        }
        
        Console.WriteLine("Valor: {0}", configuracoes["Data Source"]); // Retorna .\SQLEXPRESS
        Console.WriteLine("Valor: {0}", configuracoes["Initial Catalog"]); // Retorna Target_Database
        Console.WriteLine("Valor: {0}", configuracoes["User Id"]); // Retorna User
        Console.WriteLine("Valor: {0}", configuracoes["Password"]); // Retorna MyPassword
    }
}