replace last substring

CSharp · Console

/Program.cs

using System;
					
public class Program
{
	public static void Main()
	{
		string myText = "abc def ghi 123 abc def ghi 123 abc def";
		string searchStr = "abc";
		int lastIndex = myText.LastIndexOf(searchStr);
	    if(lastIndex>=0)
		myText = myText.Substring(0,lastIndex) + myText.Substring(lastIndex+searchStr.Length);
		Console.WriteLine(myText);
	}
}