Fast multi-string replacement

Question

I need to perform multi string replacement. I have a string where several parts need to be changed according to substitution map.

All replacement must be done in one action - it means if "a" should be replaced with "b" and also "b" must be replaced with "c" and input string is "abc", the result will be "bcc" I have a sollution based on building regex and then replacing all matches. I wrote it some time ago, now I'm refactoring the code and not so satisfied with it. Is there better (faster, simplier) sollution?

This is what I have:

public static string Replace(string s, Dictionary<string, string> substitutions)
{
    string pattern = "";
    int i = 0;
    foreach (string ch in substitutions.Keys)
    {
        if (i == 0)
           pattern += "(" + Regex.Escape(ch) + ")";
        else
           pattern += "|(" + Regex.Escape(ch) + ")";
        i++;
     }

     var r = new Regex(pattern);
     var parts = r.Split(s);

     string ret = "";
     foreach (string part in parts)
     {
         if (part.Length == 1 && substitutions.ContainsKey(part[0].ToString()))
         {
             ret += substitutions[part[0].ToString()];
         }
         else
         {
             ret += part;
         }
      }
      return ret;
} 

And test case:

 var test = "test aabbcc";
 var output = Replace(test, new Dictionary<string, string>{{"a","b"},{"b","y"}});
 Assert.That(output=="test bbyycc");

Answer 1

You can replace all this with

var r = new Regex(string.Join("|", substitutions.Keys.Select(k => "(" + k + ")")));     
return r.Replace(s, m => substitutions[m.Value]);

The key things are making use of the string.Join method rather than implementing it yourself, and making use of this Regex.Replace overload and delegates to do the replacement.