using System;
using System.Text.RegularExpressions;

public class Test
{
	public static void Main()
	{
    var sampleText = @"Mr. Grant and Ms. Kelly  starred in the film \'To Catch A Thief' but not in 'Stardust' because they'd stopped acting by then";
    var regEx = new Regex("(?<!\\\\)'");
    var result = regEx.Replace(sampleText, "\\'");
    Console.WriteLine(result);
	}
}