How to replace specific words, without knowing them C#

Question

I want to replace a specific word in my string list in c#. I created a list that contains appointmens like([9.9.2009] meeting at 9:00), and now i want to add a function to edit the appointments like "[9.9.2009]" for "[8.8.2008]".

This is my code:

int count = File.ReadLines(appointmentPath + "appointment.txt").Count();
Console.WriteLine("Which appointment you want to edit? (1 - " + count + ")");
if (count != 0)
{
    for (int i = 1; i < count + 1; i++)
    {
        List<string> linescount = File.ReadAllLines(appointmentPath + "appointment.txt").ToList();
        Console.WriteLine(i + ". " + linescount[i - 1]);
    }

    int input = Convert.ToInt32(Console.ReadLine());
    List<string> linesList = File.ReadAllLines(appointmentPath + "appointment.txt").ToList();
    Console.Clear();
    if(input != 1)
    {
        Console.WriteLine(linesList[input - 1]);
    }
    else
    {
        Console.WriteLine(linesList[0]);
    }

    Console.WriteLine("What you want to edit? (1. Date, 2. Summary, 3. Time)");
    input = Convert.ToInt32(Console.ReadLine());
    switch (input)
    {
    case 1:             break;
    case 2:             break;
    case 3:             break;
    default: Console.WriteLine("Input invalid"); break;
    }

Answer 1

I suggest using regular expressions here: we can match (and replace) all fragments like ( d here is a digit)

d.d.dddd
d.dd.dddd
dd.d.dddd
dd.dd.dddd

We can do it with a help of pattern

[0-9]{1,2}\.[0-9]{1,2}\.[0-9]{4}

which means

[0-9]{1,2} - 1 or 2 digits (symbols in 0..9 range)
\.         - dot .
[0-9]{1,2} - 1 or 2 digits (symbols in 0..9 range)
\.         - dot .
[0-9]{4}   - 4 digits (symbols in 0..9 range)

Code:

string result = Regex.Replace(
    text, 
  @"[0-9]{1,2}\.[0-9]{1,2}\.[0-9]{4}", 
   "8.8.2008");