using System;

using System.Collections.Generic;

using System.Globalization;

using System.Text.RegularExpressions;

using static System.Console;

public class Program

{

    public static void Main()

    {

        string teste = "Aprovados por autorizantes da aplicação -> 02/03/2018";     

        ExtrairDatas(teste).ForEach(d => WriteLine(d));

    }

    static List<string> ExtrairDatas(string str)

    {

        var regex = new Regex(@"(?:(?:(?:(?:0?[13578])|(1[02]))/31/(19|20)?\d\d)|(?:(?:(?:0?[13-9])|(?:1[0-2]))/(?:29|30)/(?:19|20)?\d\d)|(?:0?2/29/(?:19|20)(?:(?:[02468][048])|(?:[13579][26])))|(?:(?:(?:0?[1-9])|(?:1[0-2]))/(?:(?:0?[1-9])|(?:1\d)|(?:2[0-8]))/(?:19|20)?\d\d))"); 

        var datas = new List<string>();

        foreach(Match m in regex.Matches(str))

        {                   

            DateTime dt;

            if (DateTime.TryParseExact(m.Value,"dd/MM/yyyy", CultureInfo.InvariantCulture, DateTimeStyles.None, out dt)) datas.Add(dt.ToString("dd/MM/yyyy"));

        }

        return datas;

    }

}