(%|&&)|($)

(%|&&)

using System;
using System.Text.RegularExpressions;

public class Example
{
    public static void Main()
    {
        string pattern = @"(%|&&)|($)";
        string substitution = @";";
        string input = @"1@gmail.com%2@gmail.com
1@gmail.com&&2@gmail.com";
        RegexOptions options = RegexOptions.Multiline;

        Regex regex = new Regex(pattern, options);
        string result = regex.Replace(input, substitution);
    }
}