C# Regex - Match specific string followed by a substring1 or substring2

Question

Input : This AbT5xY\\nAppleUvW is a test AbT5xY AppleUvW is a test and AbT5xrAppleUvW and another AbT5xY\\nmangoUvW test

Following RegEx gives the output: This SomeFruitUvW is a test SomeFruitUvW is a test and AbT5xrAppleUvW and another SomeFruitUvW test.

Regex.Replace(st, "AbT5xY\\s*(Apple)|(mango)", "SomeFruit");

But what I need is that if AbT5xY is followed by Apple then replace AbT5xYApple with Fruit1 ; and if AbT5xY is followed by mango replace AbT5xYmango with Fruit2 . Hence,

Desired output : This Fruit1UvW is a test Fruit1UvW is a test and AbT5xrAppleUvW and another Fruit2UvW test.

Note :

1. I'm ignoring whitespace characters (newline, blanks, tabs etc.) between AbT5xY and Apple or AbT5xY and mango. Also AbT5xrAppleUvW is correctly not matched since it has AbT5xr and not AbT5xY before Apple.
2. I think C#'s RegEx has something called substitutions, groups, captures that need to be used here but I'm struggling with how to use these here.

Answer 1

You may capture the Apple and mango into Group 1 and when replacing, use a match evaluator, where you can check the Group 1 value, and then perform the necessary replacement based on the check result:

var pat = @"AbT5xY\s*(Apple|mango)";
var s = "This AbT5xY\nAppleUvW is a test AbT5xY AppleUvW is a test and AbT5xrAppleUvW and another AbT5xY\nmangoUvW test";
var res = Regex.Replace(s, pat, m =>
        m.Groups[1].Value == "Apple" ? "Fruit1" : "Fruit2");
Console.WriteLine(res);
// => This Fruit1UvW is a test Fruit1UvW is a test and AbT5xrAppleUvW and another Fruit2UvW test

See the C# demo .

The AbT5xY\\s*(Apple|mango) regex matches AbT5xY , then 0+ whitespaces (note a single backslash as I used a verbatim string literal) and then matches and captures either Apple or mango into Group 1. The m.Groups[1].Value == "Apple" if Group 1 value is Apple , and then proceeds to replace the match.