using System;
    using System.Text.RegularExpressions;
    using System.Collections.Specialized;
    class Program
    {
    static void Main() {
    var myRegex = new Regex(@"&#x\w+;|([257])");
    string s1 = @"hello Sam, how are you 27‏ do not worry 5
how are you  why you are not OK.ℏ  and ‍ ";
 
    string replaced = myRegex.Replace(s1, delegate(Match m) {
    switch (m.Groups[1].Value) {
    	case "2": return "a";
    	case "7": return "b";
    	case "5": return "c";
    	default: return  m.Value;
        }
    });
    Console.WriteLine("\n" + "*** Replacements ***");
    Console.WriteLine(replaced);
 
 
    Console.WriteLine("\nPress Any Key to Exit.");
    Console.ReadKey();
 
    } // END Main
    } // END Program

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