Delete the last instance of a certain string from a text file without changing the other instances of the string

Question

I have a C# program where I am using a lot of RegEx.Replace to replace text in my text file.

Here is my problem.

In my text file, I have a code such as "M6T1". This code is listed in numerous places in the text file.

However, I only want to delete it from the bottom (last instance) in the text file. There will always be a "M6T1" at the bottom of the text file, but it is not always the last line. It could be the 3rd line from the bottom, the 5th line from the bottom etc.

I only want to get rid of the last instance of "M6T1" so RegEx.Replace won't work here. I don't want to interfer with the other "M6T1"'s in the other locations in the text file.

Can someone please give me a solution to this problem?

Thanks

Answer 1

var needle = "M6T1";
var ix = str.LastIndexOf(needle);
str = str.Substring(0, ix) + str.Substring(ix + needle.Length);

Answer 2

public static string ReplaceFirstOccurrence (string Source, string Find, string Replace)
{
    int Place = Source.IndexOf(Find);
    string result = Source.Remove(Place, Find.Length).Insert(Place, Replace);
    return result;
}

public static string ReplaceLastOccurrence(string Source, string Find, string Replace)
{
    int Place = Source.LastIndexOf(Find);
    string result = Source.Remove(Place, Find.Length).Insert(Place, Replace);
    return result;
}

Answer 3

Due to my low score, I am not allowed to comment on kevingessner's answer. So, I will add my comment here, with modification to his code -

var needle = "M6T1";
var ix = str.LastIndexOf(needle);
str = str.Substring(0, ix) + str.Substring(ix + needle.Length);

This can cause an exception when index of needle = -1. That happens when the needle is not found in the string/haystack. To avoid that, I did it like this -

String needle = "M6T1";
int ix = str.LastIndexOf(needle);
if(ix != -1){
      str = str.Substring(0, ix) + str.Substring(ix + needle.Length); 
}else{//not found}